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    Object.isExtensible()

    Summary

    The Object.isExtensible() method determines if an object is extensible (whether it can have new properties added to it).

    Syntax

    Object.isExtensible(obj)

    Parameters

    obj
    The object which should be checked.

    Description

    Objects are extensible by default: they can have new properties added to them, and (in engines that support __proto__ their __proto__ property) can be modified. An object can be marked as non-extensible using Object.preventExtensions(), Object.seal(), or Object.freeze().

    Examples

    // New objects are extensible.
    var empty = {};
    assert(Object.isExtensible(empty) === true);
    
    // ...but that can be changed.
    Object.preventExtensions(empty);
    assert(Object.isExtensible(empty) === false);
    
    // Sealed objects are by definition non-extensible.
    var sealed = Object.seal({});
    assert(Object.isExtensible(sealed) === false);
    
    // Frozen objects are also by definition non-extensible.
    var frozen = Object.freeze({});
    assert(Object.isExtensible(frozen) === false);
    

    Notes

    In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES6, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.

    > Object.isExtensible(1)
    TypeError: 1 is not an object // ES5 code
    
    > Object.isExtensible(1)
    false                         // ES6 code
    

    Specifications

    Specification Status Comment
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Object.isExtensible' in that specification.
    Standard Initial definition. Implemented in JavaScript 1.8.5.
    ECMAScript 6 (ECMA-262)
    The definition of 'Object.isExtensible' in that specification.
    Release Candidate  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 6 4.0 (2.0) 9 12 5.1
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support ? ? ? ? ? ?

    Based on Kangax's compat table.

    See also

    Document Tags and Contributors

    Last updated by: Mingun,