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JavaScript WeakMap

The WeakMap object is a collection of key/value pairs in which the keys must be objects and the values can be arbitrary values. The difference between a WeakMap and a Map is that a WeakMap allows its keys to be garbage collected, while a Map does not.

Syntax

new WeakMap([iterable])
  • iterable: An iterable object whose elements are key/value pairs (e.g. arrays), or an object whose own enumerable properties will be used as the key/value pairs.

Example

const wm = new WeakMap();

let o1 = {};
let o2 = {};

wm.set(o1, "foo");
wm.set(o2, "bar");
console.log(wm.get(o1)); // output: "foo"
console.log(wm.get(o2)); // output: "bar"
wm.delete(o2);
console.log(wm.get(o2)); // output: undefined

Output

foo
bar
undefined

Explanation

In the above example, we create a new WeakMap object wm. We then create two different objects, o1 and o2. We add key/value pairs to wm using set() method, where o1 is the key for "foo" and o2 is the key for "bar". We retrieve the values using get() method.

We then delete the key/value pair where o2 is the key using delete() method. We try to retrieve the value again using get() method but it returns undefined because o2 has been deleted.

Use

WeakMap is useful when you need to associate some data with an object but you don't want to create a strong reference to it. This can help prevent memory leaks in your application.

Important Points

  • Keys must be objects and values can be arbitrary values.
  • Allows keys to be garbage collected.
  • Not iterable.

Summary

WeakMap is a special type of map that allows its keys to be garbage collected. It is useful in situations where you want to associate some data with an object, but don't want to create a strong reference to it.

WeakSetcallback
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