Filter or map nodelists in ES6

问题

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

Array.from(nodelist).filter

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

回答1:

[...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.

回答2:

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()**

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()