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    setter

    The set syntax binds an object property to a function to be called when there is an attempt to set that property.

    Syntax

    {set prop(val) { . . . }}
    {set [expression](val) { . . . }}

    Parameters

    prop
    The name of the property to bind to the given function.
    val
    An alias for the variable that holds the value attempted to be assigned to prop.
    expression
    Starting with ECMAScript 6, you can also use expressions for a computed property name to bind to the given function.

    Description

    In JavaScript, a setter can be used to execute a function whenever a specified property is attempted to be changed. Setters are most often used in conjunction with getters to create a type of pseudo-property. It is not possible to simultaneously have a setter on a property that holds an actual value.

    Note the following when working with the set syntax:

    A setter can be removed using the delete operator.

    Examples

    Defining a setter on new objects in object initializers

    This will define a pseudo-property current of object o that, when assigned a value, will update log with that value:

    var o = {
      set current (str) {
        this.log[this.log.length] = str;
      },
      log: []
    }
    

    Note that current is not defined and any attempts to access it will result in undefined.

    Removing a setter with the delete operator

    If you want to remove the setter, you can just delete it:

    delete o.current;
    

    Defining a setter on existing objects using defineProperty

    To append a setter to an existing object later at any time, use Object.defineProperty().

    var o = { a:0 };
    
    Object.defineProperty(o, "b", { set: function (x) { this.a = x / 2; } });
    
    o.b = 10; // Runs the setter, which assigns 10 / 2 (5) to the 'a' property
    console.log(o.a) // 5

    Using a computed property name

    Note: Computed properties are experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet. This will trigger a syntax error in non-supporting environments.

    var expr = "foo";
    
    var obj = {
      baz: "bar",
      set [expr](v) { this.baz = v; }
    };
    
    console.log(obj.baz); // "bar"
    obj.foo = "baz";      // run the setter
    console.log(obj.baz); // "baz"
    

    Specifications

    Specification Status Comment
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Object Initializer' in that specification.
    Standard Initial definition.
    ECMAScript 2015 (6th Edition, ECMA-262)
    The definition of 'Method definitions' in that specification.
    Standard Added computed property names.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 1 2.0 (1.8.1) 9 9.5 3
    Computed property names Not supported 34 (34) Not supported Not supported Not supported
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) 1.0 (1.8.1) (Yes) (Yes) (Yes)
    Computed property names Not supported Not supported 34.0 (34.0) Not supported Not supported Not supported

    SpiderMonkey-specific notes

    • Starting with JavaScript 1.8.1, setters are no longer called when setting properties in object and array initializers.
    • From SpiderMonkey 38 on, a setter with a rest parameter is a SyntaxError as per the ES6 specification.

    See also

    Document Tags and Contributors

    Last updated by: fscholz,
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