JS: Iterator.prototype.flatMap

(new in ECMAScript 2025)

iterator.flatMap(f)

• similar to Array.prototype.flatMap but works on Iterator.
• Return a new Generator.

f is passed args: currentElement, currentIndex.

// define a generator function
function* gf() {
 for (let x of [1, 2, 3, 4, 5, 6]) yield x;
}

// if number is even, repeat it, else delete it
const xx = gf().flatMap((x) => ((x % 2 === 0) ? [x, x] : []));

console.assert(
 JSON.stringify(
  Array.from(xx),
 ) === `[2,2,4,4,6,6]`,
);

// is iterable
console.assert(
 Reflect.has(xx, Symbol.iterator),
);

// is iterator
console.assert(
 Reflect.has(xx, "next"),
);