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How to fix this jQuery undefined value [duplicate]

How do I determine if variable is undefined or null?
My code is as follows:
var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
// DO SOMETHING
};
<div id="esd-names">
<div id="name"></div>
</div>
But if I do this, the JavaScript interpreter halts execution.

You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined is true, the above code will catch both null and undefined.

The standard way to catch null and undefined simultaneously is this:
if (variable == null) {
// do something
}
--which is 100% equivalent to the more explicit but less concise:
if (variable === undefined || variable === null) {
// do something
}
When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.
Edit again
The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.
You should not have any references to undeclared variables in your code.

Combining the above answers, it seems the most complete answer would be:
if( typeof variable === 'undefined' || variable === null ){
// Do stuff
}
This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.

if (variable == null) {
// Do stuff, will only match null or undefined, this won't match false
}

if (typeof EmpName != 'undefined' && EmpName) {
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false

Probably the shortest way to do this is:
if(EmpName == null) { /* DO SOMETHING */ };
Here is proof:
function check(EmpName) {
if(EmpName == null) { return true; };
return false;
}
var log = (t,a) => console.log(`${t} -> ${check(a)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined"
And here are more details about == (source here)
BONUS: reason why === is more clear than == (look on agc answer)

jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.
So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:
if (!EmpName) { //do something }

Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.
I've come to write my own function for this. JavaScript is weird.
It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.
function empty(v) {
let type = typeof v;
if (type === 'undefined') {
return true;
}
if (type === 'boolean') {
return !v;
}
if (v === null) {
return true;
}
if (v === undefined) {
return true;
}
if (v instanceof Array) {
if (v.length < 1) {
return true;
}
} else if (type === 'string') {
if (v.length < 1) {
return true;
}
if (v === '0') {
return true;
}
} else if (type === 'object') {
if (Object.keys(v).length < 1) {
return true;
}
} else if (type === 'number') {
if (v === 0) {
return true;
}
}
return false;
}
TypeScript-compatible.
This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)
Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.
"0.0", NaN, " ", true are considered non-empty.

The shortest and easiest:
if(!EmpName ){
// DO SOMETHING
}
this will evaluate true if EmpName is:
null
undefined
NaN
empty
string ("")
0
false

If the variable you want to check is a global, do
if (window.yourVarName) {
// Your code here
}
This way to check will not throw an error even if the yourVarName variable doesn't exist.
Example: I want to know if my browser supports History API
if (window.history) {
history.back();
}
How this works:
window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.

In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.
if (variable === undefined){
}
if (variable === null){
}
if (variable === ''){
}
Check all conditions:
if(variable === undefined || variable === null || variable === ''){
}

Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.
var s; // undefined
jQuery.isEmptyObject(s); // will return true;
s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;
// usage
if(jQuery.isEmptyObject(s)){
alert('Either variable: s is undefined or its value is null');
}else{
alert('variable: s has value ' + s);
}
s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;

I've just had this problem i.e. checking if an object is null.
I simply use this:
if (object) {
// Your code
}
For example:
if (document.getElementById("enterJob")) {
document.getElementById("enterJob").className += ' current';
}

You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).
if (x === null || x === undefined) {
// Add your response code here, etc.
}
source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/

jQuery check element not null:
var dvElement = $('#dvElement');
if (dvElement.length > 0) {
// Do something
}
else{
// Else do something else
}

With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.
SO,
if(EmpName == null){ // if Variable EmpName null or undefined
EmpName = 'some value';
};
Is equivalent to:
EmpName ??= 'some value';

The easiest way to check is:
if(!variable) {
// If the variable is null or undefined then execution of code will enter here.
}

I run this test in the Chrome console. Using (void 0) you can check undefined:
var c;
undefined
if (c === void 0) alert();
// output = undefined
var c = 1;
// output = undefined
if (c === void 0) alert();
// output = undefined
// check c value c
// output = 1
if (c === void 0) alert();
// output = undefined
c = undefined;
// output = undefined
if (c === void 0) alert();
// output = undefined

With the solution below:
const getType = (val) => typeof val === 'undefined' || !val ? null : typeof val;
const isDeepEqual = (a, b) => getType(a) === getType(b);
console.log(isDeepEqual(1, 1)); // true
console.log(isDeepEqual(null, null)); // true
console.log(isDeepEqual([], [])); // true
console.log(isDeepEqual(1, "1")); // false
etc...
I'm able to check for the following:
null
undefined
NaN
empty
string ("")
0
false

To test if a variable is null or undefined I use the below code.
if(typeof sVal === 'undefined' || sVal === null || sVal === ''){
console.log('variable is undefined or null');
}

if you create a function to check it:
export function isEmpty (v) {
if (typeof v === "undefined") {
return true;
}
if (v === null) {
return true;
}
if (typeof v === "object" && Object.keys(v).length === 0) {
return true;
}
if (Array.isArray(v) && v.length === 0) {
return true;
}
if (typeof v === "string" && v.trim().length === 0) {
return true;
}
return false;
}

(null == undefined) // true
(null === undefined) // false
Because === checks for both the type and value. Type of both are different but value is the same.

Let's look at this,
let apple; // Only declare the variable as apple
alert(apple); // undefined
In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.
let apple = null; /* Declare the variable as apple and initialized but the value is null */
alert(apple); // null
In the second one it displays null, because variable of apple value is null.
So you can check whether a value is undefined or null.
if(apple !== undefined || apple !== null) {
// Can use variable without any error
}

The foo == null check should do the trick and resolve the "undefined OR null" case in the shortest manner. (Not considering "foo is not declared" case.) But people who are used to have 3 equals (as the best practice) might not accept it. Just look at eqeqeq or triple-equals rules in eslint and tslint...
The explicit approach, when we are checking if a variable is undefined or null separately, should be applied in this case, and my contribution to the topic (27 non-negative answers for now!) is to use void 0 as both short and safe way to perform check for undefined.
Using foo === undefined is not safe because undefined is not a reserved word and can be shadowed (MDN). Using typeof === 'undefined' check is safe, but if we are not going to care about foo-is-undeclared case the following approach can be used:
if (foo === void 0 || foo === null) { ... }

You can do something like this, I think its more efficient for multiple value check on the same variable in one condition
const x = undefined;
const y = null;
const z = 'test';
if ([undefined, null].includes(x)) {
// Will return true
}
if ([undefined, null].includes(y)) {
// Will return true
}
if ([undefined, null].includes(z)) {
// Will return false
}

No one seems to have to posted this yet, so here we go:
a?.valueOf() === undefined works reliably for either null or undefined.
The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like == 😎
function check(a) {
const value = a?.valueOf();
if (value === undefined) {
console.log("a is null or undefined");
}
else {
console.log(value);
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
On a side note, a?.constructor works too:
function check(a) {
if (a?.constructor === undefined) {
console.log("a is null or undefined");
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);

Calling typeof null returns a value of “object”, as the special value null is considered to be an empty object reference. Safari through version 5 and Chrome through version 7 have a quirk where calling typeof on a regular expression returns “function” while all other browsers return “object”.

var x;
if (x === undefined) {
alert ("only declared, but not defined.")
};
if (typeof y === "undefined") {
alert ("not even declared.")
};
You can only use second one: as it will check for both definition and declaration

var i;
if (i === null || typeof i === 'undefined') {
console.log(i, 'i is undefined or null')
}
else {
console.log(i, 'i has some value')
}

I still think the best/safe way to test these two conditions is to cast the value to a string:
var EmpName = $("div#esd-names div#name").attr('class');
// Undefined check
if (Object.prototype.toString.call(EmpName) === '[object Undefined]'){
// Do something with your code
}
// Nullcheck
if (Object.prototype.toString.call(EmpName) === '[object Null]'){
// Do something with your code
}

Prevent errors when array? variable is null [duplicate]

How do I determine if variable is undefined or null?
My code is as follows:
var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
// DO SOMETHING
};
<div id="esd-names">
<div id="name"></div>
</div>
But if I do this, the JavaScript interpreter halts execution.

