JavaScript WeakMap
The WeakMap Object
A JavaScript WeakMap is a collection of key/value pairs where the keys must be objects.
A WeakMap holds weak references to its keys.
Example
// 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);
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Garbage Collection
JavaScript employs a memory management mechanism known as Garbage Collection.
The primary functiona are:
- Ensuring efficient use of memory resources
- Reclaim memory occupied by variables that are no longer in use
- Preventing memory leaks
Weak References
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.
When the key is garbage collected, its key-value pair is removed from the WeakMap.
Example
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 collected
Keys Must Be Objects
Primitive values cannot be used as keys in a WeakMap.
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.
Tracking 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>";
}
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Automatic Cleanup
If you remove all references to a visitor object:
Tracking Visitors:
John = null;
// The entry for John is now removed from the WeakMap (persons)
Not Iterable
WeakMaps are not enumerable.
You cannot iterate over the keys and values with for loops, forEach(), or keys().
You cannot access the size.
Limited Methods
WeakMap provides a limited set of methods:
| new WeakMap() | Creates a new WeakMap object |
| get(key) | 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 |
Weak Map Secret Data
Example
// 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();
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Example Explained
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.
Privacy
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).
Browser Support
WeakMap is an ES6 feature.
ES6 is fully supported in all modern browsers since June 2017:
| Chrome 51 |
Edge 15 |
Firefox 54 |
Safari 10 |
Opera 38 |
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
