Javascript: adding a key-value pair to array

Question

I noticed that I can add a key value pair to an array (not an object, an array).

var a = [];       // create the array
a[0] = "test";    // conventionally setting an index of 0 to a value
a["foo"] = "bar"; // this actually sets a "key" of the array to "bar"

If I try to get the value of a.foo or `a["foo"], I simply get "bar". No errors raised.

I know that a Javascript Array is actually an object, but with special rules, but it feels weird that this doesn't throw an error.

I'm using the latest version of Chrome.

Is there an actual use case where this is ok to do? What is common practice around this fact?

You can add props to any object, like array or even function. It's not a bug. — Egor Stambakio
– Egor Stambakio, Commented Dec 3, 2017 at 19:35

Rich Churcher · Accepted Answer · 2017-12-03 19:55:38Z

2

Now try this:

> Object.getOwnPropertyNames(a)
[ '0', 'length', 'foo' ]
> a.length
1

Welcome to JavaScript! It's such a wonderful place...

Yes, you can create named properties on an array. No, you probably shouldn't. They won't be counted in Array.length, and most developers who read your code will be experiencing a moment of confusion that is unlikely to lead to a positive outcome.

If you need a named property, use a (non-array) object.

answered Dec 3, 2017 at 19:55

Rich Churcher

7,6923 gold badges40 silver badges61 bronze badges

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

2 Comments

Sventies Over a year ago

Thanks, I like the example of length, makes it clear that it is probably not a good idea. However, I'd think it's ok to use in cases where you know what you're doing, and other options aren't optimal either?

Rich Churcher Over a year ago

I would think those occasions would be very rare, but sure. I guess you have to ask yourself, "will I still understand this code when I come back to it six months later?" :)

Collectives™ on Stack Overflow

Javascript: adding a key-value pair to array

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related