You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined is true, the above code will catch both null and undefined.

The standard way to catch null and undefined simultaneously is this:
if (variable == null) {
// do something
}
--which is 100% equivalent to the more explicit but less concise:
if (variable === undefined || variable === null) {
// do something
}
When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.
Edit again
The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.
You should not have any references to undeclared variables in your code.

Combining the above answers, it seems the most complete answer would be:
if( typeof variable === 'undefined' || variable === null ){
// Do stuff
}
This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.

if (variable == null) {
// Do stuff, will only match null or undefined, this won't match false
}

if (typeof EmpName != 'undefined' && EmpName) {
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false

Probably the shortest way to do this is:
if(EmpName == null) { /* DO SOMETHING */ };
Here is proof:
function check(EmpName) {
if(EmpName == null) { return true; };
return false;
}
var log = (t,a) => console.log(`${t} -> ${check(a)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined"
And here are more details about == (source here)
BONUS: reason why === is more clear than == (look on agc answer)

jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.
So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:
if (!EmpName) { //do something }

Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.
I've come to write my own function for this. JavaScript is weird.
It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.
function empty(v) {
let type = typeof v;
if (type === 'undefined') {
return true;
}
if (type === 'boolean') {
return !v;
}
if (v === null) {
return true;
}
if (v === undefined) {
return true;
}
if (v instanceof Array) {
if (v.length < 1) {
return true;
}
} else if (type === 'string') {
if (v.length < 1) {
return true;
}
if (v === '0') {
return true;
}
} else if (type === 'object') {
if (Object.keys(v).length < 1) {
return true;
}
} else if (type === 'number') {
if (v === 0) {
return true;
}
}
return false;
}
TypeScript-compatible.
This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)
Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.
"0.0", NaN, " ", true are considered non-empty.

The shortest and easiest:
if(!EmpName ){
// DO SOMETHING
}
this will evaluate true if EmpName is:
null
undefined
NaN
empty
string ("")
0
false

If the variable you want to check is a global, do
if (window.yourVarName) {
// Your code here
}
This way to check will not throw an error even if the yourVarName variable doesn't exist.
Example: I want to know if my browser supports History API
if (window.history) {
history.back();
}
How this works:
window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.

In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.
if (variable === undefined){
}
if (variable === null){
}
if (variable === ''){
}
Check all conditions:
if(variable === undefined || variable === null || variable === ''){
}

Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.
var s; // undefined
jQuery.isEmptyObject(s); // will return true;
s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;
// usage
if(jQuery.isEmptyObject(s)){
alert('Either variable: s is undefined or its value is null');
}else{
alert('variable: s has value ' + s);
}
s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;

I've just had this problem i.e. checking if an object is null.
I simply use this:
if (object) {
// Your code
}
For example:
if (document.getElementById("enterJob")) {
document.getElementById("enterJob").className += ' current';
}

You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).
if (x === null || x === undefined) {
// Add your response code here, etc.
}
source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/

jQuery check element not null:
var dvElement = $('#dvElement');
if (dvElement.length > 0) {
// Do something
}
else{
// Else do something else
}

With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.
SO,
if(EmpName == null){ // if Variable EmpName null or undefined
EmpName = 'some value';
};
Is equivalent to:
EmpName ??= 'some value';

The easiest way to check is:
if(!variable) {
// If the variable is null or undefined then execution of code will enter here.
}

I run this test in the Chrome console. Using (void 0) you can check undefined:
var c;
undefined
if (c === void 0) alert();
// output = undefined
var c = 1;
// output = undefined
if (c === void 0) alert();
// output = undefined
// check c value c
// output = 1
if (c === void 0) alert();
// output = undefined
c = undefined;
// output = undefined
if (c === void 0) alert();
// output = undefined

With the solution below:
const getType = (val) => typeof val === 'undefined' || !val ? null : typeof val;
const isDeepEqual = (a, b) => getType(a) === getType(b);
console.log(isDeepEqual(1, 1)); // true
console.log(isDeepEqual(null, null)); // true
console.log(isDeepEqual([], [])); // true
console.log(isDeepEqual(1, "1")); // false
etc...
I'm able to check for the following:
null
undefined
NaN
empty
string ("")
0
false

To test if a variable is null or undefined I use the below code.
if(typeof sVal === 'undefined' || sVal === null || sVal === ''){
console.log('variable is undefined or null');
}

if you create a function to check it:
export function isEmpty (v) {
if (typeof v === "undefined") {
return true;
}
if (v === null) {
return true;
}
if (typeof v === "object" && Object.keys(v).length === 0) {
return true;
}
if (Array.isArray(v) && v.length === 0) {
return true;
}
if (typeof v === "string" && v.trim().length === 0) {
return true;
}
return false;
}

(null == undefined) // true
(null === undefined) // false
Because === checks for both the type and value. Type of both are different but value is the same.

Let's look at this,
let apple; // Only declare the variable as apple
alert(apple); // undefined
In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.
let apple = null; /* Declare the variable as apple and initialized but the value is null */
alert(apple); // null
In the second one it displays null, because variable of apple value is null.
So you can check whether a value is undefined or null.
if(apple !== undefined || apple !== null) {
// Can use variable without any error
}

The foo == null check should do the trick and resolve the "undefined OR null" case in the shortest manner. (Not considering "foo is not declared" case.) But people who are used to have 3 equals (as the best practice) might not accept it. Just look at eqeqeq or triple-equals rules in eslint and tslint...
The explicit approach, when we are checking if a variable is undefined or null separately, should be applied in this case, and my contribution to the topic (27 non-negative answers for now!) is to use void 0 as both short and safe way to perform check for undefined.
Using foo === undefined is not safe because undefined is not a reserved word and can be shadowed (MDN). Using typeof === 'undefined' check is safe, but if we are not going to care about foo-is-undeclared case the following approach can be used:
if (foo === void 0 || foo === null) { ... }

You can do something like this, I think its more efficient for multiple value check on the same variable in one condition
const x = undefined;
const y = null;
const z = 'test';
if ([undefined, null].includes(x)) {
// Will return true
}
if ([undefined, null].includes(y)) {
// Will return true
}
if ([undefined, null].includes(z)) {
// Will return false
}

No one seems to have to posted this yet, so here we go:
a?.valueOf() === undefined works reliably for either null or undefined.
The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like == 😎
function check(a) {
const value = a?.valueOf();
if (value === undefined) {
console.log("a is null or undefined");
}
else {
console.log(value);
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
On a side note, a?.constructor works too:
function check(a) {
if (a?.constructor === undefined) {
console.log("a is null or undefined");
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);

Calling typeof null returns a value of “object”, as the special value null is considered to be an empty object reference. Safari through version 5 and Chrome through version 7 have a quirk where calling typeof on a regular expression returns “function” while all other browsers return “object”.

var x;
if (x === undefined) {
alert ("only declared, but not defined.")
};
if (typeof y === "undefined") {
alert ("not even declared.")
};
You can only use second one: as it will check for both definition and declaration

var i;
if (i === null || typeof i === 'undefined') {
console.log(i, 'i is undefined or null')
}
else {
console.log(i, 'i has some value')
}

I still think the best/safe way to test these two conditions is to cast the value to a string:
var EmpName = $("div#esd-names div#name").attr('class');
// Undefined check
if (Object.prototype.toString.call(EmpName) === '[object Undefined]'){
// Do something with your code
}
// Nullcheck
if (Object.prototype.toString.call(EmpName) === '[object Null]'){
// Do something with your code
}

A way to check for undefined null and 0 [duplicate]

Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not undefined or null? I've got this code, but I'm not sure if it covers all cases:
function isEmpty(val){
return (val === undefined || val == null || val.length <= 0) ? true : false;
}

You can just check if the variable has a truthy value or not. That means
if (value) {
// do something..
}
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false
The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.
Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance
if (typeof foo !== 'undefined') {
// foo could get resolved and it's defined
}
If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

The verbose method to check if value is undefined or null is:
return value === undefined || value === null;
You can also use the == operator but this expects one to know all the rules:
return value == null; // also returns true if value is undefined

function isEmpty(value){
return (value == null || value.length === 0);
}
This will return true for
undefined // Because undefined == null
null
[]
""
and zero argument functions since a function's length is the number of declared parameters it takes.
To disallow the latter category, you might want to just check for blank strings
function isEmpty(value){
return (value == null || value === '');
}
Null or whitespace
function isEmpty(value){
return (value == null || value.trim().length === 0);
}

This is the safest check and I haven't seen it posted here exactly like that:
if (typeof value !== 'undefined' && value) {
//deal with value'
};
It will cover cases where value was never defined, and also any of these:
null
undefined (value of undefined is not the same as a parameter that was never defined)
0
"" (empty string)
false
NaN
Edited: Changed to strict equality (!==) because it's the norm by now ;)

You may find the following function useful:
function typeOf(obj) {
return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
Or in ES7 (comment if further improvements)
function typeOf(obj) {
const { toString } = Object.prototype;
const stringified = obj::toString();
const type = stringified.split(' ')[1].slice(0, -1);
return type.toLowerCase();
}
Results:
typeOf(); //undefined
typeOf(null); //null
typeOf(NaN); //number
typeOf(5); //number
typeOf({}); //object
typeOf([]); //array
typeOf(''); //string
typeOf(function () {}); //function
typeOf(/a/) //regexp
typeOf(new Date()) //date
typeOf(new WeakMap()) //weakmap
typeOf(new Map()) //map
"Note that the bind operator (::) is not part of ES2016 (ES7) nor any later edition of the ECMAScript standard at all. It's currently a stage 0 (strawman) proposal for being introduced to the language." – Simon Kjellberg. the author wishes to add his support for this beautiful proposal to receive royal ascension.

A solution I like a lot:
Let's define that a blank variable is null, or undefined, or if it has length, it is zero, or if it is an object, it has no keys:
function isEmpty (value) {
return (
// null or undefined
(value == null) ||
// has length and it's zero
(value.hasOwnProperty('length') && value.length === 0) ||
// is an Object and has no keys
(value.constructor === Object && Object.keys(value).length === 0)
)
}
Returns:
true: undefined, null, "", [], {}
false: true, false, 1, 0, -1, "foo", [1, 2, 3], { foo: 1 }

This condition check
if (!!foo) {
//foo is defined
}
is all you need.

Take a look at the new ECMAScript Nullish coalescing operator
You can think of this feature - the ?? operator - as a way to “fall back” to a default value when dealing with null or undefined.
let x = foo ?? bar();
Again, the above code is equivalent to the following.
let x = (foo !== null && foo !== undefined) ? foo : bar();

The first answer with best rating is wrong. If value is undefined it will throw an exception in modern browsers. You have to use:
if (typeof(value) !== "undefined" && value)
or
if (typeof value !== "undefined" && value)

! check for empty strings (""), null, undefined, false and the number 0 and NaN. Say, if a string is empty var name = "" then console.log(!name) returns true.
function isEmpty(val){
return !val;
}
this function will return true if val is empty, null, undefined, false, the number 0 or NaN.
OR
According to your problem domain you can just use like !val or !!val.

You are a bit overdoing it. To check if a variable is not given a value, you would only need to check against undefined and null.
function isEmpty(value){
return (typeof value === "undefined" || value === null);
}
This is assuming 0, "", and objects(even empty object and array) are valid "values".

Vacuousness
I don't recommend trying to define or use a function which computes whether any value in the whole world is empty. What does it really mean to be "empty"? If I have let human = { name: 'bob', stomach: 'empty' }, should isEmpty(human) return true? If I have let reg = new RegExp('');, should isEmpty(reg) return true? What about isEmpty([ null, null, null, null ]) - this list only contains emptiness, so is the list itself empty? I want to put forward here some notes on "vacuousness" (an intentionally obscure word, to avoid pre-existing associations) in javascript - and I want to argue that "vacuousness" in javascript values should never be dealt with generically.
Truthiness/Falsiness
For deciding how to determine the "vacuousness" of values, we need to accomodate javascript's inbuilt, inherent sense of whether values are "truthy" or "falsy". Naturally, null and undefined are both "falsy". Less naturally, the number 0 (and no other number except NaN) is also "falsy". Least naturally: '' is falsy, but [] and {} (and new Set(), and new Map()) are truthy - although they all seem equally vacuous!
Null vs Undefined
There is also some discussion concerning null vs undefined - do we really need both in order to express vacuousness in our programs? I personally avoid ever having undefined appear in my code. I always use null to signify "vacuousness". Again, though, we need to accomodate javascript's inherent sense of how null and undefined differ:
Trying to access a non-existent property gives undefined
Omitting a parameter when calling a function results in that parameter receiving undefined:
let f = a => a;
console.log(f('hi'));
console.log(f());
Parameters with default values receive the default only when given undefined, not null:
let f = (v='hello') => v;
console.log(f(null));
console.log(f(undefined));
To me, null is an explicit signifier of vacuousness; "something that could have been filled in was intentionally left blank".
Really undefined is a necessary complication that allows some js features to exist, but in my opinion it should always be left behind the scenes; not interacted with directly. We can think of undefined as, for example, javascript's mechanic for implementing default function arguments. If you refrain from supplying an argument to a function it will receive a value of undefined instead. And a default value will be applied to a function argument if that argument was initially set to undefined. In this case undefined is the linchpin of default function arguments, but it stays in the background: we can achieve default argument functionality without ever referring to undefined:
This is a bad implementation of default arguments as it interacts directly with undefined:
let fnWithDefaults = arg => {
if (arg === undefined) arg = 'default';
...
};
This is a good implementation:
let fnWithDefaults = (arg='default') => { ... };
This is a bad way to accept the default argument:
fnWithDefaults(undefined);
Simply do this instead:
fnWithDefaults();
By the way: do you have a function with multiple arguments, and you want to provide some arguments while accepting defaults for others?
E.g.:
let fnWithDefaults = (a=1, b=2, c=3, d=4) => console.log(a, b, c, d);
If you want to provide values for a and d and accepts defaults for the others what to do? This seems wrong:
fnWithDefaults(10, undefined, undefined, 40);
The answer is: refactor fnWithDefaults to accept a single object:
let fnWithDefaults = ({ a=1, b=2, c=3, d=4 }={}) => console.log(a, b, c, d);
fnWithDefaults({ a: 10, d: 40 }); // Now this looks really nice! (And never talks about "undefined")
Non-generic Vacuousness
I believe that vacuousness should never be dealt with in a generic fashion. We should instead always have the rigour to get more information about our data before determining if it is vacuous - I mainly do this by checking what type of data I'm dealing with:
let isType = (value, Cls) => {
// Intentional use of loose comparison operator detects `null`
// and `undefined`, and nothing else!
return value != null && Object.getPrototypeOf(value).constructor === Cls;
};
Note that this function ignores inheritance - it expects value to be a direct instance of Cls, and not an instance of a subclass of Cls. I avoid instanceof for two main reasons:
([] instanceof Object) === true ("An Array is an Object")
('' instanceof String) === false ("A String is not a String")
Note that Object.getPrototypeOf is used to avoid an (obscure) edge-case such as let v = { constructor: String }; The isType function still returns correctly for isType(v, String) (false), and isType(v, Object) (true).
Overall, I recommend using this isType function along with these tips:
Minimize the amount of code processing values of unknown type. E.g., for let v = JSON.parse(someRawValue);, our v variable is now of unknown type. As early as possible, we should limit our possibilities. The best way to do this can be by requiring a particular type: e.g. if (!isType(v, Array)) throw new Error('Expected Array'); - this is a really quick and expressive way to remove the generic nature of v, and ensure it's always an Array. Sometimes, though, we need to allow v to be of multiple types. In those cases, we should create blocks of code where v is no longer generic, as early as possible:
if (isType(v, String)) {
/* v isn't generic in this block - It's a String! */
} else if (isType(v, Number)) {
/* v isn't generic in this block - It's a Number! */
} else if (isType(v, Array)) {
/* v isn't generic in this block - it's an Array! */
} else {
throw new Error('Expected String, Number, or Array');
}
Always use "whitelists" for validation. If you require a value to be, e.g., a String, Number, or Array, check for those 3 "white" possibilities, and throw an Error if none of the 3 are satisfied. We should be able to see that checking for "black" possibilities isn't very useful: Say we write if (v === null) throw new Error('Null value rejected'); - this is great for ensuring that null values don't make it through, but if a value does make it through, we still know hardly anything about it. A value v which passes this null-check is still VERY generic - it's anything but null! Blacklists hardly dispell generic-ness.
Unless a value is null, never consider "a vacuous value". Instead, consider "an X which is vacuous". Essentially, never consider doing anything like if (isEmpty(val)) { /* ... */ } - no matter how that isEmpty function is implemented (I don't want to know...), it isn't meaningful! And it's way too generic! Vacuousness should only be calculated with knowledge of val's type. Vacuousness-checks should look like this:
"A string, with no chars":
if (isType(val, String) && val.length === 0) ...
"An Object, with 0 props": if (isType(val, Object) && Object.entries(val).length === 0) ...
"A number, equal or less than zero": if (isType(val, Number) && val <= 0) ...
"An Array, with no items": if (isType(val, Array) && val.length === 0) ...
The only exception is when null is used to signify certain functionality. In this case it's meaningful to say: "A vacuous value": if (val === null) ...

