What is the "ownerID" in Immutable.js?

Question

I'm going through Immutable.js's source code and there's an ownerID field that I don't understand.

Here's the source for Map.asMutable() and Map.asImmutable(): https://github.com/facebook/immutable-js/blob/master/src/Map.js#L171

It seems like the only difference between a mutable and an immutable object are their ownerIDs. What is an ownerID and what is it used for?

Answer 1

If you track back the property:

L#14:

import { DELETE, SHIFT, SIZE, MASK, NOT_SET, CHANGE_LENGTH, DID_ALTER, OwnerID,
          MakeRef, SetRef, arrCopy } from './TrieUtils'

in src/TrieUtils.js :

L#36:

// A function which returns a value representing an "owner" for transient writes
// to tries. The return value will only ever equal itself, and will not equal
// the return of any subsequent call of this function.
export function OwnerID() {}

It is some property they create like hash to represent a virtual owner.

Answer 2

It is used to ensure mutability in asMutable returned instances. When asMutable is invoked, it ensures an __ownerId and returns the current instance back -

asMutable() {
    return this.__ownerID ? this : this.__ensureOwner(new OwnerID());
}

Then any supported mutating operations return the current instance back, instead of creating a new instance with the changes (which is key for immutability).

E.g., here's how the "clear" method operates based on the presence of __ownerId -

clear() {
    if (this.size === 0) {
      return this;
    }
    if (this.__ownerID) {
      this.size = 0;
      this._root = null;
      this.__hash = undefined;
      this.__altered = true;
      return this;
    }
    return emptyMap();
}

Notice that when this.__ownerID is present, the method returns the current instance (thereby mutating itself). But when it is absent, it returns a new map for ensuring immutability.