JavaScript WeakSet
The WeakSet Object
A JavaScript WeakSet is a collection of values where the values must be objects.
A WeakSet holds weak references to its values.
Examples
// Create a WeakSet
let mySet = new WeakSet();
// Create an Object
let myObj = {fname:"John", lname:"Doe"};
// Add the Object
mySet.add(myObj);
// Do I have myObj in the mySet?
let answer = mySet.has(myObj);
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// Create a WeakSet
let mySet = new WeakSet();
// Create an Object
let myObj = {fname:"John", lname:"Doe"};
// Add the Object
mySet.add(myObj);
// Delete the Object
mySet.delete(myObj);
// Do I have myObj in the mySet?
let answer = mySet.has(myObj);
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Garbage Collection
JavaScript employs a memory management mechanism known as Garbage Collection.
The primary functions 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 Set, a WeakSet does not prevent its values from being garbage collected.
If a value (an object) has no references in a program, it becomes eligible for garbage collection.
When the value is garbage collected, its is removed from the WeakSet.
Example
// Create a WeakSet
let mySet = new WeakSet();
// Create an Object
let myObj = {fname:"John", lname:"Doe"};
// Add the Object
mySet.add(myObj);
// Remove the Object from memory
myObj = null;
// Now myObj in mySet will be garbage collected
Values Must Be Objects
Primitive values cannot be values in a WeakSet.
The values must be objects.
This restriction is tied to the garbage collection mechanism; primitives are not garbage collected in the same way as objects.
Tracking Objects
A WeakSet is similar to a Set, but it only stores objects, and holds them weakly. If there are no other references to an object, it gets garbage collected automatically.
This makes it handy for tracking objects without storing extra data (like counts).
Tracking Visitors:
let text = "";
// Create a WeakSet to track persons
const persons = new WeakSet();
// 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);
// Function to track visitors
function track(visitor) {
if (persons.has(visitor)) {
text += visitor.name + " is visiting again.<br>";
} else {
persons.add(visitor);
text += visitor.name + ", age" + visitor.age +", is visiting for the first time<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
WeakSets are not enumerable.
You cannot iterate over the values with for loops, forEach(), or values().
You cannot access the size. WeakSets have no size property.
Limited Methods
A WeakSet is minimal, memory-safe, and purpose-built for tracking objects privately and efficiently.
WeakSet provides a limited set of methods:
| new WeakSet() | Creates a new WeakSet object |
| add(object) | Adds an Object to a WeakSet |
| delete(object) | Removes a WeakSet object |
| has(object) | Returns true if an object exists in a WeakSet |
WeakSet does not have:
- A size property
- A spread operator (...)
- A clear() method
- Any logic methods (union, difference, intersection)
- Any iteration methods (forEach, keys, values, entries)
The above is by design: Objects may disappear in garbage collection, it makes no sense to iterate over or count them.
Learn More:
Browser Support
WeakSet 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 |