The probably shortest answer is
val==null || val==''
if you change rigth side to val==='' then empty array will give false. Proof
function isEmpty(val){
return val==null || val==''
}
// ------------
// TEST
// ------------
var log = (name,val) => console.log(`${name} -> ${isEmpty(val)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined" - so we should get here TRUE
More details about == (source here)
BONUS: Reason why === is more clear than ==
To write clear and easy
understandable code, use explicite list of accepted values
val===undefined || val===null || val===''|| (Array.isArray(val) && val.length===0)
function isEmpty(val){
return val===undefined || val===null || val==='' || (Array.isArray(val) && val.length===0)
}
// ------------
// TEST
// ------------
var log = (name,val) => console.log(`${name} -> ${isEmpty(val)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined" - so we should get here TRUE

Here's mine - returns true if value is null, undefined, etc or blank (ie contains only blank spaces):
function stringIsEmpty(value) {
return value ? value.trim().length == 0 : true;
}

If you prefer plain javascript try this:
/**
* Checks if `value` is empty. Arrays, strings, or `arguments` objects with a
* length of `0` and objects with no own enumerable properties are considered
* "empty".
*
* #static
* #memberOf _
* #category Objects
* #param {Array|Object|string} value The value to inspect.
* #returns {boolean} Returns `true` if the `value` is empty, else `false`.
* #example
*
* _.isEmpty([1, 2, 3]);
* // => false
*
* _.isEmpty([]);
* // => true
*
* _.isEmpty({});
* // => true
*
* _.isEmpty('');
* // => true
*/
function isEmpty(value) {
if (!value) {
return true;
}
if (isArray(value) || isString(value)) {
return !value.length;
}
for (var key in value) {
if (hasOwnProperty.call(value, key)) {
return false;
}
}
return true;
}
Otherwise, if you are already using underscore or lodash, try:
_.isEmpty(value)

return val || 'Handle empty variable'
is a really nice and clean way to handle it in a lot of places, can also be used to assign variables
const res = val || 'default value'

If the variable hasn't been declared, you wont be able to test for undefined using a function because you will get an error.
if (foo) {}
function (bar) {}(foo)
Both will generate an error if foo has not been declared.
If you want to test if a variable has been declared you can use
typeof foo != "undefined"
if you want to test if foo has been declared and it has a value you can use
if (typeof foo != "undefined" && foo) {
//code here
}

You could use the nullish coalescing operator ?? to check for null and undefined values. See the MDN Docs
null ?? 'default string'; // returns "default string"
0 ?? 42; // returns 0
(null || undefined) ?? "foo"; // returns "foo"

To check Default Value
function typeOfVar (obj) {
return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
function isVariableHaveDefaltVal(variable) {
if ( typeof(variable) === 'string' ) { // number, boolean, string, object
console.log(' Any data Between single/double Quotes is treated as String ');
return (variable.trim().length === 0) ? true : false;
}else if ( typeof(variable) === 'boolean' ) {
console.log('boolean value with default value \'false\'');
return (variable === false) ? true : false;
}else if ( typeof(variable) === 'undefined' ) {
console.log('EX: var a; variable is created, but has the default value of undefined.');
return true;
}else if ( typeof(variable) === 'number' ) {
console.log('number : '+variable);
return (variable === 0 ) ? true : false;
}else if ( typeof(variable) === 'object' ) {
// -----Object-----
if (typeOfVar(variable) === 'array' && variable.length === 0) {
console.log('\t Object Array with length = ' + [].length); // Object.keys(variable)
return true;
}else if (typeOfVar(variable) === 'string' && variable.length === 0 ) {
console.log('\t Object String with length = ' + variable.length);
return true;
}else if (typeOfVar(variable) === 'boolean' ) {
console.log('\t Object Boolean = ' + variable);
return (variable === false) ? true : false;
}else if (typeOfVar(variable) === 'number' ) {
console.log('\t Object Number = ' + variable);
return (variable === 0 ) ? true : false;
}else if (typeOfVar(variable) === 'regexp' && variable.source.trim().length === 0 ) {
console.log('\t Object Regular Expression : ');
return true;
}else if (variable === null) {
console.log('\t Object null value');
return true;
}
}
return false;
}
var str = "A Basket For Every Occasion";
str = str.replace(/\s/g, "-");
//The "g" flag in the regex will cause all spaces to get replaced.
check Result:
isVariableHaveDefaltVal(' '); // string
isVariableHaveDefaltVal(false); // boolean
var a;
isVariableHaveDefaltVal(a);
isVariableHaveDefaltVal(0); // number
isVariableHaveDefaltVal(parseInt('')); // NAN isNAN(' '); - true
isVariableHaveDefaltVal(null);
isVariableHaveDefaltVal([]);
isVariableHaveDefaltVal(/ /);
isVariableHaveDefaltVal(new Object(''));
isVariableHaveDefaltVal(new Object(false));
isVariableHaveDefaltVal(new Object(0));
typeOfVar( function() {} );
I used #Vix function() to check the object of which type.
using instansof «
var prototypes_or_Literals = function (obj) {
switch (typeof(obj)) {
// object prototypes
case 'object':
if (obj instanceof Array)
return '[object Array]';
else if (obj instanceof Date)
return '[object Date]';
else if (obj instanceof RegExp)
return '[object regexp]';
else if (obj instanceof String)
return '[object String]';
else if (obj instanceof Number)
return '[object Number]';
else
return 'object';
// object literals
default:
return typeof(obj);
}
};
output test «
prototypes_or_Literals( '' ) // "string"
prototypes_or_Literals( new String('') ) // "[object String]"
Object.prototype.toString.call("foo bar") //"[object String]"

function isEmpty(obj) {
if (typeof obj == 'number') return false;
else if (typeof obj == 'string') return obj.length == 0;
else if (Array.isArray(obj)) return obj.length == 0;
else if (typeof obj == 'object') return obj == null || Object.keys(obj).length == 0;
else if (typeof obj == 'boolean') return false;
else return !obj;
}
In ES6 with trim to handle whitespace strings:
const isEmpty = value => {
if (typeof value === 'number') return false
else if (typeof value === 'string') return value.trim().length === 0
else if (Array.isArray(value)) return value.length === 0
else if (typeof value === 'object') return value == null || Object.keys(value).length === 0
else if (typeof value === 'boolean') return false
else return !value
}

It may be usefull.
All values in array represent what you want to be (null, undefined or another things) and you search what you want in it.
var variablesWhatILookFor = [null, undefined, ''];
variablesWhatILookFor.indexOf(document.DocumentNumberLabel) > -1

If you are using TypeScript and don't want to account for "values those are false" then this is the solution for you:
First: import { isNullOrUndefined } from 'util';
Then: isNullOrUndefined(this.yourVariableName)
Please Note: As mentioned below this is now deprecated, use value === undefined || value === null instead. ref.

