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?

+ this feels like bad design to me. Just make two functions executeOneQuery and executeManyQueries. — georg
– georg, Commented May 27, 2013 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
– Felix Kling, Commented May 27, 2013 at 11:36

Janus Troelsen · Accepted Answer · 2013-05-27 11:12:37Z

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, 2013 at 11:12

answered May 27, 2013 at 11:02

Janus Troelsen

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4 Comments

Mehmet Ince Over a year ago

I couldn't understand your syntax

David Hellsing Over a year ago

This can fail in multi-frame DOM environments. It’s more safe to use Object.prototype.toString, see here: perfectionkills.com/…

Janus Troelsen Over a year ago

@MehmetInce: See this for an explanation of the literal array syntax. As you can see, there are reasons to prefer it.

David Hellsing Over a year ago

frames = bad. iFrames = still very common (f.ex video embeds).

Steven Moseley · Accepted Answer · 2013-05-27 11:03:28Z

2

Like this:

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

answered May 27, 2013 at 11:03

Steven Moseley

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Comments

Pranav · Accepted Answer · 2013-05-27 11:03:40Z

2

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

answered May 27, 2013 at 11:03

Pranav

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1 Comment

David Hellsing Over a year ago

+1. This is also what jQuery uses under the hood for $.isArray().

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