Checking if an object is an array

Array.isArray(obj) returns true if the object is an Array, otherwise false.

Array.isArray([])           // true
Array.isArray([1, 2, 3])    // true
Array.isArray({})           // false
Array.isArray(1)            // false

In most cases you can instanceof to check if an object is an Array.

[] instanceof Array; // true
{} instanceof Array; // false

Array.isArray has the an advantage over using a instanceof check in that it will still return true even if the prototype of the array has been changed and will return false if a non-arrays prototype was changed to the Array prototype.

var arr = [];
Object.setPrototypeOf(arr, null);
Array.isArray(arr);   // true
arr instanceof Array; // false