Check if object exists in JavaScript - javascript

How do I verify the existence of an object in JavaScript?
The following works:
if (!null)
alert("GOT HERE");
But this throws an Error:
if (!maybeObject)
alert("GOT HERE");
The Error:
maybeObject is not defined.

You can safely use the typeof operator on undefined variables.
If it has been assigned any value, including null, typeof will return something other than undefined. typeof always returns a string.
Therefore
if (typeof maybeObject != "undefined") {
alert("GOT THERE");
}

There are a lot of half-truths here, so I thought I make some things clearer.
Actually you can't accurately tell if a variable exists (unless you want to wrap every second line into a try-catch block).
The reason is Javascript has this notorious value of undefined which strikingly doesn't mean that the variable is not defined, or that it doesn't exist undefined !== not defined
var a;
alert(typeof a); // undefined (declared without a value)
alert(typeof b); // undefined (not declared)
So both a variable that exists and another one that doesn't can report you the undefined type.
As for #Kevin's misconception, null == undefined. It is due to type coercion, and it's the main reason why Crockford keeps telling everyone who is unsure of this kind of thing to always use strict equality operator === to test for possibly falsy values. null !== undefined gives you what you might expect. Please also note, that foo != null can be an effective way to check if a variable is neither undefined nor null. Of course you can be explicit, because it may help readability.
If you restrict the question to check if an object exists, typeof o == "object" may be a good idea, except if you don't consider arrays objects, as this will also reported to be the type of object which may leave you a bit confused. Not to mention that typeof null will also give you object which is simply wrong.
The primal area where you really should be careful about typeof, undefined, null, unknown and other misteries are host objects. They can't be trusted. They are free to do almost any dirty thing they want. So be careful with them, check for functionality if you can, because it's the only secure way to use a feature that may not even exist.

You can use:
if (typeof objectName == 'object') {
//do something
}

Two ways:
typeof for local variables
You can test for a local object using typeof:
if (typeof object !== "undefined") {}
window for global variables
You can test for a global object (one defined on the global scope) by inspecting the window object:
if (window.FormData) {}

If that's a global object, you can use if (!window.maybeObject)

You could use "typeof".
if(typeof maybeObject != "undefined")
alert("GOT HERE");

If you care about its existence only ( has it been declared ? ), the approved answer is enough :
if (typeof maybeObject != "undefined") {
alert("GOT THERE");
}
If you care about it having an actual value, you should add:
if (typeof maybeObject != "undefined" && maybeObject != null ) {
alert("GOT THERE");
}
As typeof( null ) == "object"
e.g. bar = { x: 1, y: 2, z: null}
typeof( bar.z ) == "object"
typeof( bar.not_present ) == "undefined"
this way you check that it's neither null or undefined, and since typeof does not error if value does not exist plus && short circuits, you will never get a run-time error.
Personally, I'd suggest adding a helper fn somewhere (and let's not trust typeof() ):
function exists(data){
data !== null && data !== undefined
}
if( exists( maybeObject ) ){
alert("Got here!");
}

I used to just do a if(maybeObject) as the null check in my javascripts.
if(maybeObject){
alert("GOT HERE");
}
So only if maybeObject - is an object, the alert would be shown.
I have an example in my site.
https://sites.google.com/site/javaerrorsandsolutions/home/javascript-dynamic-checkboxes

The thread was opened quite some time ago. I think in the meanwhile the usage of a ternary operator is the simplest option:
maybeObject ? console.log(maybeObject.id) : ""

I've just tested the typeOf examples from above and none worked for me, so instead I've used this:
btnAdd = document.getElementById("elementNotLoadedYet");
if (btnAdd) {
btnAdd.textContent = "Some text here";
} else {
alert("not detected!");
}

Apart from checking the existence of the object/variable you may want to provide a "worst case" output or at least trap it into an alert so it doesn't go unnoticed.
Example of function that checks, provides alternative, and catch errors.
function fillForm(obj) {
try {
var output;
output = (typeof obj !== 'undefined') ? obj : '';
return (output);
}
catch (err) {
// If an error was thrown, sent it as an alert
// to help with debugging any problems
alert(err.toString());
// If the obj doesn't exist or it's empty
// I want to fill the form with ""
return ('');
} // catch End
} // fillForm End
I created this also because the object I was passing to it could be x , x.m , x.m[z] and typeof x.m[z] would fail with an error if x.m did not exist.
I hope it helps. (BTW, I am novice with JS)

