Object.prototype.__defineSetter__()

The __defineSetter__ method binds an object's property to a function to be called when an attempt is made to set that property.

Syntax

obj.__defineSetter__(prop, fun)

Parameters

prop

A string containing the name of the property to be bound to the given function.

fun

A function to be called when there is an attempt to set the specified property. This function takes the form

function(val) { . . . }

val

An alias for the variable that holds the value attempted to be assigned to prop.

Return value

undefined.

Description

The __defineSetter__ method allows a setter to be defined on a pre-existing object.

Examples

// Non-standard and deprecated way

var o = {};
o.__defineSetter__('value', function(val) { this.anotherValue = val; });
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5


// Standard-compliant ways

// Using the set operator
var o = { set value(val) { this.anotherValue = val; } };
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5

// Using Object.defineProperty
var o = {};
Object.defineProperty(o, 'value', {
  set: function(val) {
    this.anotherValue = val;
  }
});
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5

Specifications

Browser compatibility

Compatibility notes

• Starting with Firefox 48 (Firefox 48 / Thunderbird 48 / SeaMonkey 2.45), this method can no longer be called at the global scope without any object. A TypeError will be thrown otherwise. Previously, the global object was used in these cases automatically, but this is no longer the case (bug 1253016).

See also

• Object.prototype.__defineGetter__()
• set operator
• Object.defineProperty()
• Object.prototype.__lookupGetter__()
• Object.prototype.__lookupSetter__()
• JS Guide: Defining Getters and Setters
• [Blog Post] Deprecation of __defineGetter__ and __defineSetter__
• bug 647423