Try With Different Logic. You can use bellow code for check all four(4) condition for validation like not null, not blank, not undefined and not zero only use this code (!(!(variable))) in javascript and jquery.
function myFunction() {
var data; //The Values can be like as null, blank, undefined, zero you can test
if(!(!(data)))
{
alert("data "+data);
}
else
{
alert("data is "+data);
}
}

The optional chaining operator provides a way to simplify accessing values through connected objects when it's possible that a reference or function may be undefined or null.
let customer = {
name: "Carl",
details: {
age: 82,
location: "Paradise Falls" // detailed address is unknown
}
};
let customerCity = customer.details?.address?.city;
The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:
let customer = {
name: "Carl",
details: { age: 82 }
};
const customerCity = customer?.city ?? "Unknown city";
console.log(customerCity); // Unknown city

function isEmpty(val){
return !val;
}
but this solution is over-engineered, if you dont'want to modify the function later for busines-model needings, then is cleaner to use it directly in code:
if(!val)...

var myNewValue = myObject && myObject.child && myObject.child.myValue;
This will never throw an error. If myObject, child, or myValue is null then myNewValue will be null. No errors will be thrown

For everyone coming here for having similar question, the following works great and I have it in my library the last years:
(function(g3, $, window, document, undefined){
g3.utils = g3.utils || {};
/********************************Function type()********************************
* Returns a lowercase string representation of an object's constructor.
* #module {g3.utils}
* #function {g3.utils.type}
* #public
* #param {Type} 'obj' is any type native, host or custom.
* #return {String} Returns a lowercase string representing the object's
* constructor which is different from word 'object' if they are not custom.
* #reference http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/
* http://stackoverflow.com/questions/3215046/differentiating-between-arrays-and-hashes-in-javascript
* http://javascript.info/tutorial/type-detection
*******************************************************************************/
g3.utils.type = function (obj){
if(obj === null)
return 'null';
else if(typeof obj === 'undefined')
return 'undefined';
return Object.prototype.toString.call(obj).match(/^\[object\s(.*)\]$/)[1].toLowerCase();
};
}(window.g3 = window.g3 || {}, jQuery, window, document));

If you want to avoid getting true if the value is any of the following, according to jAndy's answer:
null
undefined
NaN
empty string ("")
0
false
One possible solution that might avoid getting truthy values is the following:
function isUsable(valueToCheck) {
if (valueToCheck === 0 || // Avoid returning false if the value is 0.
valueToCheck === '' || // Avoid returning false if the value is an empty string.
valueToCheck === false || // Avoid returning false if the value is false.
valueToCheck) // Returns true if it isn't null, undefined, or NaN.
{
return true;
} else {
return false;
}
}
It would be used as follows:
if (isUsable(x)) {
// It is usable!
}
// Make sure to avoid placing the logical NOT operator before the parameter (isUsable(!x)) and instead, use it before the function, to check the returned value.
if (!isUsable(x)) {
// It is NOT usable!
}
In addition to those scenarios, you may want to return false if the object or array is empty:
Object: {} (Using ECMA 7+)
Array: [] (Using ECMA 5+)
You would go about it this way:
function isEmptyObject(valueToCheck) {
if(typeof valueToCheck === 'object' && !Object.keys(valueToCheck).length){
// Object is empty!
return true;
} else {
// Object is not empty!
return false;
}
}
function isEmptyArray(valueToCheck) {
if(Array.isArray(valueToCheck) && !valueToCheck.length) {
// Array is empty!
return true;
} else {
// Array is not empty!
return false;
}
}
If you wish to check for all whitespace strings (" "), you may do the following:
function isAllWhitespace(){
if (valueToCheck.match(/^ *$/) !== null) {
// Is all whitespaces!
return true;
} else {
// Is not all whitespaces!
return false;
}
}
Note: hasOwnProperty returns true for empty strings, 0, false, NaN, null, and undefined, if the variable was declared as any of them, so it might not be the best to use. The function may be modified to use it to show that it was declared, but is not usable.

Code on GitHub
const isEmpty = value => (
(!value && value !== 0 && value !== false)
|| (Array.isArray(value) && value.length === 0)
|| (isObject(value) && Object.keys(value).length === 0)
|| (typeof value.size === 'number' && value.size === 0)
// `WeekMap.length` is supposed to exist!?
|| (typeof value.length === 'number'
&& typeof value !== 'function' && value.length === 0)
);
// Source: https://levelup.gitconnected.com/javascript-check-if-a-variable-is-an-object-and-nothing-else-not-an-array-a-set-etc-a3987ea08fd7
const isObject = value =>
Object.prototype.toString.call(value) === '[object Object]';
Poor man's tests 😁
const test = () => {
const run = (label, values, expected) => {
const length = values.length;
console.group(`${label} (${length} tests)`);
values.map((v, i) => {
console.assert(isEmpty(v) === expected, `${i}: ${v}`);
});
console.groupEnd();
};
const empty = [
null, undefined, NaN, '', {}, [],
new Set(), new Set([]), new Map(), new Map([]),
];
const notEmpty = [
' ', 'a', 0, 1, -1, false, true, {a: 1}, [0],
new Set([0]), new Map([['a', 1]]),
new WeakMap().set({}, 1),
new Date(), /a/, new RegExp(), () => {},
];
const shouldBeEmpty = [
{undefined: undefined}, new Map([[]]),
];
run('EMPTY', empty, true);
run('NOT EMPTY', notEmpty, false);
run('SHOULD BE EMPTY', shouldBeEmpty, true);
};
Test results:
EMPTY (10 tests)
NOT EMPTY (16 tests)
SHOULD BE EMPTY (2 tests)
Assertion failed: 0: [object Object]
Assertion failed: 1: [object Map]

function notEmpty(value){
return (typeof value !== 'undefined' && value.trim().length);
}
it will also check white spaces (' ') along with following:
null ,undefined ,NaN ,empty ,string ("") ,0 ,false

How to check a null value with an extension method using Object.prototype

There are a variety of places where I need to check if a JavaScript variable is null or empty string so I wrote an extension method that looks like this:
Object.prototype.IsNullOrEmptyString = function()
{
return (this == null || (typeof this === "string" && this.length == 0));
}
I then call it like so:
var someVariable = null;
if (someVariable.IsNullOrEmptyString())
alert("do something");
But that doesn't work because, at the point of evaluation in the if statement, someVariable is null. The error I keep getting is "someVariable is null".
You can see it live here: http://jsfiddle.net/AhnkF/ (run in Firefox and notice the error console)
Is there anyway to do what I want and have a single extension method check for null and other things at the same time?

