how javascript add property to an object?

Question

I was reading javascript questions where i found this piece of code

var a={},
    b={key:'b'},
    c={key:'c'};

a[b] = 123;
a[c] = 456;

console.log(a[b]);      // o/p - 456

can anybody make me understand this code why and how it is printing 456?

And I think we can use dot i.e a.b = 123 and string a['b'] = 123 approach to add property to an object.

b is an object, when used as key converted to string as [object Object], c is again an object which will be converted to string when used as key. This will overwrite the previous value, thus 456. — Tushar
– Tushar, Commented Sep 20, 2016 at 13:23
When using dot notation and bracket notation with string, you're actually adding b property on the object. — Tushar
– Tushar, Commented Sep 20, 2016 at 13:24

MinusFour · Accepted Answer · 2016-09-20 13:32:59Z

3

Both b and c resolve to the same string ([object Object]). Hence you are overwriting the same key.

And I think we can use dot i.e a.b = 123 and string a['b'] = 123 approach to add property to an object.

Yes you can, but a['b'] is very different from a[b]. The first one resolves to a key with a string value just as it shows ('b'), where as the other one will depend on the stringified value of the variable b (in this case is [object Object]).

edited Sep 20, 2016 at 13:32

answered Sep 20, 2016 at 13:23

MinusFour

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Comments

Nina Scholz · Accepted Answer · 2016-09-20 13:29:00Z

0

For really using the content of the object, you may use the stringified version.

var a = {},
    b = { key: 'b' },
    c = { key: 'c' };

a[JSON.stringify(b)] = 123;
a[JSON.stringify(c)] = 456;

console.log(a[JSON.stringify(b)]);

edited Sep 20, 2016 at 13:29

answered Sep 20, 2016 at 13:27

Nina Scholz

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