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

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.

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;

@SomeKittens was right, but that's a week ago. I patched node.js core and the docs. Now they're both the same thing, or will be in due time.

> Array.isArray === util.isArray;true