In if statement,undefined equals with false

Question

I'm confused with the code below:

if(undefined){
   //code will not be executed
}

and

if(!undefined){
   //code will be executed
}

Is that mean the "undefined" equals with false?

Here the question related,but no one point above situation out.

The truthy value of `undefined` in Javascript is `false`. Hence the behaviour that you see. — stackErr, May 11 '16 at 15:42
`Boolean(undefined)` is `false`.but your comments should be interchanged — Madhawa Priyashantha, May 11 '16 at 15:43
@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right? — Xheldon Cao, May 12 '16 at 02:25
@XheldonCao if you pass a none Boolean value then yes.javascript use `Boolean()` function to get a boolean value because if condition need a boolean value — Madhawa Priyashantha, May 12 '16 at 03:18

Andreas Louv · Accepted Answer · 2020-12-07T08:20:29.847

46

It means that undefined is a falsy value, list of falsy values are:

""        // Empty string
null      // null
undefined // undefined, which you get when doing: var a;
false     // Boolean false
0         // Number 0
NaN       // Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true
!null      === true
!undefined === true
!0         === true
!NaN       === true

And when you nagate a truthy value you will get false:

!"hello" === false
!1       === false

But undefined is not equal false:

undefined === false // false
undefined  == false // false

And just for the fun if it:

undefined == null // true

edited Dec 07 '20 at 08:20

answered May 11 '16 at 15:43

Andreas Louv

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I am shocked that `undefined == false` gives `false`! How come? Where I can read about it? – Marecky Jul 08 '20 at 11:54
@Marecky there is a nice write up on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness – Andreas Louv Jul 30 '20 at 18:13
is it safe to use undefined to test for falsey this way? is `if (company.list_of_buildings) {` as good as: `if (Array.isArray(company.list_of_buildings) && company.list_of_buildings.length) {` ?? – Zaffer Jun 22 '21 at 20:46
1

@Zaffer The later ensures that `company.list_of_buildings` is an array with at least one item in it, while the other just ensures that `company.list_of_buildings` is non falsy, which means it could be a string, non zero number among other things. – Andreas Louv Jun 23 '21 at 07:11

score 7 · Answer 2 · answered May 11 '16 at 15:47

In javascript strict mode, undefined is not false, but javascript try to convert the object or var to a boolean value (this is called in javascript truthy value), that's the reason you got an undefined as false. This happens with null also, for example.

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

Bablu Ahmed · Answer 3 · 2019-03-03T13:51:35.990

Please take a look below checked falsy values:

""==false?
    Ans: true
null == false?
    Ans: false
undefined == false?
    Ans: false
0 == false?
    Ans: true
NaN == false?
    Ans: false
null == NaN?
    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true That means they are not equal. From the above result, we got 3 falsy values group:

The False group
The Null group and
The NaN group

But a negative falsy value is always true:

!""        === true
!null      === true
!undefined === true
!0         === true
!NaN       === true

For example: To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))
{
    console.log('hi');
}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)
{
    console.log('hi');
}

//or 

if(dataTitle==null || dataTitle===false)
  console.log('hi');

Shaya · Answer 4 · 2022-02-24T17:43:31.540

In javascript, undefined and null are empty properties declared on the global scope. Their value doesn't equal false; they both have the initial value of primitive undefined.

undefined == false // false

With that said, both will result in false value upon a programmatic true/false evaluation. In your example, you used a logical NOT operator (MDN):

Returns false if its single operand can be converted to true; otherwise, returns true

The negation operator first evaluated undefined as false, and then negated the value to true. You can illustrate the process roughly like this:

if (!(!!undefined)) {
    // code will be executed
}

In if statement,undefined equals with false

4 Answers4