JavaScript: detect if argument is array instead of object (Node.JS)


How should I detect if the argument is an array because typeof [] returns 'object' and I want to distinguish between arrays and objects.

It is possible that object will look like {"0":"string","1":"string","length":"2"} but I don't want it to come out as an array if it is in fact an object looking like an array.

JSON.parse and JSON.stringify are able to make this distinction. How can I do it?

I am using Node.JS which is based on V8 the same as Chrome.

5/9/2011 7:46:04 PM

Accepted Answer

  • Array.isArray

native V8 function. It's fast, it's always correct. This is part of ES5.

  • arr instanceof Array

Checks whether the object was made with the array constructor.

A method from underscore. Here is a snippet taken from the their source

var toString = Object.prototype.toString,
    nativeIsArray = Array.isArray;
_.isArray = nativeIsArray || function(obj) {
    return === '[object Array]';

This method takes an object and calls the Object.prototype.toString method on it. This will always return [object Array] for arrays.

In my personal experience I find asking the toString method is the most effective but it's not as short or readable as instanceof Array nor is it as fast as Array.isArray but that's ES5 code and I tend to avoid using it for portability.

I would personally recommend you try using underscore, which is a library with common utility methods in it. It has a lot of useful functions that DRY up your code.

11/10/2015 3:50:44 PM

Try this code:


Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow