javascript - 未处理的 promise 拒绝？

Question

This example (repl.it) (来自 this answer )在我看来它遵循了所有关于 promise 的规则。然而，运行它会记录有关未处理的 promise 拒绝的异常以及相关的控制台消息。 (这也发生在 FF、Chrome 和 Node v10 中。)

try/catch block 显然在那里并包装了被拒绝的 promise ，那么这是怎么回事，我该如何解决它？

async function example() {
  const start = Date.now()
  let i = 0
  function res(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve()
        console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }

  function rej(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        reject()
        console.log(`rej #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }

  try {
    const delay1 = res(3000)
    const delay2 = res(2000)
    const delay3 = rej(1000)

    const data1 = await delay1
    const data2 = await delay2
    const data3 = await delay3
  } catch (error) {
    console.log(`await finished`, Date.now() - start)
  }
}

example()

Answer 1

问题在于，rej调用拒绝，解释器还没有到达 await 的那一行这是 rej 创建的 promise ，所以被拒绝的 Promise 就是一个被拒绝的 Promise，而不是 当前线程是 await 的 Promise。 ing:

try {
  const delay1 = res(3000)
  const delay2 = res(2000)
  const delay3 = rej(1000)

  const data1 = await delay1
  // The interpreter is paused on the line above when `rej` rejects
  const data2 = await delay2
  const data3 = await delay3

因此，行为与在没有 catch 的情况下声明被拒绝的 Promise 相同。处理程序。 (Promises 抛出的错误只会在 async 函数中被捕获，如果它们是 await ed 在 Promise 拒绝的点 - 否则，它只会导致未处理的 promise拒绝。)

我建议在您 await 的同一点上声明 Promises。他们:

const data1 = await res(3000)

(注意:上述方法的时间不会和原来的代码一样)

或使用 await Promise.all对于所有 Promise，这意味着 Promise口译员目前是await一旦 一个 Promise 拒绝，ing 将抛出(并因此进入 catch block ):

const [data1, data2, data3] = await Promise.all([
  res(3000),
  res(2000),
  rej(1000)
]);

async function example() {
  const start = Date.now()
  let i = 0
  function res(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve()
        console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }
  
  function rej(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        reject()
        console.log(`rej #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }
  
  try {
    const [data1, data2, data3] = await Promise.all([
      res(3000),
      res(2000),
      rej(1000),
    ]);
  } catch (error) {
    console.log(`error caught: await finished`, Date.now() - start)
  }
}

example()

要在三个 Promise 进行时做额外的工作，并从这些 Promise 和主线程中捕获错误，将第四个项传递给 Promise.all ，一个 IIFE，它可以完成您想做的额外工作:

async function example() {
  const start = Date.now()
  let i = 0
  function res(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve()
        console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }
  
  function rej(n) {
    const id = ++i
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        reject()
        console.log(`rej #${id} called after ${n} milliseconds`, Date.now() - start)
      }, n)
    })
  }
  
  try {
    const [data1, data2, data3] = await Promise.all([
      res(3000),
      res(2000),
      rej(1000),
      (() => {
        console.log('doing work...');
      })()
    ]);
  } catch (error) {
    console.log(`error caught: await finished`, Date.now() - start)
  }
}

example()