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    Object.seal()

    Summary

    The Object.seal() method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable.

    Syntax

    Object.seal(obj)

    Parameters

    obj
    The object which should be sealed.

    Description

    By default, objects are extensible (new properties can be added to them). Sealing an object prevents new properties from being added and marks all existing properties as non-configurable. This has the effect of making the set of properties on the object fixed and immutable. Making all properties non-configurable also prevents them from being converted from data properties to accessor properties and vice versa, but it does not prevent the values of data properties from being changed. Attempting to delete or add properties to a sealed object, or to convert a data property to accessor or vice versa, will fail, either silently or by throwing a TypeError (most commonly, although not exclusively, when in strict mode code).

    The prototype chain remains untouched. However, the __proto__ property is sealed as well.

    Examples

    var obj = {
      prop: function() {},
      foo: 'bar'
    };
    
    // New properties may be added, existing properties may be changed or removed.
    obj.foo = 'baz';
    obj.lumpy = 'woof';
    delete obj.prop;
    
    var o = Object.seal(obj);
    
    assert(o === obj);
    assert(Object.isSealed(obj) === true);
    
    // Changing property values on a sealed object still works.
    obj.foo = 'quux';
    
    // But you can't convert data properties to accessors, or vice versa.
    Object.defineProperty(obj, 'foo', { get: function() { return 'g'; } }); // throws a TypeError
    
    // Now any changes, other than to property values, will fail.
    obj.quaxxor = 'the friendly duck'; // silently doesn't add the property
    delete obj.foo; // silently doesn't delete the property
    
    // ...and in strict mode such attempts will throw TypeErrors.
    function fail() {
      'use strict';
      delete obj.foo; // throws a TypeError
      obj.sparky = 'arf'; // throws a TypeError
    }
    fail();
    
    // Attempted additions through Object.defineProperty will also throw.
    Object.defineProperty(obj, 'ohai', { value: 17 }); // throws a TypeError
    Object.defineProperty(obj, 'foo', { value: 'eit' }); // changes existing property value
    

    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 sealed ordinary object, simply return it.

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

    Specifications

    Specification Status Comment
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Object.seal' in that specification.
    Standard Initial definition. Implemented in JavaScript 1.8.5.
    ECMAScript 2015 (6th Edition, ECMA-262)
    The definition of 'Object.seal' in that specification.
    Standard  

    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

    Contributors to this page:
    Last updated by: MichaelTheriot,