So my uploaded media file (event.target.files[0]) does not equal to true or false.
It has a typeof object.
It's part of some form state and I'd like to check the object whether all fields are not empty "".
I thought a JS object should always === true, but maybe this is different for 'files' objects?
=== checks for strict equality, so the two values must be exactly the same.
An object is truthy, but does not equal true, so what you are really doing is { ... } === true, which is false.
If you want to check if none of the object's values are empty, you can filter for empty values:
const empty = Object.keys(theObject).length === 0 || Object.values(theObject).filter(value => {
return value.trim() === '';
}).length > 0;
=== tests for equal value and equal type (ref). typeof(true) is boolean but a file is not a boolean. So the comparison will never yield true.
See also https://stackoverflow.com/a/8511350/4640820
To check the type of a value, you must write
if( (typeof <your value>) == ("<expected type>")){
...
}
For example, a statement like this:
if( (typeof 42)=="number" )
is true.
Reference for most cases of typeof
I like to use the !! in Javascript to ensure that a variable is set and that no error will be thrown.
However today I have a variable with 0 value that is valid for me. I need to ensure that it is not NaN nor undefined, is there really no short way to do it without the boring if (variable !== NaN && variable !== undefined?
If that may help, I am using angular.
Thx.
let a = 'value';
if (isNaN(a) || a == null) {
console.log('a is NaN or null, undefined');
} else {
// business logic ;)
}
To handle null also, caz isNaN(null) // false
You can use isNan. It will return true for undefined and NAN value but not for ZERO.
const variable = undefined;
if(isNaN(variable)){
console.log('I am undefined / NAN');
}else{
console.log('I am something / zero');
}
The correct way to check for undefined, null, NaN is
if(a == null || Number.isNaN(a)) {
// we did it!
}
please notice the use of Number.isNaN instead of just isNaN.
If you did use just isNaN then your check will not do what you want for values like strings or empty object
if(a == null || isNaN(a)) {
// {} comes in because `isNaN({})` is `true`
// strings that can't be coerced into numbers
// some other funny things
}
Read more at MDN
As we all know null & undefined are falsy values.
But why does the first snippet of code work and the second does not?
// #1
if (!undefined) { // or (!null)
console.log("hi"); // => hi
}
enter code here
// #2
if (undefined == false) { // or (null == false)
console.log("hi"); => never gets executed
}
Is there a specific reason for this or is it just a language specification?
Other falsy values such as 0, "", false (except NaN) work and I guess they are being converted to false.
Because being "falsey" is not the same as being equal to false, which is why that term needed to be invented.
undefined only equals null, but not false by definition:
undefined == null // true
undefined == false // false
And yes thats not really for a specific reason, just to confuse people:
[] == false // true
So all you can learn from thisis, is that you should always use ===
First of all, undefined and null are two different primitive values.
undefined means that the variable is created but not been assigned any value
and so it acts as a falsy value, while null represents a non-existing reference and so it's falsy too,
Now in your case,
if(undefined==false){
//the code in here will never be executed
}
->As undefined has no value while false does.so,
undefined==false
//is false as false(with a value) can't be
//equal to undefined which has no value
I am getting the following javascript error:
'value' is null or not an object
Can someone please let me know what is the best way to check whether an object's value is NULL in javascript as I have been using:
if ((pNonUserID !== "") || (pExtUserID !== "")){
Is this correct or is there a better way?
Thanks.
You don't have to do that:
var n=null;
if(n)alert('Not null.'); // not shown
if(!n)alert('Is null.'); // popup is shown
Your error implies otherwise:
var n=null;
alert(n.something); // Error: n is null or not an object.
In the case above, something like this should be used:
if(n)alert(n.something);
The !== operator returns true when two variables are not the same object. It doesn't look at the values of the objects at all
To test if something is null:
myVar == null
Your code was testing to see if the variable 'pNonUserId' referred to the same object as "", which can never be true as "" will always be a new instance of the empty string.
As an aside, a test such as:
var n = something();
// do stuff
if (n)
doSomethingElse();
Is a bad idea. If n was a boolean and false, but you were expecting the if block to test nullify you'll be in for a shock.
if (pNonUserID && pExtUserID)
{
// neither pNonUserId nor pExtUserID are null here
}
Any Javascript variable automatically evaluates to true when it references an object.
What you were doing are comparisons to empty strings, which are not the same as null.
null, undefined and empty string is consider as false in conditional statement.
so
if(!n) alert("n is null or undefined or empty string");
if(n) alert("n has some value");
therefor, inflagranti suggested condition will work perfectly for you
if(pNonUserID && pExtUserID) {
}
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.");
}
}