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Formula
WeakMap is a collection of key/value pairs where the keys must be objects.A WeakMap holds weak references to its keys.
// Create a WeakMap let myMap = new WeakMap();
// Create an Object let myObj = {fname:"John", lname:"Doe"};
// Set a WeakMap value myMap.set(myObj, "player");
// Get the WeakMap value let type = myMap.get(myObj);JavaScript employs a memory management mechanism known as Garbage Collection.
Reclaim memory occupied by variables that are no longer in use
Unlike a regular Map, a WeakMap does not prevent its keys from being garbage collected. If a key (an object) has no references to it in a program, it becomes eligible for garbage collection.
Formula
When the key is garbage collected, its key - value pair is removed from the WeakMap.let myMap = new WeakMap();
let myObj = {fname:"John", lname:"Doe"};
myMap.set(myObj, "secret");
myObj = null;
// now myObj (and its values) in myMap can be garbage collectedPrimitive values cannot be used as keys in a WeakMap.
Formula
The keys must be objects or non - registered symbols.This restriction is tied to the garbage collection mechanism;primitives are not garbage collected in the same way as objects.
The entries in a WeakMap are weakly held: if the object key becomes unreachable, its mapping is removed automatically. This is perfect for tracking data about objects without preventing garbage collection.
Tracking Visitors let text = "";
// Create a WeakMap to store visit counts const visitsCount = new WeakMap();
// Create Visitor Objects const John = {name:"John", age:40};
const Paul = {name:"Paul", age:41};
const Ringo = {name:"Ringo", age:42};
const George = {name:"George", age:43};
// Track visits track(Paul);
track(Ringo);
track(Paul);
track(Paul);
track(John);
// Function to track visitors function track(visitor) {
let count = visitsCount.get(visitor) || 0;
count++;
visitsCount.set(visitor, count);
text += visitor.name + ", age " + visitor.age + ", has visited " + count + " time(s).<br>";
}If you remove all references to a visitor object:
John = null;
// The entry for John is now removed from the WeakMap (persons)WeakMaps are not enumerable. You cannot iterate over the keys and values with for loops, forEach(), or keys(). You cannot access the size.
WeakMap provides a limited set of methods: new WeakMap()
Gets the value for a key in a WeakMap set(key, value) Sets the value for a key in a WeakMap delete(key) Removes an element specified by a key has(key) Returns true if a key exists in a WeakMap
// Create WeakMap const myMap = new WeakMap();
// Private Fields Simulation class User {
constructor(name) {
myMap.set(this, {secret:"hidden data"});
this.name = name;
}
getSecret() {
return myMap.get(this).secret;
}
}
const user1 = new User("John");
secret = user1.getSecret();A WeakMap does not allow iteration. Outside code can not "discover" what objects are stored inside a WeakMap. To get the secret, you need the this reference that was used in the constructor. External code has access to user1 and myMap, bot not to the this reference inside myMap, unless you explicitly expose it, like via getSecret(), the secret value is unreachable.
WeakMap was intentionally designed for privacy: you can set, get, has, and delete using an object key, but not inspect what is inside. This was a great tool for simulating private properties in JavaScript classes
(before #private fields were added to the language).