生成器函数

1. 生成器函数基础

function* simpleGenerator() {
  yield 1;
  yield 2;
  return 3;
}

const gen = simpleGenerator();
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: 3, done: true }

1.1 yield 接收值

function* dialog() {
  const name = yield '你叫什么名字？';
  const age = yield `你好 ${name}，你多大了？`;
  return `${name}，${age}岁`;
}

const it = dialog();
console.log(it.next()); // { value: '你叫什么名字？', done: false }
console.log(it.next('Alice')); // { value: '你好 Alice，你多大了？', done: false }
console.log(it.next(25)); // { value: 'Alice，25岁', done: true }

1.2 yield* 委托

function* inner() {
  yield 'a';
  yield 'b';
}
function* outer() {
  yield 1;
  yield* inner();
  yield 2;
}

for (const value of outer()) {
  console.log(value); // 1, 'a', 'b', 2
}

2. 迭代协议

// 自定义可迭代对象
const range = {
  from: 1, to: 5,
  [Symbol.iterator]() {
    let current = this.from;
    return {
      next() {
        return current <= this.to
          ? { value: current++, done: false }
          : { done: true };
      }.bind(this);
    }.bind({ to: this.to });
  }
};

3. 异步生成器

async function* fetchPages(baseUrl) {
  let page = 1;
  let hasMore = true;
  while (hasMore) {
    const response = await fetch(`${baseUrl}?page=${page}`);
    const data = await response.json();
    yield data.items;
    hasMore = data.hasNext;
    page++;
  }
}

// 消费
async function processAll() {
  for await (const items of fetchPages('/api/posts')) {
    console.log(items);
  }
}

4. 生成器方法