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isNaN(value)

Arguments

value

The expression to test.

Returns

The Boolean true if value is the special numeric value NaN; otherwise, false.

Description

To test whether or not a value is equal to the special numeric value NaN, we must use the isNaN( ) function, because NaN does not test equal to itself in an ordinary equality test. For example, the expression:

NaN =  = NaN;

yields the value false. The isNaN( ) function is often used to check whether a mathematical error (such as zero divided by itself) has occurred, or whether converting a value to a legitimate number has failed. Because isNaN( ) returns true when the expression is not a valid numeric expression, you'll often use the logical NOT operator (!) along with isNaN( ) (i.e., something that is not not-a-number is a number). Note that 0/0 yields NaN, but any positive number divided by 0 yields Infinity, and any negative number divided by 0 yields -Infinity.

Example

// Set a value
var x = "test123";
// Check if x is a legitimate number before using it is in a math expression. This 
// is a handy for user input in text fields, which are always treated as strings.
if (!isNaN(parseFloat(x))) {
  var y = parseFloat(x) * 2;
}

See Also

isFinite( ), NaN; "Special Values of the Number Datatype," in Chapter 4

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