Lelaran Async — for await...of dan Penjana Async

// Drive an async iterator manually with await it.next()
(async function () {
  async function* two() {
    yield "A";
    yield "B";
  }
  const it = two();
  console.log(await it.next()); // { value: "A", done: false }
  console.log(await it.next()); // { value: "B", done: false }
  console.log(await it.next()); // { value: undefined, done: true }
})();

// Mark a plain object as async-iterable via Symbol.asyncIterator
(async function () {
  const AsyncTwice = {
    from: function (a, b) {
      return {
        a: a, b: b,
        [Symbol.asyncIterator]: async function () {
          let step = 0;
          return {
            next: async function () {
              if (step === 0) { step = 1; return { value: a, done: false }; }
              if (step === 1) { step = 2; return { value: b, done: false }; }
              return { value: undefined, done: true };
            }
          };
        }
      };
    }
  };

  for await (const v of AsyncTwice.from("X", "Y")) {
    console.log("from object:", v);
  }
})();

// Async map: transform values lazily while awaiting work
(async function () {
  async function* countAsync(n) {
    for (let i = 1; i <= n; i = i + 1) {
      yield i;
    }
  }

  async function* asyncMap(iterable, fn) {
    for await (const v of iterable) {
      // fn can be async; await its result
      const out = await fn(v);
      yield out;
    }
  }

  const doubled = asyncMap(countAsync(4), async function (x) {
    // simulate async transform (no real delay for demo)
    return x * 2;
  });

  const collected = [];
  for await (const v of doubled) {
    collected.push(v);
  }
  console.log("doubled async:", collected);
})();