How to check if an argument is an object (and not an array) in JavaScript

Question

After testing out instasnceof I found that it will return true if the argument is an array or an object literal.

function test(options){
  if(options instanceof Object){alert('yes')}//this will alert for both arrays and object literals
}
test({x:11})// alerts
test([11])// alerts as well but I do not want it to

Is there a way to test if the argument "options" is an object literal?

P.S. I am creating a module that will allow the user to access its configuration options, and I want to test if the argument is only an object literal or not?

"Object literal" is just a syntax for creating objects. I assume you're trying to specifically distinguish between an Array object and a non-Array object. I'm certain this has been covered on SO. — user1106925
– user1106925, Commented May 24, 2012 at 16:33
Distinguish between array and hash in javascript with typeof() — user1106925
– user1106925, Commented May 24, 2012 at 16:36

T.J. Crowder · Accepted Answer · 2014-12-12 19:16:18Z

12

is there a way to test if the argument "options" is an object literal?

No, because it makes no sense. You can test whether it's an object, but how it was created (via a literal in the call to your function, via a literal elsewhere, through new Object, by deserializing a JSON string, ...) is not information that's maintained.

after testing out instasnceof i found that it will return true if the argument is an array or an object literal

Correct. Arrays in JavaScript are objects (and not really arrays).

If you want to test that an object is a plain old object, you can do this:

if (Object.prototype.toString.call(options) === "[object Object]") {
    // It's a plain object
}

But there's really no reason to do that. It's not your problem. As long as what they pass you has the properties you expect, don't try to limit the object further.

p.s. i'm making a module that will allow the user to pass it configuration options and i want to test to make sure that the argument is only an object literal.

Why? If the user wants to use an object that hasn't been declared as a literal right there and then, why would you care? If they want to use an object that they've created via a different constructor function (e.g., rather than just a plain object), again, why would you care?

edited Dec 12, 2014 at 19:16

answered May 24, 2012 at 16:35

T.J. Crowder

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6 Comments

zero Over a year ago

so then i should be checking for properties and if they have values or not instead of what the argument is?

T.J. Crowder Over a year ago

@codewombat: Yes, that's it exactly. Just test for the things you want to see. (This is sometimes called "duck typing" -- e.g., does it walk like a duck, and quack like a duck? Then it's a duck.)

Bergi Over a year ago

I wouldn't say that arrays in JavaScript are not Arrays... But they are Objects, too.

T.J. Crowder Over a year ago

@Bergi: I didn't say they weren't Arrays, I said they weren't arrays (lower case). See the link for an explanation of why I say that.

zero Over a year ago

excellent answer. i'm trying to do more advance things with javascript before delving into another language i want to make sure my "P's and Q's" (i.e. best/correct practices) are straight before proceeding and this diffidently helped with that.

|

Bergi · Accepted Answer · 2012-05-24 16:35:24Z

2

function isPlainObject(o) {
     return Object(o) === o && Object.getPrototypeOf(o) === Object.prototype;
}

However, you can't test wether o was declared as a literal or instantiated somehow else - you can just test whether it's a plain object without any constructor than Object.

answered May 24, 2012 at 16:35

Bergi

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10 Comments

Phrogz Over a year ago

Not that I personally care, but this does not work when testing a value passed across frames/windows.

Bergi Over a year ago

Yes, that's true. However you can't test such an object anyway, can you?

Phrogz Over a year ago

Yes, you can but it's reasonably horrific: function isArray(o) { return Object.prototype.toString.call(o) === '[object Array]'; }

Bergi Over a year ago

I know that one, but how would you code isObject? Oh, wait, Object.getPrototypeOf(Object.getPrototypeOf(o))=== null should work - much more horrible than the toString-trick :-)

Phrogz Over a year ago

No, just replace Array in the string-based version with Object.

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Paul Phillips · Accepted Answer · 2012-05-24 16:34:54Z

1

If you're trying to forbid arrays, you can just do this:

var isObject = options instanceof Object;
var isArray = options instanceof Array;
if(isObject && !isArray)
{
    alert('yes');
}

answered May 24, 2012 at 16:34

Paul Phillips

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1 Comment

Bergi Over a year ago

check out Array.isArray() and its common shims.

James Porter · Accepted Answer · 2022-12-13 14:33:08Z

0

An alternative solution would be to use Lodash:

_.isPlainObject(value)

Here is the documentation: https://lodash.com/docs/4.17.15#isPlainObject

answered Dec 13, 2022 at 14:33

James Porter

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