js - 如何在Promise中调用异步函数.then()

Question

首先,我必须提到我已经在stackoverflow中查看了很多问题,但很多问题都没有回答我的问题.更不用说很多甚至没有答案.

如何实现以下功能,确保完成functionB()后执行functionA()？

注意:我不想将我的异步函数转换为,new Promise(resolve=>{...})
因为我必须转换someServiceThatMakesHTTPCall()调用堆栈中的以及任何其他异步函数,这是一个很大的改变.

  function functionThatCannotHaveAsyncKeyword() {
      functionA()
        .then(async function() {
            await functionB();
        })
        .then(function() {
            console.log('last');
        });
  }

  async function functionA() {
      console.log('first');
      await someServiceThatMakesHTTPCall();
  }

  async function functionB() {
      console.log('second');
      await someServiceThatMakesHTTPCall();
  }

Answer 1

用你的方法await在async then回调将工作,但它是不必要的复杂,如果所有你想要做的是调用async函数有它的结果通过链传播.但是,如果你正在做其他事情并希望async函数的语法优势,那很好.我马上回过头来看看.

async 函数返回promises,所以你只需返回调用函数的结果:

function functionThatCannotHaveAsyncKeyword() {
  functionA()
    .then(function() {
        return functionB(someArgument);
    })
    .then(function() {
        console.log('last');
    }); // <=== Note: You need a `catch` here, or this function needs
        // to return the promise chain to its caller so its caller can
        // handle errors
}

如果你想传递functionA分辨率值functionB,你可以更直接地做到:

functionA()
  .then(functionB)
  // ...

当您从then回调中返回承诺时,调用创建的承诺将从您返回的承诺开始then.

例:

const wait = (duration, ...args) => new Promise(resolve => {
  setTimeout(resolve, duration, ...args);
});

async function functionA() {
  await wait(500);
  return 42;
}

async function functionB() {
  await wait(200);
  return "answer";
}

functionB()
.then(result => {
  console.log(result); // "answer"
  return functionA();
})
.then(result => {
  console.log(result); // 42
})
.catch(error => {
  // ...handle error...
});

使用async then回调回到你的方法:这也有效,当你做更多的东西时有意义:

const wait = (duration, ...args) => new Promise(resolve => {
  setTimeout(resolve, duration, ...args);
});

async function functionA() {
  await wait(500);
  return 42;
}

async function functionB() {
  await wait(200);
  return "answer";
}

functionB()
.then(async (result) => {
  console.log(result); // "answer"
  const v = await functionA();
  if (v < 60) {
    console.log("Waiting 400ms...");
    await wait(400);
    console.log("Done waiting");
  }
  console.log(v);      // 42
})
.catch(error => {
  // ...handle error...
});