for me this worked for a DOM-object:
if(document.getElementsById('IDname').length != 0 ){
alert("object exist");
}

if (n === Object(n)) {
// code
}

if (maybeObject !== undefined)
alert("Got here!");

set Textbox value to one frame to inline frame using div alignmnt tabbed panel.
So first of all, before set the value we need check selected tabbed panels frame available or not using following codes:
Javascript Code :
/////////////////////////////////////////
<script>
function set_TextID()
{
try
{
if(!parent.frames["entry"])
{
alert("Frame object not found");
}
else
{
var setText=document.getElementById("formx").value;
parent.frames["entry"].document.getElementById("form_id").value=setText;
}
if(!parent.frames["education"])
{
alert("Frame object not found");
}
else
{
var setText=document.getElementById("formx").value;
parent.frames["education"].document.getElementById("form_id").value=setText;
}
if(!parent.frames["contact"])
{
alert("Frame object not found");
}
else
{
var setText=document.getElementById("formx").value;
parent.frames["contact"].document.getElementById("form_id").value=setText;
}
}catch(exception){}
}
</script>

zero and null are implicit pointers. If you arn't doing arithmetic, comparing, or printing '0' to screen there is no need to actually type it. Its implicit. As in implied. Typeof is also not required for the same reason. Watch.
if(obj) console.log("exists");
I didn't see request for a not or else there for it is not included as. As much as i love extra content which doesn't fit into the question. Lets keep it simple.

Think it's easiest like this
if(myobject_or_myvar)
alert('it exists');
else
alert("what the hell you'll talking about");

Or, you can all start using my exclusive exists() method instead and be able to do things considered impossible. i.e.:
Things like: exists("blabla"), or even: exists("foreignObject.guessedProperty.guessNext.propertyNeeded") are also possible...

Related

How do I check if a specific key is undefined if the object is undefined?

What am I trying to do?
I want to check if a specific key is undefined, but not able to because the higher level of the object is undefined.
What is the code that currently tries to do that?
The structure of the character object is (that's inside another object that exists):
{
...
character: {
...,
archer: {
...,
description: "shoots things"
}
}
}
What I'd like to simply do is this, but results in an error.
if (character.archer.description !== undefined) { ... }
A fix I have implemented, but hoping to optimize is this:
...
if (character !== undefined) {
if (character.archer !== undefined) {
if (character.archer.description !== undefined) {
setDescription(character.archer.description);
}
}
}
...
What do I expect the result to be?
I expect the code to realize that the if statement should run, even if undefined of undefined, that should mean undefined in general.
What is the actual result?
TypeError: Cannot read property 'undefined' of undefined.
What I think the problem could be?
Since character.archer is undefined (for now), I can not proceed to check character.archer.description.
You can use the optional chaining operator that all evergreen browsers today support. You simply prepend ? before the . for every level that could potentially returned undefined (or null, actually, that also works).
Example with your code:
if (character?.archer?.description !== undefined) { ... }
This is exactly what the optional chaining operator is for! It looks like this ?. and it will return the righthand argument if the lefthand is defined, else it will return undefined
useEffect(() => {
const description = character?.archer?.description;
if (description !== undefined) {
setDescription(description);
}
}, []);
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

What is the javascript equivalant for vbscript `Is Nothing`

If Not (oResponse.selectSingleNode("BigGroupType") Is Nothing) Then
End If
I need to convert this to javascript. Is that enough to check null?
This was my lead's answer, plz verify this,
if(typeof $(data).find("BigGroupType").text() != "undefined" && $(data).find("BigGroupType").text() != null) {
}
JavaScript has two values which mean "nothing", undefined and null. undefined has a much stronger "nothing" meaning than null because it is the default value of every variable. No variable can be null unless it is set to null, but variables are undefined by default.
var x;
console.log(x === undefined); // => true
var X = { foo: 'bar' };
console.log(X.baz); // => undefined
If you want to check to see if something is undefined, you should use === because == isn't good enough to distinguish it from null.
var x = null;
console.log(x == undefined); // => true
console.log(x === undefined); // => false
However, this can be useful because sometimes you want to know if something is undefined or null, so you can just do if (value == null) to test if it is either.
Finally, if you want to test whether a variable even exists in scope, you can use typeof. This can be helpful when testing for built-ins which may not exist in older browsers, such as JSON.
if (typeof JSON == 'undefined') {
// Either no variable named JSON exists, or it exists and
// its value is undefined.
}
You need to check for both null and undefined, this implicitly does so
if( oResponse.selectSingleNode("BigGroupType") != null ) {
}
It is the equivalent of:
var node = oResponse.selectSingleNode("BigGroupType");
if( node !== null &&
node !== void 0 ) {
}
void 0 being a bulletproof expression to get undefined
In JavaScript equvalent for Nothing is undefined
if(oResponse.selectSingleNode("BigGroupType") != undefined){
}
This logic:
If Not (oResponse.selectSingleNode("BigGroupType") Is Nothing)
Can be written like this in JavaScript:
if (typeof oResponse.selectSingleNode("BigGroupType") != 'undefined')
Nothing would equal undefined, but checking against undefined is not recommended for several reasons, it’s generally safer to use typeof.
However, if the selectSingleNode can return other falsy values such as null, it’s probably OK to just do a simple check if it is truthy:
if (oResponse.selectSingleNode("BigGroupType"))
JavaScript:-
(document.getElementById(“BigGroupType”) == undefined) // Returns true
JQuery:-
($(“#BigGroupType”).val() === “undefined”) // Returns true
Note in above examples undefined is a keyword in JavaScript, where as in JQuery it is just a string.

