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.

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