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

What is the difference between if (!x) and if (x == nul开发者_如何学Gol); that is, when can their results be different?

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!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

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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):

1. If Type(x) is the same as Type(y), then
   (...)
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.
4. (...)

(...)

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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.

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if (!x) 

coerces x uses the internal ToBoolean function

if (x==null)

coerces both operands using the internal ToPrimitive function (which generally resolves each side to a number, occasionally a string, depending on the operands)

For full explanantion of ToBoolean vs ToPrimitive see http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/

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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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!x tests for a false value. This will be true for any value that can propagate to false for whatever reason. This will be true for permutations of false, 0, etc etc.

x == null is different because var x = 0 will NOT be null... but WILL be false.