Can you use objects as Object keys in JavaScript?#

The short answer is "no". All JavaScript object keys are strings. Even if you pass an object as a key, the object's toString() will be called on it, and the key will be stringified to [object Object].

If the above explanation did not make things obvious for you, or you in the mood for more probing, read on. Also, there is a way to actually use objects as key to values, read till the end.

You might be wondering, "Hey I have used objects as keys before!". Well, it sure might have looked like you used objects as keys, but it was not what you thought it was.

Run the following code:

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You get the expected result. But not obvious to you, something is happening behind the scenes.

When you do o[x] = 'Chuck Norris', the object x is getting stringified to [object Object]. And when you do console.log(o[x]), x is again getting stringified to [object Object]. But since you getting expected result for console.log(o[x]), you start to think you can actually use objects as keys.

Now get ready for the eye-opener. Run the following code:

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There you are! You thought you were setting an object as key when actually you were setting values for the string key [object Object]. See what happens when you try to use many objects as keys.

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The object keys, after getting stringified, overwrite each other. o[x] and o[y] both are references to the string [object Object]. o.x is a true references to key named x of the object o, similarly o.y.

How to use object as key to a value#

There is a way to use objects and keys to values, just not values of a conventional Object. What you need is a WeakMap.

A WeakMap is a key-value object, where the key must, in fact, be an object - just what you needed.

Here is an example of using objects as keys with a WeakMap.

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References#

  1. MDN - WeakMap
  2. MDN - Object