In node.js, why is there a util.isArray and an Array.isArray?

Question

I just noticed the API docs for Node.js v0.10.26 provide for util.isArray,

util.isArray(object)# Returns true if the given "object" is an Array.

false otherwise.

var util = require('util'); util.isArray([])   // true util.isArray(new Array)   // true util.isArray({})   // false 

But, how is that different ecmascripts normal, Array.isArray?

> Array.isArray([]); true > Array.isArray(new Array); true > Array.isArray({}); false 

Answer 1

To actually answer why util.isArray exists, we need a bit of a history lesson.

When it was first added to node, it did a bit more than call Array.isArray.

function isArray (ar) {   return ar instanceof Array       || Array.isArray(ar)       || (ar && ar !== Object.prototype && isArray(ar.__proto__)); } 

This was a local function in utils and actually wasn't exported until v0.6.0.

In this form, util.isArray handled a case that Array.isArray doesn't:

> x = [1,2,3] [ 1, 2, 3 ] > y = Object.create(x) [ , ,  ] > Array.isArray(y) false > Array.isArray(Object.getPrototypeOf(y)) true 

There's some discussion here about this behavior of util.isArray, and consensus was that this behavior is actually bad because y is not really an Array.

Thus, the prototype-checking functionality was soon removed and replaced with a check that used both Array.isArray and a check of the argument's [[Class]].

function isArray(ar) {   return Array.isArray(ar) ||          (typeof ar === 'object' && objectToString(ar) === '[object Array]'); } 

However, checking the [[Class]] is actually duplicate effort because Array.isArray also checks the [[Class]], so it too was eventually removed – leaving only a call to Array.isArray.

Today, util.isArray is just an alias of Array.isArray.

So in other words, the existence of util.isArray is mostly a legacy thing and can be safely ignored.

Answer 2

While the Node.js source uses them both, util.isArray uses Array.isArray internally (source)

In util.js:

function isArray(ar) {   return Array.isArray(ar); } exports.isArray = isArray;