null does not have any properties or methods. You have to create a function, and pass the variable, so it can be tested. Note: To test whether a variable is really an empty string, using === "" is recommended.*
function isNullOrEmpty(test) {
return test === null || test === "";
}
var someVariable = null;
if (isNullOrEmpty(someVariable)) alert("Do something");
* about == and ===. The following comparisons are true:
null == undefined
"" == 0;
"" == false
"" ==

Because your variable is not an object, so its not working. Do it this way.
Object.prototype.IsNullOrEmptyString = function(obj)
{
return (obj == null || (typeof this === "string" && this.length == 0));
}
if(Object.IsNullOrEmptyString(null))
alert('yes');

just make it a function call.
function IsEmptyString(value)
{
return (value == null ||
value === "");
}
Here is a fiddle
Edit: consolidated (=== undefined and === null) into (== null)

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not undefined or null? I've got this code, but I'm not sure if it covers all cases:
function isEmpty(val){
return (val === undefined || val == null || val.length <= 0) ? true : false;
}

You can just check if the variable has a truthy value or not. That means
if (value) {
// do something..
}
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false
The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.
Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance
if (typeof foo !== 'undefined') {
// foo could get resolved and it's defined
}
If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

The verbose method to check if value is undefined or null is:
return value === undefined || value === null;
You can also use the == operator but this expects one to know all the rules:
return value == null; // also returns true if value is undefined

function isEmpty(value){
return (value == null || value.length === 0);
}
This will return true for
undefined // Because undefined == null
null
[]
""
and zero argument functions since a function's length is the number of declared parameters it takes.
To disallow the latter category, you might want to just check for blank strings
function isEmpty(value){
return (value == null || value === '');
}
Null or whitespace
function isEmpty(value){
return (value == null || value.trim().length === 0);
}

This is the safest check and I haven't seen it posted here exactly like that:
if (typeof value !== 'undefined' && value) {
//deal with value'
};
It will cover cases where value was never defined, and also any of these:
null
undefined (value of undefined is not the same as a parameter that was never defined)
0
"" (empty string)
false
NaN
Edited: Changed to strict equality (!==) because it's the norm by now ;)

You may find the following function useful:
function typeOf(obj) {
return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
Or in ES7 (comment if further improvements)
function typeOf(obj) {
const { toString } = Object.prototype;
const stringified = obj::toString();
const type = stringified.split(' ')[1].slice(0, -1);
return type.toLowerCase();
}
Results:
typeOf(); //undefined
typeOf(null); //null
typeOf(NaN); //number
typeOf(5); //number
typeOf({}); //object
typeOf([]); //array
typeOf(''); //string
typeOf(function () {}); //function
typeOf(/a/) //regexp
typeOf(new Date()) //date
typeOf(new WeakMap()) //weakmap
typeOf(new Map()) //map
"Note that the bind operator (::) is not part of ES2016 (ES7) nor any later edition of the ECMAScript standard at all. It's currently a stage 0 (strawman) proposal for being introduced to the language." – Simon Kjellberg. the author wishes to add his support for this beautiful proposal to receive royal ascension.

A solution I like a lot:
Let's define that a blank variable is null, or undefined, or if it has length, it is zero, or if it is an object, it has no keys:
function isEmpty (value) {
return (
// null or undefined
(value == null) ||
// has length and it's zero
(value.hasOwnProperty('length') && value.length === 0) ||
// is an Object and has no keys
(value.constructor === Object && Object.keys(value).length === 0)
)
}
Returns:
true: undefined, null, "", [], {}
false: true, false, 1, 0, -1, "foo", [1, 2, 3], { foo: 1 }

This condition check
if (!!foo) {
//foo is defined
}
is all you need.

Take a look at the new ECMAScript Nullish coalescing operator
You can think of this feature - the ?? operator - as a way to “fall back” to a default value when dealing with null or undefined.
let x = foo ?? bar();
Again, the above code is equivalent to the following.
let x = (foo !== null && foo !== undefined) ? foo : bar();

The first answer with best rating is wrong. If value is undefined it will throw an exception in modern browsers. You have to use:
if (typeof(value) !== "undefined" && value)
or
if (typeof value !== "undefined" && value)

! check for empty strings (""), null, undefined, false and the number 0 and NaN. Say, if a string is empty var name = "" then console.log(!name) returns true.
function isEmpty(val){
return !val;
}
this function will return true if val is empty, null, undefined, false, the number 0 or NaN.
OR
According to your problem domain you can just use like !val or !!val.

You are a bit overdoing it. To check if a variable is not given a value, you would only need to check against undefined and null.
function isEmpty(value){
return (typeof value === "undefined" || value === null);
}
This is assuming 0, "", and objects(even empty object and array) are valid "values".

Vacuousness
I don't recommend trying to define or use a function which computes whether any value in the whole world is empty. What does it really mean to be "empty"? If I have let human = { name: 'bob', stomach: 'empty' }, should isEmpty(human) return true? If I have let reg = new RegExp('');, should isEmpty(reg) return true? What about isEmpty([ null, null, null, null ]) - this list only contains emptiness, so is the list itself empty? I want to put forward here some notes on "vacuousness" (an intentionally obscure word, to avoid pre-existing associations) in javascript - and I want to argue that "vacuousness" in javascript values should never be dealt with generically.
Truthiness/Falsiness
For deciding how to determine the "vacuousness" of values, we need to accomodate javascript's inbuilt, inherent sense of whether values are "truthy" or "falsy". Naturally, null and undefined are both "falsy". Less naturally, the number 0 (and no other number except NaN) is also "falsy". Least naturally: '' is falsy, but [] and {} (and new Set(), and new Map()) are truthy - although they all seem equally vacuous!
Null vs Undefined
There is also some discussion concerning null vs undefined - do we really need both in order to express vacuousness in our programs? I personally avoid ever having undefined appear in my code. I always use null to signify "vacuousness". Again, though, we need to accomodate javascript's inherent sense of how null and undefined differ:
Trying to access a non-existent property gives undefined
Omitting a parameter when calling a function results in that parameter receiving undefined:
let f = a => a;
console.log(f('hi'));
console.log(f());
Parameters with default values receive the default only when given undefined, not null:
let f = (v='hello') => v;
console.log(f(null));
console.log(f(undefined));
To me, null is an explicit signifier of vacuousness; "something that could have been filled in was intentionally left blank".
Really undefined is a necessary complication that allows some js features to exist, but in my opinion it should always be left behind the scenes; not interacted with directly. We can think of undefined as, for example, javascript's mechanic for implementing default function arguments. If you refrain from supplying an argument to a function it will receive a value of undefined instead. And a default value will be applied to a function argument if that argument was initially set to undefined. In this case undefined is the linchpin of default function arguments, but it stays in the background: we can achieve default argument functionality without ever referring to undefined:
This is a bad implementation of default arguments as it interacts directly with undefined:
let fnWithDefaults = arg => {
if (arg === undefined) arg = 'default';
...
};
This is a good implementation:
let fnWithDefaults = (arg='default') => { ... };
This is a bad way to accept the default argument:
fnWithDefaults(undefined);
Simply do this instead:
fnWithDefaults();
By the way: do you have a function with multiple arguments, and you want to provide some arguments while accepting defaults for others?
E.g.:
let fnWithDefaults = (a=1, b=2, c=3, d=4) => console.log(a, b, c, d);
If you want to provide values for a and d and accepts defaults for the others what to do? This seems wrong:
fnWithDefaults(10, undefined, undefined, 40);
The answer is: refactor fnWithDefaults to accept a single object:
let fnWithDefaults = ({ a=1, b=2, c=3, d=4 }={}) => console.log(a, b, c, d);
fnWithDefaults({ a: 10, d: 40 }); // Now this looks really nice! (And never talks about "undefined")
Non-generic Vacuousness
I believe that vacuousness should never be dealt with in a generic fashion. We should instead always have the rigour to get more information about our data before determining if it is vacuous - I mainly do this by checking what type of data I'm dealing with:
let isType = (value, Cls) => {
// Intentional use of loose comparison operator detects `null`
// and `undefined`, and nothing else!
return value != null && Object.getPrototypeOf(value).constructor === Cls;
};
Note that this function ignores inheritance - it expects value to be a direct instance of Cls, and not an instance of a subclass of Cls. I avoid instanceof for two main reasons:
([] instanceof Object) === true ("An Array is an Object")
('' instanceof String) === false ("A String is not a String")
Note that Object.getPrototypeOf is used to avoid an (obscure) edge-case such as let v = { constructor: String }; The isType function still returns correctly for isType(v, String) (false), and isType(v, Object) (true).
Overall, I recommend using this isType function along with these tips:
Minimize the amount of code processing values of unknown type. E.g., for let v = JSON.parse(someRawValue);, our v variable is now of unknown type. As early as possible, we should limit our possibilities. The best way to do this can be by requiring a particular type: e.g. if (!isType(v, Array)) throw new Error('Expected Array'); - this is a really quick and expressive way to remove the generic nature of v, and ensure it's always an Array. Sometimes, though, we need to allow v to be of multiple types. In those cases, we should create blocks of code where v is no longer generic, as early as possible:
if (isType(v, String)) {
/* v isn't generic in this block - It's a String! */
} else if (isType(v, Number)) {
/* v isn't generic in this block - It's a Number! */
} else if (isType(v, Array)) {
/* v isn't generic in this block - it's an Array! */
} else {
throw new Error('Expected String, Number, or Array');
}
Always use "whitelists" for validation. If you require a value to be, e.g., a String, Number, or Array, check for those 3 "white" possibilities, and throw an Error if none of the 3 are satisfied. We should be able to see that checking for "black" possibilities isn't very useful: Say we write if (v === null) throw new Error('Null value rejected'); - this is great for ensuring that null values don't make it through, but if a value does make it through, we still know hardly anything about it. A value v which passes this null-check is still VERY generic - it's anything but null! Blacklists hardly dispell generic-ness.
Unless a value is null, never consider "a vacuous value". Instead, consider "an X which is vacuous". Essentially, never consider doing anything like if (isEmpty(val)) { /* ... */ } - no matter how that isEmpty function is implemented (I don't want to know...), it isn't meaningful! And it's way too generic! Vacuousness should only be calculated with knowledge of val's type. Vacuousness-checks should look like this:
"A string, with no chars":
if (isType(val, String) && val.length === 0) ...
"An Object, with 0 props": if (isType(val, Object) && Object.entries(val).length === 0) ...
"A number, equal or less than zero": if (isType(val, Number) && val <= 0) ...
"An Array, with no items": if (isType(val, Array) && val.length === 0) ...
The only exception is when null is used to signify certain functionality. In this case it's meaningful to say: "A vacuous value": if (val === null) ...

