#Iterator unique S1

let uniques = new Set(iter).values();

let objSeen = new WeakSet,
  primSeen = new Set,
  seenNegZero = false;

let uniques = iter.filter(e => {
  if (e === 0 && 1/e < 0) {
    if (seenNegZero) return false;
    seenNegZero = true;
    return true;
  }
  let seen = Object(e) === e ? objSeen : primSeen;
  if (seen.has(e)) return false;
  seen.add(e);
  return true;
});

let uniques = iter
  .map(e => [e, f(e)])
  .filter([, criteria] => { ... })
  .map(([e]) => e);

let uniques = iter.uniqBy();
let uniques = iter.uniqBy(obj => obj.field);