Note: This feature is deprecated in favor of defining getters 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 __defineGetter__() method of Object instances binds an object's property to a function to be called when that property is looked up.
Syntax
__defineGetter__(prop, func)
Parameters
prop- : A string containing the name of the property that the getter
funcis bound to.
- : A string containing the name of the property that the getter
func- : A function to be bound to a lookup of the specified property.
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 __defineGetter__() method. This method allows a getter to be defined on a pre-existing object. This is equivalent to Object.defineProperty(obj, prop, { get: func, configurable: true, enumerable: true }), which means the property is enumerable and configurable, and any existing setter, if present, is preserved.
__defineGetter__() 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 __defineGetter__(), it also needs to implement the __lookupGetter__(), __lookupSetter__(), and __defineSetter__() methods.
Examples
Using __defineGetter__()
const o = {};
o.__defineGetter__("gimmeFive", function () {
return 5;
});
console.log(o.gimmeFive); // 5
Defining a getter property in standard ways
You can use the get syntax to define a getter when the object is first initialized.
const o = {
get gimmeFive() {
return 5;
},
};
console.log(o.gimmeFive); // 5
You may also use Object.defineProperty to define a getter on an object after it's been created. Compared to __defineGetter__(), this method allows you to control the getter'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 __defineGetter__() method.
const o = {};
Object.defineProperty(o, "gimmeFive", {
get() {
return 5;
},
configurable: true,
enumerable: true,
});
console.log(o.gimmeFive); // 5