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Get object reference IDs in JavaScript/TypeScript

September 28, 2022

Sometimes you want to get an object reference in JavaScript. While other languages like Java allow for the object reference to be printed, in JS this is not possible.
I want to show a quick & easy way how to get an id per object.

Disclaimer: It’s not possible to get the internal used object references, but this method rather assigns IDs to objects and makes sure for the same object you will always get the same ID.

You can find a StackBlitz example here

TypeScript

let count: number = 1;
const idMap: WeakMap<Record<string, unknown> | Array<unknown>, number> = new WeakMap<Record<string, unknown> | Array<unknown>, number>();
function getObjectId(object: Record<string, unknown> | Array<unknown>): number {
  const objectId: number | undefined = idMap.get(object);
  if (objectId === undefined) {
    count += 1;
    idMap.set(object, count);

    return count;
  }

  return objectId;
}

JavaScript

let count = 1;
const idMap = new WeakMap();
function getObjectId(object) {
  const objectId = idMap.get(object);
  if (objectId === undefined) {
    count += 1;
    idMap.set(object, count);

    return count;
  }

  return objectId;
}

How does it work

A WeakMap can only have objects as keys (regular Maps can have any data type). But the interesting part here actually is that WeakMaps do not create strong references to the objects they are using. This means you don’t need to worry about garbage collection / memory leaks. Because once an object does no longer exist, it will automatically be removed from the WeakMap as well. For more information see MDN docs
As long as the object exists the WeakMap will keep the reference and the linked counter ID to it. And creating a similar object will still result in a different ID because they have a different reference.

Alternative IDs

This version is using a number to keep it simple. There are two other data types I used as well in the past.
uuid are by design also always unique, so also a great fit here (npm package)

If you want to always get the same ID for the same object (with a counter) you can do a combination of JSON.toString() and a basic string hash algorithm - result could then look something like 1-fbced512asd. And if you try to get it with the same object 2-fbced512asd etc. (Keep in mind this requires some additional effort like proper & fast hashing algorithm and sorting of the object properties)


Personal Blog written by Nicolas Gehlert, software developer from Freiburg im Breisgau. Developer & Papa. Github | Twitter

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