Javascript - Shorthand notation to safely check/access a property on a potential undefined object

What's the shortest syntax to check if jsonObject is not undefined before accessing its errorMessage property?
var jsonObject = SomeMethodReturningAnObject();
if (jsonObject.errorMessage === undefined) // jsonObject can be undefined and this will throw an error
/* success! */
else
alert(jsonObject.errorMessage);
You can use the && operator, since it doesn't evaluate the right-hand side if the left-hand side is undefined:
if (jsonObject && jsonObject.errorMessage === undefined)
Another way to do this is to use the typeof operator.
In JS if a variable has been declared but not set a value, such as:
var x;
Then x is set to undefined so you can check for it easily by:
if(x) //x is defined
if(!x) //x is undefined
However if you try to do if(x) on a variable that hasn't even been declared, you'll get the error you allude to in your post, "ReferenceError: x is not defined".
In this case we need to use typeof - MSDN Docs - to check.
So in your case something like:
if(typeof jsonObject !== "undefined") {
//jsonObject is set
if(jsonObject.errorMessage) {
//jsonObject set, errorMessage also set
} else {
//jsonObject set, no errorMessage!
}
} else {
//jsonObject didn't get set
}
This works because if you have a variable set to an empty object, x={}, and try to get at a variable within that object that doesn't exist, eg x.y, you get undefined returned, you don't get a ReferenceError.
Be aware that the typeof operator returns a string denoting the variable type, not the type itself. So it would return "undefined" not undefined.
Also, this very similar question on SO that could help you: How to check a not-defined variable in JavaScript
Hope this helps.
Jack.
var jsonObject = SomeMethodReturningAnObject();
if (jsonObject && jsonObject.errorMessage === undefined)
/* success! */
else
alert(!jsonObject ? "jsonObject not defined" : jsonObject.errorMessage);
if(jsonObject)
{
if (!jsonObject.errorMessage)
// success..
foo()
else
alert(jsonObject.errorMessage);
}
I'll answer the shorthand notation aspect as your specific situation is better served by an existing answer. As of ECMAScript 2018 we have spread syntax, which makes the whole thing much more concise:
if ( {...jsonObject}.errorMessage ) {
// we have errorMessage
} else {
// either jsonObject or jsonObject.errorMessage are undefined / falsey
// in either case, we don't get an exception
}
A straight if / else is not the perfect fit for your situation because you actually have 3 states, 2 of which are crammed into the same if branch above.
No jsonObject: Failure
Has jsonObject, has no errorMessage: Success
Has jsonObject, has errorMessage: Failure
Why this works:
Accessing a property foo from an object obj, assuming the object is undefined, is essentially doing this:
undefined.foo //exception, obviously
Spread syntax copies properties to a new object, so you end up with an object no matter what:
typeof {...undefined} === 'object'
So by spreading obj you avoid operating directly on a potentially undefined variable.
Some more examples:
({...undefined}).foo // === undefined, no exception thrown
({...{'foo': 'bar'}}).foo // === 'bar'

is there something like isset of php in javascript/jQuery? [duplicate]

This question already has answers here:
JavaScript isset() equivalent
(28 answers)
Closed 6 years ago.
Is there something in javascript/jQuery to check whether variable is set/available or not? In php, we use isset($variable) to check something like this.
thanks.
Try this expression:
typeof(variable) != "undefined" && variable !== null
This will be true if the variable is defined and not null, which is the equivalent of how PHP's isset works.
You can use it like this:
if(typeof(variable) != "undefined" && variable !== null) {
bla();
}
JavaScript isset() on PHP JS
function isset () {
// discuss at: http://phpjs.org/functions/isset
// + original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
// + improved by: FremyCompany
// + improved by: Onno Marsman
// + improved by: Rafał Kukawski
// * example 1: isset( undefined, true);
// * returns 1: false
// * example 2: isset( 'Kevin van Zonneveld' );
// * returns 2: true
var a = arguments,
l = a.length,
i = 0,
undef;
if (l === 0) {
throw new Error('Empty isset');
}
while (i !== l) {
if (a[i] === undef || a[i] === null) {
return false;
}
i++;
}
return true;
}
typeof will serve the purpose I think
if(typeof foo != "undefined"){}
If you want to check if a property exists: hasOwnProperty is the way to go
And since most objects are properties of some other object (eventually leading to the window object) this can work well for checking if values have been declared.
Some parts of each of these answers work. I compiled them all down into a function "isset" just like the question was asking and works like it does in PHP.
// isset helper function
var isset = function(variable){
return typeof(variable) !== "undefined" && variable !== null && variable !== '';
}
Here is a usage example of how to use it:
var example = 'this is an example';
if(isset(example)){
console.log('the example variable has a value set');
}
It depends on the situation you need it for but let me break down what each part does:
typeof(variable) !== "undefined" checks if the variable is defined at all
variable !== null checks if the variable is null (some people explicitly set null and don't think if it is set to null that that is correct, in that case, remove this part)
variable !== '' checks if the variable is set to an empty string, you can remove this if an empty string counts as set for your use case
Hope this helps someone :)
Not naturally, no... However, a googling of the thing gave this: http://phpjs.org/functions/isset:454
http://phpjs.org/functions/isset:454
phpjs project is a trusted source. Lots of js equivalent php functions available there. I have been using since a long time and found no issues so far.
The problem is that passing an undefined variable to a function causes an error.
This means you have to run typeof before passing it as an argument.
The cleanest way I found to do this is like so:
function isset(v){
if(v === 'undefined'){
return false;
}
return true;
}
Usage:
if(isset(typeof(varname))){
alert('is set');
} else {
alert('not set');
}
Now the code is much more compact and readable.
This will still give an error if you try to call a variable from a non instantiated variable like:
isset(typeof(undefVar.subkey))
thus before trying to run this you need to make sure the object is defined:
undefVar = isset(typeof(undefVar))?undefVar:{};
Here :)
function isSet(iVal){
return (iVal!=="" && iVal!=null && iVal!==undefined && typeof(iVal) != "undefined") ? 1 : 0;
} // Returns 1 if set, 0 false
in addition to #emil-vikström's answer, checking for variable!=null would be true for variable!==null as well as for variable!==undefined (or typeof(variable)!="undefined").
You can just:
if(variable||variable===0){
//Yes it is set
//do something
}
else {
//No it is not set
//Or its null
//do something else
}

How to check for an undefined or null variable in JavaScript?

