WeakSet
A WeakSet
is a collection of garbage-collectable values, including objects and non-registered symbols. A value in the WeakSet
may only occur once. It is unique in the WeakSet
's collection.
Description
Values of WeakSets must be garbage-collectable. Most primitive data types can be arbitrarily created and don't have a lifetime, so they cannot be stored. Objects and non-registered symbols can be stored because they are garbage-collectable.
The main differences to the Set
object are:
WeakSet
s are collections of objects and symbols only. They cannot contain arbitrary values of any type, asSet
s can.- The
WeakSet
is weak, meaning references to objects in aWeakSet
are held weakly. If no other references to a value stored in theWeakSet
exist, those values can be garbage collected.Note: This also means that there is no list of current values stored in the collection.
WeakSets
are not enumerable.
Use case: Detecting circular references
Functions that call themselves recursively need a way of guarding against circular data structures by tracking which objects have already been processed.
WeakSet
s are ideal for this purpose:
js
// Execute a callback on everything stored inside an object
function execRecursively(fn, subject, _refs = new WeakSet()) {
// Avoid infinite recursion
if (_refs.has(subject)) {
return;
}
fn(subject);
if (typeof subject === "object") {
_refs.add(subject);
for (const key in subject) {
execRecursively(fn, subject[key], _refs);
}
_refs.delete(subject);
}
}
const foo = {
foo: "Foo",
bar: {
bar: "Bar",
},
};
foo.bar.baz = foo; // Circular reference!
execRecursively((obj) => console.log(obj), foo);
Here, a WeakSet
is created on the first run, and passed along with every subsequent function call (using the internal _refs
parameter).
The number of objects or their traversal order is immaterial, so a WeakSet
is more suitable (and performant) than a Set
for tracking object references, especially if a very large number of objects is involved.
Constructor
WeakSet()
-
Creates a new
WeakSet
object.
Instance properties
These properties are defined on WeakSet.prototype
and shared by all WeakSet
instances.
WeakSet.prototype.constructor
-
The constructor function that created the instance object. For
WeakSet
instances, the initial value is theWeakSet
constructor. WeakSet.prototype[@@toStringTag]
-
The initial value of the
@@toStringTag
property is the string"WeakSet"
. This property is used inObject.prototype.toString()
.
Instance methods
WeakSet.prototype.add()
-
Appends
value
to theWeakSet
object. WeakSet.prototype.delete()
-
Removes
value
from theWeakSet
.WeakSet.prototype.has(value)
will returnfalse
afterwards. WeakSet.prototype.has()
-
Returns a boolean asserting whether
value
is present in theWeakSet
object or not.
Examples
Using the WeakSet object
js
const ws = new WeakSet();
const foo = {};
const bar = {};
ws.add(foo);
ws.add(bar);
ws.has(foo); // true
ws.has(bar); // true
ws.delete(foo); // removes foo from the set
ws.has(foo); // false, foo has been removed
ws.has(bar); // true, bar is retained
Note that foo !== bar
. While they are similar objects, they are not the same object. And so they are both added to the set.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-weakset-objects |
Browser compatibility
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