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javascriptif-statementundefined

In if statement,undefined equals with false

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.

Solution

  • 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