set
The set
syntax binds an object property to a function to be called when there is an attempt to set that property. It can also be used in classes.
Try it
Syntax
js
{ set prop(val) { /* … */ } }
{ set [expression](val) { /* … */ } }
There are some additional syntax restrictions:
- A setter must have exactly one parameter.
Parameters
prop
-
The name of the property to bind to the given function. In the same way as other properties in object initializers, it can be a string literal, a number literal, or an identifier.
val
-
An alias for the variable that holds the value attempted to be assigned to
prop
. expression
-
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.
Examples
Defining a setter on new objects in object initializers
The following example define a pseudo-property current
of object
language
. When current
is assigned a value, it updates
log
with that value:
js
const language = {
set current(name) {
this.log.push(name);
},
log: [],
};
language.current = "EN";
console.log(language.log); // ['EN']
language.current = "FA";
console.log(language.log); // ['EN', 'FA']
Note that current
is not defined, and any attempts to access it will
result in undefined
.
Using setters in classes
You can use the exact same syntax to define public instance setters that are available on class instances. In classes, you don't need the comma separator between methods.
js
class ClassWithGetSet {
#msg = "hello world";
get msg() {
return this.#msg;
}
set msg(x) {
this.#msg = `hello ${x}`;
}
}
const instance = new ClassWithGetSet();
console.log(instance.msg); // "hello world"
instance.msg = "cake";
console.log(instance.msg); // "hello cake"
Setter properties are defined on the prototype
property of the class and are thus shared by all instances of the class. Unlike setter properties in object literals, setter properties in classes are not enumerable.
Static setters and private setters use similar syntaxes, which are described in the static
and private class features pages.
Removing a setter with the delete
operator
If you want to remove the setter, you can just delete
it:
js
delete language.current;
Defining a setter on existing objects using defineProperty
To append a setter to an existing object, use
Object.defineProperty()
.
js
const o = { a: 0 };
Object.defineProperty(o, "b", {
set(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
js
const expr = "foo";
const 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 |
---|
ECMAScript Language Specification # sec-method-definitions |
Browser compatibility
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