JavaScript: What is the difference between `if (!x)` and `if (x == null)`?

 

What is the difference between if (!x) and if (x == null) ; that is, when can their results be different?

!x will return true for every "falsy" value (empty string, 0 , null , false , undefined , NaN ) whereas x == null will only return true if x is null (edit: or apparently undefined (see below)).

Try with x = 0 , there is a difference.

You can say that the NOT operator ! converts a value into its opposite boolean equivalent. This is different than actually comparing two values.

In addition, if you compare values with == , JavaScript does type conversion which can lead to unexpected behavior (like undefined == null ). It is better to always use strict comparison === (value and type must be the same) and make use of type conversion only if you really know what you are doing.

Something to read:

  • Data Type Conversion
  • Comparison Operators
  • Logical Operators

    • Update:

    For more information about the non-strict comparison of null and undefined (or the comparison in general), it is worth having a look at the specification. The comparison algorithm is defined there (the comparison is x == y ):

  • If Type(x) is the same as Type(y), then
    (...)
  • If x is null and y is undefined , return true .
  • If x is undefined and y is null , return true .
  • (...)

    • (...)

    The results can be different if x is false, NaN, '' (empty string), undefined (using the strict comparison operator ===), or 0 (zero).

    See Felix Kling's answer for an excellent summary of type comparison.

    Say x is a string. 

    x = undefined;
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

x = "";
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

x = null;
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

    You'll notice that "X is not a truthy value" is shown in all three cases, but only in the case of X being undefined or null is "X is null" shown.

    When X is a boolean value, then (!x) will be true when X is false but (x == null) will not be. For numbers 0 and NaN are considered false values, so not X is truthy.

    See it in action, including the difference between == (equality using type conversion) and === (strict equality)

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