Javascript Objects: Using Arguments as object property

Question

I was reimplementing a memoize function using the Arguments object. I am wondering why this would work. For example, passing in 2 arguments:

var args = Array.prototype.slice.call(arguments); // input: 1,2

Then I would store the args array into an object. For Example,

var obj = {};
    obj[args] = 2;

If you were to call Object, you would see this as:

{ 1,2: 2 }/*shows as this*/  { [1,2]: 2 }/*but not as this*/

Not that I wanted the second object, just curious what is happening under the hood. Is this what you call Javascript coercion?

All property names are strings, so args is getting converted to a string, yes. — Ry-
– Ry- ♦, Commented Jan 5, 2016 at 21:54
Using an array as an object property name won't really work very well. The array will be converted to a string first, and that result will be the actual name of the property. — Pointy
– Pointy, Commented Jan 5, 2016 at 21:54
Converting an array to a string works by doing array.join(",") — Barmar
– Barmar, Commented Jan 5, 2016 at 21:55
Ah, I see! Thanks you very much for the information. I wasn't sure what was happening, learning something new everyday! — DSK
– DSK, Commented Jan 5, 2016 at 21:57

Barmar · Accepted Answer · 2016-01-05 21:58:42Z

1

Object property names are always strings. So when you do:

obj[args] = 2;

it treats this as

obj[args.toString()] = 2;

And the toString() method of arrays is equivalent to .join(","), so the above is equivalent to:

obj[args.join(",")] = 2;

which matches the result you saw.

answered Jan 5, 2016 at 21:58

Barmar

789k57 gold badges554 silver badges669 bronze badges

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

1 Comment

Thank you so much, it makes a lot of sense! I was scratching my head for awhile lol.