The probably shortest answer is
val==null || val==''
if you change rigth side to val==='' then empty array will give false. Proof
function isEmpty(val){
return val==null || val==''
}
// ------------
// TEST
// ------------
var log = (name,val) => console.log(`${name} -> ${isEmpty(val)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined" - so we should get here TRUE
More details about == (source here)
BONUS: Reason why === is more clear than ==
To write clear and easy
understandable code, use explicite list of accepted values
val===undefined || val===null || val===''|| (Array.isArray(val) && val.length===0)
function isEmpty(val){
return val===undefined || val===null || val==='' || (Array.isArray(val) && val.length===0)
}
// ------------
// TEST
// ------------
var log = (name,val) => console.log(`${name} -> ${isEmpty(val)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined" - so we should get here TRUE

Here's mine - returns true if value is null, undefined, etc or blank (ie contains only blank spaces):
function stringIsEmpty(value) {
return value ? value.trim().length == 0 : true;
}

If you prefer plain javascript try this:
/**
* Checks if `value` is empty. Arrays, strings, or `arguments` objects with a
* length of `0` and objects with no own enumerable properties are considered
* "empty".
*
* #static
* #memberOf _
* #category Objects
* #param {Array|Object|string} value The value to inspect.
* #returns {boolean} Returns `true` if the `value` is empty, else `false`.
* #example
*
* _.isEmpty([1, 2, 3]);
* // => false
*
* _.isEmpty([]);
* // => true
*
* _.isEmpty({});
* // => true
*
* _.isEmpty('');
* // => true
*/
function isEmpty(value) {
if (!value) {
return true;
}
if (isArray(value) || isString(value)) {
return !value.length;
}
for (var key in value) {
if (hasOwnProperty.call(value, key)) {
return false;
}
}
return true;
}
Otherwise, if you are already using underscore or lodash, try:
_.isEmpty(value)

return val || 'Handle empty variable'
is a really nice and clean way to handle it in a lot of places, can also be used to assign variables
const res = val || 'default value'

If the variable hasn't been declared, you wont be able to test for undefined using a function because you will get an error.
if (foo) {}
function (bar) {}(foo)
Both will generate an error if foo has not been declared.
If you want to test if a variable has been declared you can use
typeof foo != "undefined"
if you want to test if foo has been declared and it has a value you can use
if (typeof foo != "undefined" && foo) {
//code here
}

You could use the nullish coalescing operator ?? to check for null and undefined values. See the MDN Docs
null ?? 'default string'; // returns "default string"
0 ?? 42; // returns 0
(null || undefined) ?? "foo"; // returns "foo"

To check Default Value
function typeOfVar (obj) {
return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
function isVariableHaveDefaltVal(variable) {
if ( typeof(variable) === 'string' ) { // number, boolean, string, object
console.log(' Any data Between single/double Quotes is treated as String ');
return (variable.trim().length === 0) ? true : false;
}else if ( typeof(variable) === 'boolean' ) {
console.log('boolean value with default value \'false\'');
return (variable === false) ? true : false;
}else if ( typeof(variable) === 'undefined' ) {
console.log('EX: var a; variable is created, but has the default value of undefined.');
return true;
}else if ( typeof(variable) === 'number' ) {
console.log('number : '+variable);
return (variable === 0 ) ? true : false;
}else if ( typeof(variable) === 'object' ) {
// -----Object-----
if (typeOfVar(variable) === 'array' && variable.length === 0) {
console.log('\t Object Array with length = ' + [].length); // Object.keys(variable)
return true;
}else if (typeOfVar(variable) === 'string' && variable.length === 0 ) {
console.log('\t Object String with length = ' + variable.length);
return true;
}else if (typeOfVar(variable) === 'boolean' ) {
console.log('\t Object Boolean = ' + variable);
return (variable === false) ? true : false;
}else if (typeOfVar(variable) === 'number' ) {
console.log('\t Object Number = ' + variable);
return (variable === 0 ) ? true : false;
}else if (typeOfVar(variable) === 'regexp' && variable.source.trim().length === 0 ) {
console.log('\t Object Regular Expression : ');
return true;
}else if (variable === null) {
console.log('\t Object null value');
return true;
}
}
return false;
}
var str = "A Basket For Every Occasion";
str = str.replace(/\s/g, "-");
//The "g" flag in the regex will cause all spaces to get replaced.
check Result:
isVariableHaveDefaltVal(' '); // string
isVariableHaveDefaltVal(false); // boolean
var a;
isVariableHaveDefaltVal(a);
isVariableHaveDefaltVal(0); // number
isVariableHaveDefaltVal(parseInt('')); // NAN isNAN(' '); - true
isVariableHaveDefaltVal(null);
isVariableHaveDefaltVal([]);
isVariableHaveDefaltVal(/ /);
isVariableHaveDefaltVal(new Object(''));
isVariableHaveDefaltVal(new Object(false));
isVariableHaveDefaltVal(new Object(0));
typeOfVar( function() {} );
I used #Vix function() to check the object of which type.
using instansof «
var prototypes_or_Literals = function (obj) {
switch (typeof(obj)) {
// object prototypes
case 'object':
if (obj instanceof Array)
return '[object Array]';
else if (obj instanceof Date)
return '[object Date]';
else if (obj instanceof RegExp)
return '[object regexp]';
else if (obj instanceof String)
return '[object String]';
else if (obj instanceof Number)
return '[object Number]';
else
return 'object';
// object literals
default:
return typeof(obj);
}
};
output test «
prototypes_or_Literals( '' ) // "string"
prototypes_or_Literals( new String('') ) // "[object String]"
Object.prototype.toString.call("foo bar") //"[object String]"

