New semantics to the `of ` operator

💡 Ideas
1

It would be nice if we could see whether an element is present in an array using the of operator.
like: 3 of [5, 4, 3, 2, 1] // true
there's of course the Array.prototype.includes() method but the new semantics to of seems to be more intuitive.

1 Like
2

This doesn't appear very intuitive to me. I'd expect 3 of x to be equivalent to x.slice(0, 3) or x[Symbol.iterator]().take(3).
I could understand using 3 in x like Python does it, unfortunately that already has (different) semantics in JS. After all, x.includes(3) is very unambiguous.

3

I'd rather see someone add .includes to https://github.com/tc39/proposal-iterator-helpers than this syntax.

Though now that I think about it, a in b + for (let a in b) does provide some very strong precedent for this - a in b is basically GetEnumerableProperties(b).includes(a) (except it bails out early), and so desugaring like this might actually be a good idea:

// Source
const result = a of b

// Implementation
function _binaryOf(a, b) {
  for (const item of b) if (SameValueZero(item, a)) return true
  return false
}
const result = _binaryOf(a, b)

It's an open question how to handle async iterables syntactically, though.