Generator函数深度理解与异步应用

function* gen() {
  const a = yield 1;
  console.log('a:', a);
  const b = yield 2;
  console.log('b:', b);
  return 3;
}
const it = gen();
console.log(it.next());
console.log(it.next('x'));
console.log(it.next('y'));

function run(genFn) {
  const it = genFn();
  function handle(result) {
    if (result.done) return Promise.resolve(result.value);
    return Promise.resolve(result.value).then(
      val => handle(it.next(val)),
      err => handle(it.throw(err))
    );
  }
  return handle(it.next());
}

run(function* () {
  const data1 = yield fetch('/api/user').then(r => r.json());
  const data2 = yield fetch(`/api/posts/${data1.id}`).then(r => r.json());
  console.log(data2);
});

关键点：1)Generator函数返回遍历器对象；2)next()传参会成为上一个yield表达式的值；3)yield*委托给另一个Generator。