Why is object[key] not equal to key, if key is an object, in JavaScript?

Question

var a = new Object; var b = new Object; var c = new Object;  c[a] = a; c[b] = b;  console.log(c[a] === a); 

I tested the code above and get false. If I try console.log(c[a] === b), then true is printed.

Why?

Answer 1

The problem here has to do with how an Object's keys are set. From MDN:

Parameters

nameValuePair1, nameValuePair2, ... nameValuePairN

• Pairs of names (strings) and values (any value) where the name is separated from the value by a colon.

value

• Any value.

An object's values can be accessed (via the appropriate key) in three ways:

var o = {}; var key = "fun";  // method 1: o[key]    = "the key will be equal to `key.toString()" // method 2: o.key     = "the key will be equal to 'key'" // method 3: o["key2"] = "the key will be equal to `key2`" /* {     "fun" : "the key will be...",    // method 1     "key" : "the key will be...",    // method 2     "key2": "the key will be..."     // method 3 } */ 

When using bracket notation, you need to mind the gap...between the brackets! Objects set their keys and values using the toString method, unless they're passed a string (then there's no point in toString). When using the dot notation, they use .key as the key.

Let's look at your case:

var a = {}   , b = {}   , c = {}   ;  c[a] = a; // `a` is not a string, and we're using brackets, so the key // will be equal to `key.toString()`: // a.toString() === "[object Object]" // Try the following in your console: `{}.toString()` // Note how this is different from console.log({}), since // the console exposes your object (that's why the dev console is useful) // c is now: `{ "[object Object]" : a }`  c[b] = b; // b is also an object, so `b.toString()` is the same as `a.toString()` // that means c is now `{ "[object Object]" : b }`  assert c[a] === a // a.toString() == b.toString() == "[object Object]" // and we just noted that c was `{ "[object Object]" : b }` // so of course this is false assert c[b] === b // true because c[b] == b; assert c["[object Object]"] === b; // also true assert c.b === b // false, since `c` has no "b" key (c.b is `undefined`)