在 if 语句中，undefined 等于 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.

Answer 1

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

Answer 2

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"); 

Answer 3

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:

1. The False group
2. The Null group and
3. 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');

Answer 4

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
}