function isEmpty(obj) {
if (typeof obj == 'number') return false;
else if (typeof obj == 'string') return obj.length == 0;
else if (Array.isArray(obj)) return obj.length == 0;
else if (typeof obj == 'object') return obj == null || Object.keys(obj).length == 0;
else if (typeof obj == 'boolean') return false;
else return !obj;
}
In ES6 with trim to handle whitespace strings:
const isEmpty = value => {
if (typeof value === 'number') return false
else if (typeof value === 'string') return value.trim().length === 0
else if (Array.isArray(value)) return value.length === 0
else if (typeof value === 'object') return value == null || Object.keys(value).length === 0
else if (typeof value === 'boolean') return false
else return !value
}

It may be usefull.
All values in array represent what you want to be (null, undefined or another things) and you search what you want in it.
var variablesWhatILookFor = [null, undefined, ''];
variablesWhatILookFor.indexOf(document.DocumentNumberLabel) > -1

If you are using TypeScript and don't want to account for "values those are false" then this is the solution for you:
First: import { isNullOrUndefined } from 'util';
Then: isNullOrUndefined(this.yourVariableName)
Please Note: As mentioned below this is now deprecated, use value === undefined || value === null instead. ref.

Try With Different Logic. You can use bellow code for check all four(4) condition for validation like not null, not blank, not undefined and not zero only use this code (!(!(variable))) in javascript and jquery.
function myFunction() {
var data; //The Values can be like as null, blank, undefined, zero you can test
if(!(!(data)))
{
alert("data "+data);
}
else
{
alert("data is "+data);
}
}

The optional chaining operator provides a way to simplify accessing values through connected objects when it's possible that a reference or function may be undefined or null.
let customer = {
name: "Carl",
details: {
age: 82,
location: "Paradise Falls" // detailed address is unknown
}
};
let customerCity = customer.details?.address?.city;
The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:
let customer = {
name: "Carl",
details: { age: 82 }
};
const customerCity = customer?.city ?? "Unknown city";
console.log(customerCity); // Unknown city

function isEmpty(val){
return !val;
}
but this solution is over-engineered, if you dont'want to modify the function later for busines-model needings, then is cleaner to use it directly in code:
if(!val)...

var myNewValue = myObject && myObject.child && myObject.child.myValue;
This will never throw an error. If myObject, child, or myValue is null then myNewValue will be null. No errors will be thrown

For everyone coming here for having similar question, the following works great and I have it in my library the last years:
(function(g3, $, window, document, undefined){
g3.utils = g3.utils || {};
/********************************Function type()********************************
* Returns a lowercase string representation of an object's constructor.
* #module {g3.utils}
* #function {g3.utils.type}
* #public
* #param {Type} 'obj' is any type native, host or custom.
* #return {String} Returns a lowercase string representing the object's
* constructor which is different from word 'object' if they are not custom.
* #reference http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/
* http://stackoverflow.com/questions/3215046/differentiating-between-arrays-and-hashes-in-javascript
* http://javascript.info/tutorial/type-detection
*******************************************************************************/
g3.utils.type = function (obj){
if(obj === null)
return 'null';
else if(typeof obj === 'undefined')
return 'undefined';
return Object.prototype.toString.call(obj).match(/^\[object\s(.*)\]$/)[1].toLowerCase();
};
}(window.g3 = window.g3 || {}, jQuery, window, document));

If you want to avoid getting true if the value is any of the following, according to jAndy's answer:
null
undefined
NaN
empty string ("")
0
false
One possible solution that might avoid getting truthy values is the following:
function isUsable(valueToCheck) {
if (valueToCheck === 0 || // Avoid returning false if the value is 0.
valueToCheck === '' || // Avoid returning false if the value is an empty string.
valueToCheck === false || // Avoid returning false if the value is false.
valueToCheck) // Returns true if it isn't null, undefined, or NaN.
{
return true;
} else {
return false;
}
}
It would be used as follows:
if (isUsable(x)) {
// It is usable!
}
// Make sure to avoid placing the logical NOT operator before the parameter (isUsable(!x)) and instead, use it before the function, to check the returned value.
if (!isUsable(x)) {
// It is NOT usable!
}
In addition to those scenarios, you may want to return false if the object or array is empty:
Object: {} (Using ECMA 7+)
Array: [] (Using ECMA 5+)
You would go about it this way:
function isEmptyObject(valueToCheck) {
if(typeof valueToCheck === 'object' && !Object.keys(valueToCheck).length){
// Object is empty!
return true;
} else {
// Object is not empty!
return false;
}
}
function isEmptyArray(valueToCheck) {
if(Array.isArray(valueToCheck) && !valueToCheck.length) {
// Array is empty!
return true;
} else {
// Array is not empty!
return false;
}
}
If you wish to check for all whitespace strings (" "), you may do the following:
function isAllWhitespace(){
if (valueToCheck.match(/^ *$/) !== null) {
// Is all whitespaces!
return true;
} else {
// Is not all whitespaces!
return false;
}
}
Note: hasOwnProperty returns true for empty strings, 0, false, NaN, null, and undefined, if the variable was declared as any of them, so it might not be the best to use. The function may be modified to use it to show that it was declared, but is not usable.

Code on GitHub
const isEmpty = value => (
(!value && value !== 0 && value !== false)
|| (Array.isArray(value) && value.length === 0)
|| (isObject(value) && Object.keys(value).length === 0)
|| (typeof value.size === 'number' && value.size === 0)
// `WeekMap.length` is supposed to exist!?
|| (typeof value.length === 'number'
&& typeof value !== 'function' && value.length === 0)
);
// Source: https://levelup.gitconnected.com/javascript-check-if-a-variable-is-an-object-and-nothing-else-not-an-array-a-set-etc-a3987ea08fd7
const isObject = value =>
Object.prototype.toString.call(value) === '[object Object]';
Poor man's tests 😁
const test = () => {
const run = (label, values, expected) => {
const length = values.length;
console.group(`${label} (${length} tests)`);
values.map((v, i) => {
console.assert(isEmpty(v) === expected, `${i}: ${v}`);
});
console.groupEnd();
};
const empty = [
null, undefined, NaN, '', {}, [],
new Set(), new Set([]), new Map(), new Map([]),
];
const notEmpty = [
' ', 'a', 0, 1, -1, false, true, {a: 1}, [0],
new Set([0]), new Map([['a', 1]]),
new WeakMap().set({}, 1),
new Date(), /a/, new RegExp(), () => {},
];
const shouldBeEmpty = [
{undefined: undefined}, new Map([[]]),
];
run('EMPTY', empty, true);
run('NOT EMPTY', notEmpty, false);
run('SHOULD BE EMPTY', shouldBeEmpty, true);
};
Test results:
EMPTY (10 tests)
NOT EMPTY (16 tests)
SHOULD BE EMPTY (2 tests)
Assertion failed: 0: [object Object]
Assertion failed: 1: [object Map]

function notEmpty(value){
return (typeof value !== 'undefined' && value.trim().length);
}
it will also check white spaces (' ') along with following:
null ,undefined ,NaN ,empty ,string ("") ,0 ,false