Want an array’s values rather than its indices? `for..in` visits the indices; `for..of` visits values. An iterable is an object that can supply values one at a time through JavaScript’s iterable protocol. Arrays, strings, sets, and maps are built-in examples. Spread syntax can collect an iterable’s values into an array. section 23.0 section 23.1
The key is a `[Symbol.iterator]` method, also called `@@iterator`. It returns a new iterator whose `next()` method supplies a `{value, done}` result. Each call advances the sequence; `done: true` says there are no more values. `for..of` and spread keep requesting values until then. An infinite iterable never reaches that point, so spreading one will not finish. section 23.4 section 23.5
function* primesIterator () {
yield 2
yield 3
yield 5
}
const primesIterable = {
[Symbol.iterator]: primesIterator
}
const myPrimes = [ ...primesIterable ] 1
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A generator makes a short custom iterable: each `yield` supplies the next value, and `[...primesIterable]` produces `[2, 3, 5]`. It avoids writing `next()` and its result objects by hand. section 23.4
Try a bounded read
Write `take(n, iterable)` to collect at most `n` values. Test it with an infinite iterable and request ten values. Stop after ten requests—or sooner if `next()` reports `done: true`—rather than spreading the entire iterable. section 23.5
