Question

I have often used this code to determine if an array is really an array…

Object.prototype.toString.call(array) == '[object Array]'

I recently started changing my callback tests from typeof callback == 'function' to callback instanceof Function, because I read that Safari and Chrome will tell you a regex literal is a function using the former (and it did when I tested it).

Now, I decided to check this code to see if I can replace the verbose code above…

array instanceof Array

It worked.

Comparison of both on jsFiddle.

So, is there any issues with the latter method? I assumed there may be because the first code example came up much more often in Google.

Answer 1

The two problems with using instanceof for an array are:

• Returns false when the array is from another window or frame
• Breaks if the Array constructor has been overwritten

The following is an article by kangax where I first saw your first technique (also used in jQuery): http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/