How to determine if parameter is an Array or an Object in JavaScript?

Question

I have a prototype like this;

LocalDataEngine.prototype.ExecuteNonQuery = function (query) { }

And I call this prototype within two different argument like below;

By using object array;

var createQueries = new Array();
createQueries.push(new SQLiteQuery(""));
createQueries.push(new SQLiteQuery(""));
createQueries.push(new SQLiteQuery(""));

new LocalDataEngine().ExecuteNonQuery(createQueries);

By using only object;

new LocalDataEngine().ExecuteNonQuery(new SQLiteQuery(""));

My question is, how can I determine query argument in prototype is object array or object?

http://stackoverflow.com/questions/4775722/javascript-check-if-object-is-array — georg, May 27 '13 at 11:05
+ this feels like bad design to me. Just make two functions `executeOneQuery` and `executeManyQueries`. — georg, May 27 '13 at 11:09
Regarding the design, why don't you make `ExecuteNonQuery` variadic, i.e. accept multiple arguments where each one is a query. Then always iterate over all arguments. — Felix Kling, May 27 '13 at 11:36

Janus Troelsen · Accepted Answer · 2013-05-27T11:12:37.103

4

You can use instanceof:

% js
> [] instanceof Array
true
> {} instanceof Array
false

It will work flawlessly if you are not using frames (which is probably a bad idea anyway). If you are using frames and ECMAScript 5, use Array.isArray:

> Array.isArray({})
false
> Array.isArray([])
true

See the duplicate question linked by thg435 for additional solutions.

edited May 27 '13 at 11:12

answered May 27 '13 at 11:02

Janus Troelsen

17,537
13
121
177

I couldn't understand your syntax – Mehmet Ince May 27 '13 at 11:08
This can fail in multi-frame DOM environments. It’s more safe to use `Object.prototype.toString`, see here: http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/ – David Hellsing May 27 '13 at 11:09
@MehmetInce: See [this](http://stackoverflow.com/q/1094723/309483) for an explanation of the literal array syntax. As you can see, there are reasons to prefer it. – Janus Troelsen May 27 '13 at 11:10
frames = bad. iFrames = still very common (f.ex video embeds). – David Hellsing May 27 '13 at 12:38

score 2 · Answer 2 · answered May 27 '13 at 11:03

2

Like this:

if (query instanceof Array) {
    return 'array';
} else if (query instanceof Object) {
    return 'object';
} else {
    return 'scalar';
}

answered May 27 '13 at 11:03

Steven Moseley

13,831
4
32
42

score 2 · Answer 3 · answered May 27 '13 at 11:03

2

if( Object.prototype.toString.call( yourObj) === '[object Array]' ) {
    alert( 'Array!' );
}

answered May 27 '13 at 11:03

Pranav

7,315
4
23
40

+1. This is also what jQuery uses under the hood for `$.isArray()`. – David Hellsing May 27 '13 at 11:11

How to determine if parameter is an Array or an Object in JavaScript?

3 Answers3

Linked