Day 27: WeakMaps

Welcome back to the 27th day of the DailyJS series. Yesterday we discussed about the concept of Map in JS, and today’s topic is WeakMap.

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced.

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. The keys must be objects and the values can be arbitrary values.

I know, you might be confused by the above lines. Explaining it in layman terms, Weak map is very similar to map, it is also a data structure which allows us to hold key-value pairs, with the difference being that the keys of a WeakMap are of type Object only, unlike Map, using primitive data types as keys is not allowed.

Syntax

Very similar to Map, a WeakMap can be created using the constructor, and it also comes with several methods like set and get.

let weakMap = new WeakMap ();

Some important methods -

1. WeakMap.prototype.set(key, value)
2. WeakMap.prototype.get(key)
3. WeakMap.prototype.delete(key)
4. WeakMap.prototype.has(key)

But why is it used?

Use cases of WeakMap

Source: https://stackoverflow.com/questions/29413222/what-are-the-actual-uses-of-es6-weakmap

• Keeping private data about a specific object and only giving access to it to people with a reference to the Map. A more ad-hoc approach is coming with the private-symbols proposal but that’s a long time from now.
• Keeping data about library objects without changing them or incurring overhead.
• Keeping data about a small set of objects where many objects of the type exist to not incur problems with hidden classes JS engines use for objects of the same type.
• Keeping data about host objects like DOM nodes in the browser.
• Adding a capability to an object from the outside (like the event emitter example in the other answer).

Example

var weakMap = new WeakMap ();

// weakMap.set ("a", 1); - ERROR!

// Set
var myObj = {};
weakMap.set (myObj, 'My Object');
console.log (weakMap);

// Get
var val = weakMap.get (myObj);
console.log ("Value is: ", val);

// Has
console.log ("Has myObj?", weakMap.has (myObj));

// Delete
weakMap.delete (myObj);

That’s it for today, see you tomorrow!