JS: Iterator.prototype.flatMap

(new in ECMAScript 2025)

iterator.flatMap(f)

• similar to Array.prototype.flatMap but works on Iterator Object.
• 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;
}

// create a generator. a generator is both iterable and iterator
const xgen = gf();

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

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

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

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

• JS: Generator Function (asterisk)