Can we use object as a key of an object in Javascript?

Question

I was given a short javascript code in an interview to give the output of that but I was not sure what will be the output of that. He creates a var object and assigns objects as indexes of that object using the array data structure. I did console that code after the interview but still do not understand how it is showing the output.

Here is the code

var a = {};
b = { key: 'b'};
c = { key: 'c'};
a[b] = 123;
a[c] = 456;

console.log(a[b]); //what is the output of this console statement and explain why

Can anyone explain with javascript logic behind the output it shows? Thanks in advance!

There's no array in your example. a, b and c are all objects — Andreas
– Andreas, Commented Aug 23, 2021 at 10:51
Does this answer your question? Dynamically access object property using variable — David Arias
– David Arias, Commented Aug 23, 2021 at 10:52
yes but when assigning it with number there is array data structure is used — Suleman Ahmad
– Suleman Ahmad, Commented Aug 23, 2021 at 10:52
You can access the properties of an object with the dot notation (b.key) or with the bracket notation (b["key"]). The bracket notation has to be used when the property name is not a valid identifier (e.g. when there's a space in the name) -> Property accessors - JavaScript | MDN — Andreas
– Andreas, Commented Aug 23, 2021 at 10:54

mplungjan · Accepted Answer · 2021-08-23 10:55:56Z

4

The object used as key's toString is [object Object] and that is what is used each time

var a = {};
b = { key: 'b'};
c = { key: 'c'};
a[b] = 123; // sets a["[object Object]"]
console.log(b,a[b])
a[c] = 456; // ALSO sets a["[object Object]"]
console.log(b,a[b])
console.log(Object.keys(a))
console.log(a[b]);

console.log(a["[object Object]"]);

answered Aug 23, 2021 at 10:55

mplungjan

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Nina Scholz · Accepted Answer · 2021-08-23 10:55:00Z

1

You could have a look to the object a, where you see the stringed object (with toString())

[object Object]

as accessor.

Objects can have only strings as properties and with the latest Symbol, it could be this one, too.

var a = {};
b = { key: 'b'};
c = { key: 'c'};
a[b] = 123;
a[c] = 456;

console.log(a[b]); //what is the output of this console statement and explain why
console.log(a);

answered Aug 23, 2021 at 10:55

Nina Scholz

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