Author: Nicholas Yang
Stage: -1
Right now, iterating through a range of numbers requires a traditional for (let i = 0; i < n; i ++) loop. This is both verbose and not interchangable with other iterators.
The only way to functionally iterate over a range is to use an array, for instance Array(n).fill().map((_, i) => console.log(i)), but that's rather inefficient.
Furthermore, with the iterator helpers proposal, we can use common methods such as map/filter/reduce on Number.range. This will allow users to write expressions that iterate over a range, which will be very useful in expression based cases such as JSX.
Higher Order Iterators
Let's say we want to write a higher order iterator (or HOI) that takes in an unspecified iterator, does some transformation and returns a new iterator. The classic example is a map function. Our map function will take a function and an iterator, then return an iterator that applies the function to each value.
function* map(f, iter) {
for (const val of iter) {
yield f(val)
}
}
map can take any generic iterator, allowing it to work over arrays, streams, etc. However it cannot work over a range of numbers unless we pass an array filled with our range, or write our own range implementation. Note that a lot of these higher order iterators will be implemented in the iterator helpers proposal.
Lazy Streams
Number ranges also allows for lazy streams of numbers, similar to those of Haskell. For instance, Number.range(0, Infinity) is completely valid, as the iterator is lazy itself.
const infinitePosts = map(() => <Post />, Number.range(0, Infinity))
Creating an Array
Using array spread, it's quite easy to create an array with a range of numbers:
const evenNumbers = [...Number.range(0, 100, 2)]
Number.range(start, end, step) will produce an iterator that iterates from start to end by adding step for each iteration. It will terminate when the index is greater than or equal to end.
Q: What about Number.range(-Infinity, Infinity)/Number.range(NaN, undefined)/insert-edgecase-here?
A: The guiding principle of Number.range(start, end) is that it is functionally equivalent to a for (let i = start; i < end; i++). While this can lead to some unusual behavior, people already use for (let i = start; i < end; i++) loops regularly in codebases and are familiar with their pitfalls.