Filter or map nodelists in ES6

What is the most efficient way to filter or map a nodelist in ES6?

Based on my readings, I would use one of the following options:

[...nodelist].filter

or

Array.from(nodelist).filter

Which one would you recommend? And are there better ways, for example without involving arrays?

• [...nodelist] will make an array of out of an object if the object is iterable.
• Array.from(nodelist) will make an array out of an object if the object is iterable or if the object is array-like (has .length and numeric props)

Your two examples will be identical if NodeList.prototype[Symbol.iterator] exists, because both cases cover iterables. If your environment has not been configured such that NodeList is iterable however, your first example will fail, and the second will succeed. Babel currently does not handle this case properly.

So if your NodeList is iterable, it is really up to you which you use. I would likely choose on a case-by-case basis. One benefit of Array.from is that it takes a second argument of a mapping function, whereas the first [...iterable].map(item => item) would have to create a temporary array, Array.from(iterable, item => item) would not. If you are not mapping the list however, it does not matter.

TL;DR;

Array.prototype.slice.call(nodelist).filter

The slice() method returns an array. That returned array is a shallow copy of collection (NodeList) So it works faster than Array.from() So it works as fast as Array.from()

Elements of the original collection are copied into the returned array as follows:

• For object references (and not the actual object), slice copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.
• For strings, numbers and booleans (not String, Number and Boolean objects), slice copies the values into the new array. Changes to the string, number or boolean in one array do not affect the other array.

Short explanation regarding arguments

Array.prototype.slice(beginIndex, endIndex)

• takes optional args beginIndex and endIndex. If they are not provided slices uses beginIndex == 0, thus it extracts all the items from the collection

Array.prototype.slice.call(namespace, beginIndex, endIndex)

• takes an object as the first argument. If we use a collection as an object it literally means that we call slice method directly from that object namespace.slice()

I found a reference that uses map directly on the NodeList by

Array.prototype.map.call(nodelist, fn)

I haven't tested it, but it seems plausible that this would be faster because it should access the NodeList directly.

How about this:

// Be evil. Extend the prototype.
if (window.NodeList && !NodeList.prototype.filter) {
  NodeList.prototype.filter = Array.prototype.filter;
}

// Use it like you'd expect:
const noClasses = document
  .querySelectorAll('div')
  .filter(div => div.classList.length === 0)

It's the same approach as mentioned in the MDN docs for NodeList.forEach (under 'Polyfill'), it works for IE11, Edge, Chrome and FF.

Filter or map nodelists in ES6

I came out of this simple function. @see https://developer.mozilla.org/fr/docs/Web/API/NodeList/entries#exemple

function filterNodeList(NodeList, callback) {
if (!typeof callback === "function") callback = (i) => i; // Any have better idear?

const Result = document.createElement("div");
//# No need to filter empty NodeList
if (Node.length === 0) return Node;

for (let i = 0; i < Node.length; i++) {
  if (callback(Node.item(i))) Result.appendChild(Node.item(i));
}

return Result.childNodes;}

Im willing to learn more :>

[...a].filter vs Array.from(a).filter

Not a "real" difference in performance, Array.from could be a very very tiny bit faster because you're not creating a new Array on the "JS level", but it happens directly in the native code.

Performance - consider not using either

However for performance (and to also avoid "Array-ing") you should consider why are you filtering a NodeList and where/how did you get it. In many cases, you just want a particular element either by id or by class or other CSS selector.

document.querySelectorAll is like 10x - 200x faster and works for any CSS selector
document.getElementById is even faster (but of course requires an id)

You can even optimize querySelectorAll or bypass "not-yet-known" case if you provide a pre-stored parent to look in, let me give you an example:

let mainbar = document.getElementById('mainbar');
mainbar.querySelectorAll('.flex--item');

is almost 10x faster than

Array.from(a).filter(el => el.classList.contains("flex--item"))

Also be aware that document.querySelectorAll('#mainbar .flex--item'); is still about 5x faster than Array filtering, but about 2x slower than pre-storing the parent with an id.

Besides better performance, you will also always get NodeList (it may be empty, but it still will be NodeList) and that goes for both document.querySelectorAll() and Element.querySelectorAll()

Using ECMAS 2016:

let nodes = [...document.querySelector('__SELECTOR__').childNodes].filter(item => item.nodeType === 1);