We are frequently using the following code pattern in our JavaScript code
if (typeof(some_variable) != 'undefined' && some_variable != null)
{
// Do something with some_variable
}
Is there a less verbose way of checking that has the same effect?
According to some forums and literature saying simply the following should have the same effect.
if (some_variable)
{
// Do something with some_variable
}
Unfortunately, Firebug evaluates such a statement as error on runtime when some_variable is undefined, whereas the first one is just fine for it. Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways?
I think the most efficient way to test for "value is null or undefined" is
if ( some_variable == null ){
// some_variable is either null or undefined
}
So these two lines are equivalent:
if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}
Note 1
As mentioned in the question, the short variant requires that some_variable has been declared, otherwise a ReferenceError will be thrown. However in many use cases you can assume that this is safe:
check for optional arguments:
function(foo){
if( foo == null ) {...}
check for properties on an existing object
if(my_obj.foo == null) {...}
On the other hand typeof can deal with undeclared global variables (simply returns undefined). Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained.
Note 2
This - even shorter - variant is not equivalent:
if ( !some_variable ) {
// some_variable is either null, undefined, 0, NaN, false, or an empty string
}
so
if ( some_variable ) {
// we don't get here if some_variable is null, undefined, 0, NaN, false, or ""
}
Note 3
In general it is recommended to use === instead of ==.
The proposed solution is an exception to this rule. The JSHint syntax checker even provides the eqnull option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;
You have to differentiate between cases:
Variables can be undefined or undeclared. You'll get an error if you access an undeclared variable in any context other than typeof.
if(typeof someUndeclaredVar == whatever) // works
if(someUndeclaredVar) // throws error
A variable that has been declared but not initialized is undefined.
let foo;
if (foo) //evaluates to false because foo === undefined
Undefined properties , like someExistingObj.someUndefProperty. An undefined property doesn't yield an error and simply returns undefined, which, when converted to a boolean, evaluates to false. So, if you don't care about
0 and false, using if(obj.undefProp) is ok. There's a common idiom based on this fact:
value = obj.prop || defaultValue
which means "if obj has the property prop, assign it to value, otherwise assign the default value defautValue".
Some people consider this behavior confusing, arguing that it leads to hard-to-find errors and recommend using the in operator instead
value = ('prop' in obj) ? obj.prop : defaultValue
Checking null with normal equality will also return true for undefined.
if (window.variable == null) alert('variable is null or undefined');
This is the only case in which == and != should be used:
if (val == null) console.log('val is null or undefined')
if (val != null) console.log('val is neither null nor undefined')
For any other comparisons, the strict comparators (=== and !==) should be used.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
https://2ality.com/2011/12/strict-equality-exemptions.html
In newer JavaScript standards like ES5 and ES6 you can just say
> Boolean(0) //false
> Boolean(null) //false
> Boolean(undefined) //false
all return false, which is similar to Python's check of empty variables.
So if you want to write conditional logic around a variable, just say
if (Boolean(myvar)){
// Do something
}
here "null" or "empty string" or "undefined" will be handled efficiently.
If you try and reference an undeclared variable, an error will be thrown in all JavaScript implementations.
Properties of objects aren't subject to the same conditions. If an object property hasn't been defined, an error won't be thrown if you try and access it. So in this situation you could shorten:
if (typeof(myObj.some_property) != "undefined" && myObj.some_property != null)
to
if (myObj.some_property != null)
With this in mind, and the fact that global variables are accessible as properties of the global object (window in the case of a browser), you can use the following for global variables:
if (window.some_variable != null) {
// Do something with some_variable
}
In local scopes, it always useful to make sure variables are declared at the top of your code block, this will save on recurring uses of typeof.
Firstly you have to be very clear about what you test. JavaScript has all sorts of implicit conversions to trip you up, and two different types of equality comparator: == and ===.
A function, test(val) that tests for null or undefined should have the following characteristics:
test(null) => true
test(undefined) => true
test(0) => false
test(1) => false
test(true) => false
test(false) => false
test('s') => false
test([]) => false
Let's see which of the ideas here actually pass our test.
These work:
val == null
val === null || val === undefined
typeof(val) == 'undefined' || val == null
typeof(val) === 'undefined' || val === null
These do not work:
typeof(val) === 'undefined'
!!val
I created a jsperf entry to compare the correctness and performance of these approaches. Results are inconclusive for the time being as there haven't been enough runs across different browsers/platforms. Please take a minute to run the test on your computer!
At present, it seems that the simple val == null test gives the best performance. It's also pretty much the shortest. The test may be negated to val != null if you want the complement.
here's another way using the Array includes() method:
[undefined, null].includes(value)
Since there is no single complete and correct answer, I will try to summarize:
In general, the expression:
if (typeof(variable) != "undefined" && variable != null)
cannot be simplified, because the variable might be undeclared so omitting the typeof(variable) != "undefined" would result in ReferenceError. But, you can simplify the expression according to the context:
If the variable is global, you can simplify to:
if (window.variable != null)
