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    isNaN()

    Summary

    The isNaN() function determines whether a value is NaN or not. Be careful, this function is broken. You may be interested in Number.isNaN() as defined in ECMAScript 6 or you can use typeof to determine if the value is Not-A-Number.

    Syntax

    isNaN(testValue)

    Parameters

    testValue
    The value to be tested.

    Description

    The necessity of an isNaN function

    Unlike all other possible values in JavaScript, it is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function.

    Origin of NaN values

    NaN values are generated when arithmetic operations result in undefined or unrepresentable values. Such values do not necessarily represent overflow conditions. A NaN also results from attempted coercion to numeric values of non-numeric values for which no primitive numeric value is available.

    For example, dividing zero by zero results in a NaN — but dividing other numbers by zero does not.

    Confusing special-case behavior

    Since the very earliest versions of the isNaN function specification, its behavior for non-numeric arguments has been confusing. When the argument to the isNaN function is not of type Number, the value is first coerced to a Number. The resulting value is then tested to determine whether it is NaN. Thus for non-numbers that when coerced to numeric type result in a valid non-NaN numeric value (notably the empty string and boolean primitives, which when coerced give numeric values zero or one), the "false" returned value may be unexpected; the empty string, for example, is surely "not a number." The confusion stems from the fact that the term, "not a number", has a specific meaning for numbers represented as IEEE-794 floating-point values. The function should be interpreted as answering the question, "is this value, when coerced to a numeric value, an IEEE-794 'Not A Number' value?"

    The next version of ECMAScript (ES6) contains the Number.isNaN() function. Number.isNaN(x) will be a reliable way to test whether x is NaN or not. Even with Number.isNaN, however, the meaning of NaN remains the precise numeric meaning, and not simply, "not a number". Alternatively, in absense of Number.isNaN, the expression (x != x) is a more reliable way to test whether variable x is NaN or not, as the result is not subject to the false positives that make isNaN unreliable.

    Examples

    isNaN(NaN);       // true
    isNaN(undefined); // true
    isNaN({});        // true
    
    isNaN(true);      // false
    isNaN(null);      // false
    isNaN(37);        // false
    
    // strings
    isNaN("37");      // false: "37" is converted to the number 37 which is not NaN
    isNaN("37.37");   // false: "37.37" is converted to the number 37.37 which is not NaN
    isNaN("");        // false: the empty string is converted to 0 which is not NaN
    isNaN(" ");       // false: a string with spaces is converted to 0 which is not NaN
    
    // dates
    isNaN(new Date());                // false
    isNaN(new Date().toString());     // true
    
    // This is a false positive and the reason why isNaN is not entirely reliable
    isNaN("blabla")   // true: "blabla" is converted to a number. 
                      // Parsing this as a number fails and returns NaN
    

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition.
    ECMAScript 5.1 (ECMA-262)
    The definition of 'isNaN' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'isNaN' in that specification.
    Draft  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

    See also

    Document Tags and Contributors

    Tags:
    Last updated by: Troynov,