class ReverseArray {
  constructor(arr) {
    this.items = [...arr];
  }
<p>[Symbol.iterator]() {
let index = this.items.length - 1;
return {
next: () => {
if (index >= 0) {
return { value: this.items[index--], done: false };
} else {
return { done: true };
}
}
};
}
}</p><p>const reverseArr = new ReverseArray([1, 2, 3]);
for (const item of reverseArr) {
console.log(item); // 输出：3, 2, 1
}</p>

function* fibonacci() {
  let a = 0, b = 1;
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}
<p>const fib = fibonacci();
console.log(fib.next().value); // 0
console.log(fib.next().value); // 1
console.log(fib.next().value); // 1
console.log(fib.next().value); // 2</p><p>// 或者用 for...of 取前几项
for (const n of fibonacci()) {
if (n > 100) break;
console.log(n);
}</p>

class EvenCollection {
  constructor(numbers) {
    this.numbers = numbers.filter(n => n % 2 === 0);
  }
<p>[Symbol.iterator]() {
let idx = 0;
const values = this.numbers;
return {
next() {
if (idx < values.length) {
return { value: values[idx++], done: false };
}
return { done: true };
}
};
}
}</p><p>const evens = new EvenCollection([1, 2, 3, 4, 5, 6]);
for (const n of evens) {
console.log(n); // 2, 4, 6
}</p>