If it is local, you can probably avoid situations when this variable is undeclared, and also simplify to:
if (variable != null)
If it is object property, you don't have to worry about ReferenceError:
if (obj.property != null)
This is an example of a very rare occasion where it is recommended to use == instead of ===. Expression somevar == null will return true for undefined and null, but false for everything else (an error if variable is undeclared).
Using the != will flip the result, as expected.
Modern editors will not warn for using == or != operator with null, as this is almost always the desired behavior.
Most common comparisions:
undeffinedVar == null // true
obj.undefinedProp == null // true
null == null // true
0 == null // false
'0' == null // false
'' == null // false
Try it yourself:
let undefinedVar;
console.table([
{ test : undefinedVar, result: undefinedVar == null },
{ test : {}.undefinedProp, result: {}.undefinedProp == null },
{ test : null, result: null == null },
{ test : false, result: false == null },
{ test : 0, result: 0 == null },
{ test : '', result: '' == null },
{ test : '0', result: '0' == null },
]);
You can just check if the variable has a value or not. Meaning,
if( myVariable ) {
//mayVariable is not :
//null
//undefined
//NaN
//empty string ("")
//0
//false
}
If you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. e.g.
if( typeof myVariable !== 'undefined' ) {
// myVariable will get resolved and it is defined
}
Similar to what you have, you could do something like
if (some_variable === undefined || some_variable === null) {
do stuff
}
This is also a nice (but verbose) way of doing it:
if((someObject.someMember ?? null) === null) {
// bladiebla
}
It's very clear what's happening and hard to misunderstand. And that can be VERY important! :-)
This uses the ?? operator (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator). If the value of someObject.someMember is null or undefined, the ?? operator kicks in and will make the value null.
TBH, I like the explicitness of this thing, but I usualle prefer someObject.someMember == null, it's more readable and skilled JS developers probably know what's going on here.
whatever yyy is undefined or null, it will return true
if (typeof yyy == 'undefined' || !yyy) {
console.log('yes');
} else {
console.log('no');
}
yes
if (!(typeof yyy == 'undefined' || !yyy)) {
console.log('yes');
} else {
console.log('no');
}
no
Open the Developer tools in your browser and just try the code shown in the below image.
If the purpose of the if statement is to check for null or undefined values before assigning a value to a variable, you can make use of the Nullish Coalescing Operator. According to the data from caniuse, it should be supported by around 85% of the browsers(as of January 2021). An example of the operator is shown below:
const a = some_variable ?? '';
This will ensure that the variable will be assigned to an empty string (or any other default value) if some_variable is null or undefined.
This operator is most suited for your use case, as it does not return the default value for other types of falsy value such as 0 and ''.
As mentioned in one of the answers, you can be in luck if you are talking about a variable that has a global scope. As you might know, the variables that you define globally tend to get added to the windows object. You can take advantage of this fact so lets say you are accessing a variable called bleh, just use the double inverted operator (!!)
!!window['bleh'];
This would return a false while bleh has not been declared AND assigned a value.
In order to understand, Let's analyze what will be the value return by the Javascript Engine when converting undefined , null and ''(An empty string also). You can directly check the same on your developer console.
You can see all are converting to false , means All these three are assuming ‘lack of existence’ by javascript. So you no need to explicitly check all the three in your code like below.
if (a === undefined || a === null || a==='') {
console.log("Nothing");
} else {
console.log("Something");
}
Also I want to point out one more thing.
What will be the result of Boolean(0)?
Of course false. This will create a bug in your code when 0 is a valid value in your expected result. So please make sure you check for this when you write the code.
With Ramda, you can simply do R.isNil(yourValue)
Lodash and other helper libraries have the same function.
I have done this using this method
save the id in some variable
var someVariable = document.getElementById("someId");
then use if condition
if(someVariable === ""){
//logic
} else if(someVariable !== ""){
//logic
}
In ES5 or ES6 if you need check it several times you cand do:
const excluded = [null, undefined, ''];
if (!exluded.includes(varToCheck) {
// it will bee not null, not undefined and not void string
}
let varToCheck = ""; //U have to define variable firstly ,or it throw error
const excluded = [null, undefined, ""];
if (!excluded.includes(varToCheck)) {
// it will bee not null, not undefined and not void string
console.log("pass");
} else {
console.log("fail");
}
for example I copy vladernn's answer to test, u can just click button "Copy snippets to answer" to test too .
Testing nullity (if (value == null)) or non-nullity (if (value != null)) is less verbose than testing the definition status of a variable.
Moreover, testing if (value) (or if( obj.property)) to ensure the existence of your variable (or object property) fails if it is defined with a boolean false value. Caveat emptor :)
Best way to compare undefined or null or 0 with ES5 and ES6 standards
if ((Boolean(some_variable_1) && Boolean(some_variable_2)) === false) {
// do something
}
You can make use of lodash library.
_.isNil(value) gives true for both null and undefined
Test on - https://bazinga.tools/lodash
You must define a function of this form:
validate = function(some_variable){
return(typeof(some_variable) != 'undefined' && some_variable != null)
}
Both values can be easily distinguished by using the strict comparison operator.
Sample Code:
function compare(){
var a = null; //variable assigned null value
var b; // undefined
if (a === b){
document.write("a and b have same datatype.");
}
else{
document.write("a and b have different datatype.");
}
}

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