什么 AsyncGenerator TypeScript 类型会产生 Promise？

Posted 2023-02-23

【中文标题】什么 AsyncGenerator TypeScript 类型会产生 Promise？

【英文标题】：What AsyncGenerator TypeScript type yields a Promise?

【发布时间】：2020-06-26 03:22:00

【问题描述】：

我正在尝试为以下函数分配返回类型：

async function *sleepyNumbers()   // what TypeScript type is this?
  let n = 0;
  while (true) 
    yield new Promise(resolve => resolve(n++));
    await new Promise(resolve => setTimeout(resolve, 500));
  


(async () => 
  for await (const i of sleepyNumbers())
    console.log(i);
)();

生成器正在生成一个解析为 number 的 Promise。将类型设置为 Promise<number> 失败并显示以下错误消息：

TS2739：“AsyncGenerator”类型缺少“Promise”类型的以下属性：then、catch、[Symbol.toStringTag]、finally

Iterable 导致了类似的错误。

我可以将类型设置为AsyncGenerator，但这还不够具体。该函数的返回类型的正确 TypeScript 语法是什么？

【参考方案1】：

它将是AsyncGenerator<number, never, void>：

number - next 结果never 返回void - next 没有得到任何参数

您还需要显式键入一个 promise resolve：

yield new Promise<number>(resolve => resolve(n++));

大家一起：

async function *sleepyNumbers(): AsyncGenerator<number, never, void> 
    let n = 0;
    while (true) 
        yield new Promise<number>(resolve => resolve(n++));
        await new Promise(resolve => setTimeout(resolve, 500));
    


(async () => 
    for await (const i of sleepyNumbers())
        console.log(i);
)();

【讨论】：

让 n = 0;产量 n++; while (true) yield new Promise(resolve => setTimeout(resolve, 500, n++));

【参考方案2】：

因为很多人看这里都需要一些退出条件，所以我将问题和Aleksey L.'s answer 调整为：

在 4 次迭代后停止： 修复生成的error TS2534: A function returning 'never' cannot have a reachable end point.（使用库“es2018.asyncgenerator”测试）。

async function* sleepyNumbers(count: number): AsyncGenerator<number, void, void> 
  let n = 0;
  while (n < count) 
    yield new Promise<number>(resolve => resolve(n++));
    await new Promise(resolve => setTimeout(resolve, 250));
  


(async () => 
  for await (const i of sleepyNumbers(4))
    console.log(i);
)();

#2 需要将第二个模板 arg 设置为 AsyncGenerator 为 void，因为生成器函数（function*）在没有返回的情况下落到最后，调用者返回：

 value: undefined, done: true 

调整生成器以在完成后传递一个最终值，我们看到了第二个模板参数的使用：

async function* sleepyNumbers(count: number): AsyncGenerator<number, string, void> 
  let n = 0;
  while (n < count) 
    yield new Promise<number>(resolve => resolve(n++));
    await new Promise(resolve => setTimeout(resolve, 250));
  
  return 'some string';


(async () => 
  const it = sleepyNumbers(4);
  let res;
  for (res = await it.next(); !res.done; res = await it.next())
    console.log(res);
  console.log('Finished with:', res);
  console.log('past end 1:', await it.next());
  console.log('past end 2:', await it.next());
)();

，产生输出：

 value: 0, done: false 
 value: 1, done: false 
 value: 2, done: false 
 value: 3, done: false 
Finished with:  value: 'some string', done: true 
past end 1:  value: undefined, done: true 
past end 2:  value: undefined, done: true 

显然，在生成器完成后敲击迭代器总是会返回 value: undefined。

tldr;（好像这在 tldr-land 中还没有），我们一直在使用 AsyncGenerator 模板的 TReturn 参数：

interface AsyncGenerator<T = unknown, TReturn = any, TNext = unknown> extends AsyncIterator<T, TReturn, TNext> 
    // NOTE: 'next' is defined using a tuple to ensure we report the correct assignability errors in all places.
    next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn>>;
    return(value: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<T, TReturn>>;
    throw(e: any): Promise<IteratorResult<T, TReturn>>;
    [Symbol.asyncIterator](): AsyncGenerator<T, TReturn, TNext>;

（根据node_modules/typescript/lib/lib.es2018.asyncgenerator.d.ts），映射到TReturn in

interface IteratorYieldResult<TYield> 
    done?: false;
    value: TYield;


interface IteratorReturnResult<TReturn> 
    done: true;
    value: TReturn;


type IteratorResult<T, TReturn = any> = IteratorYieldResult<T> | IteratorReturnResult<TReturn>;

（根据 lib.es2015.iterable.d.ts）