Using an object as a property key in JavaScript

Question

What is going on in this code?

var a = {a:1};
var b = {b:2};
var c = {};

c[a] = 1;
c[b] === 1 // true!

c[b] = 2;
c[a] === 2 // true!

Specifically, why does using looking up b in c return the value that was stored in a property of a?

What does it mean to use an object as a key to a property in JavaScript?

I've tested this in Chrome/Node and in Firefox.

currently javascript does not have support for object as a key it probably get coherse (converted to) string which will be [object Object] and store the value hence it gives true to every object as a key, but in future 'Map' is something you can use to store object as a key. — Sushil
– Sushil, Commented Sep 12, 2012 at 10:49
Sounds interesting @Sushil. Do you have a reference to information about this potential future Map class? I couldn't find it mentioned anywhere after a few minutes of searching. — Drew Noakes
– Drew Noakes, Commented Sep 12, 2012 at 11:05
its a future implementation for ECMAScript 6 developer.mozilla.org/en-US/docs/JavaScript/Reference/… here u go and additionally there is a very nice WeakMap concept (i personally like it) developer.mozilla.org/en-US/docs/JavaScript/Reference/… here u can read about it — Sushil
– Sushil, Commented Sep 12, 2012 at 11:09
Specification: Property Accessors → EvaluatePropertyAccessWithExpressionKey → ToPropertyKey. A property key is obtained by either keeping the symbol value or coercing the value to a string. — Sebastian Simon
– Sebastian Simon, Commented Jun 8, 2023 at 1:52

Linus Thiel · Accepted Answer · 2012-09-17 10:22:25Z

13

What does it mean to use an object as a key to a property in JavaScript?

Javascript objects only allow string keys, so your object will first be coerced to a string.

Specifically, why does using looking up b in c return the value that was stored in a property of a?

The string representation of {a: 1} and {b: 2} are both "[object Object]", thus, the property is overwritten.

Edit: If you really need to use objects as keys (I would prefer another solution, if possible), you could use the object's JSON representation:

c[JSON.stringify(a)] = 1
c[JSON.stringify(b)] = 2

But, again, try to think of a different approach. Perhaps the objects have unique identifiers other than the object itself.

edited Sep 17, 2012 at 10:22

answered Sep 12, 2012 at 10:49

Linus Thiel

39.3k9 gold badges112 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Drew Noakes Over a year ago

Cheers Linus, this makes good sense. I do need to use objects as keys in a sense, so an actual Map class would be useful. However I won't have very many in my set, so I think I'll just create an array of key/value pairs, and on 'get' loop through them, testing keys with ===. It'll be O(N), but my N will be around 10 so I'm not bothered. Thanks again.

xdazz · Accepted Answer · 2012-09-12 10:50:49Z

2

Why you use an object as a key, the key is becoming the object.toString() 's result which is [Object Object],

So what you are dothing is set a value to the property "[Object Object]", and get the value by the property "[Object Object]".

answered Sep 12, 2012 at 10:50

xdazz

161k38 gold badges255 silver badges278 bronze badges

Collectives™ on Stack Overflow

Using an object as a property key in JavaScript

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related