Why returns Object.keys() an object (looks like array)?

Question

I have come across this:

var stdin = {123:1,423:1};

var stdout = Object.keys(stdin);

console.log(stdout);             //["123", "423"] 
console.log(typeof(stdout));     //object
console.log(stdout[0])           //123

ECMAScript® Language Specification says:

15.2.3.14 Object.keys ( O )

When the keys function is called with argument O, the following steps are taken:

... 6. Return array.

JSFIDDLE: http://jsfiddle.net/wpVvv/1/

Tested on Chrome and Firefox on Windows 7.

What is going on? Should be array, looks like array to me, is Object?

Edit:
typeof(). Arrgh.

Community · Accepted Answer · 2017-05-23 11:56:06Z

1

Arrays are objects.

Try typing typeof([1,2,3]) into your console - you'll also get object as the result.

Now, if you type Object.prototype.toString.call( [1,2,3] ), you'll get [object Array], which is somewhat useful (and you'll get the same result for the return value of Object.keys).

If you want a boolean result, just use Array.isArray (see this related answer)

edited May 23, 2017 at 11:56

CommunityBot

11 silver badge

answered Feb 8, 2014 at 6:22

Krease

16.3k8 gold badges58 silver badges92 bronze badges

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

1 Comment

loveNoHate Over a year ago

Yeah this is useful. Just wanted to have the output as array and run first time into it. ;)

Tyler McGinnis · Accepted Answer · 2014-02-08 06:20:41Z

1

Why are you saying it's an object? Running typeof on an Array will always return 'object'.

var arr = [1,2,3];
typeof arr --> "object"

Try instead

Array.isArray(arr) --> true

Here's the test you wanted.

var stdin = {123:1,423:1};
Array.isArray(stdin) --> false

answered Feb 8, 2014 at 6:20

Tyler McGinnis

35.3k16 gold badges74 silver badges79 bronze badges

2 Comments

loveNoHate Over a year ago

OOps, as easy as this? Everything is an object in Javascript, right, hmm? OK: ecma-international.org/ecma-262/5.1/#sec-11.4.3. Arrgh. ;) Think this question should run: How does typeof() work? Can I delete it please? ;)

Tyler McGinnis Over a year ago

@dollarvar Check out that last minute edit I made. Yeah, it's pretty basic.

Collectives™ on Stack Overflow

Why returns Object.keys() an object (looks like array)?

JSFIDDLE: http://jsfiddle.net/wpVvv/1/

2 Answers 2

1 Comment

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

JSFIDDLE: http://jsfiddle.net/wpVvv/1/

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related