Note: This feature is deprecated in favor of defining setters using the object initializer syntax or the Object.defineProperty API. This method's behavior is only specified for web compatibility, and is not required to be implemented in any platform. It may not work everywhere.
The __defineSetter__() method of Object instances binds an object's property to a function to be called when an attempt is made to set that property.
Syntax
__defineSetter__(prop, func)
Parameters
prop- : A string containing the name of the property that the setter
funcis bound to.
- : A string containing the name of the property that the setter
func- : A function to be called when there is an attempt to set the specified property. This function receives the following parameter:
val- : The value attempted to be assigned to
prop.
- : The value attempted to be assigned to
- : A function to be called when there is an attempt to set the specified property. This function receives the following parameter:
Return value
None (undefined).
Exceptions
- TypeError
- : Thrown if
funcis not a function.
- : Thrown if
Description
All objects that inherit from Object.prototype (that is, all except null-prototype objects) inherit the __defineSetter__() method. This method allows a setter to be defined on a pre-existing object. This is equivalent to Object.defineProperty(obj, prop, { set: func, configurable: true, enumerable: true }), which means the property is enumerable and configurable, and any existing getter, if present, is preserved.
__defineSetter__() is defined in the spec as "normative optional", which means no implementation is required to implement this. However, all major browsers implement it, and due to its continued usage, it's unlikely to be removed. If a browser implements __defineSetter__(), it also needs to implement the __lookupGetter__(), __lookupSetter__(), and __defineGetter__() methods.
Examples
Using __defineSetter__()
const o = {};
o.__defineSetter__("value", function (val) {
this.anotherValue = val;
});
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5
Defining a setter property in standard ways
You can use the set syntax to define a setter when the object is first initialized.
const o = {
set value(val) {
this.anotherValue = val;
},
};
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5
You may also use Object.defineProperty to define a setter on an object after it's been created. Compared to __defineSetter__(), this method allows you to control the setter's enumerability and configurability, as well as defining symbol properties. The Object.defineProperty() method also works with null-prototype objects, which don't inherit from Object.prototype and therefore don't have the __defineSetter__() method.
const o = {};
Object.defineProperty(o, "value", {
set(val) {
this.anotherValue = val;
},
configurable: true,
enumerable: true,
});
o.value = 5;
console.log(o.value); // undefined
console.log(o.anotherValue); // 5