在同步函数中正确执行异步函数

Question

const mainTasks = ['mainTask1', 'mainTask2', 'mainTask3', 'mainTask4']
const subTasks = ['subTask1', 'subTask2', 'subTask3']
const defaultTimeout = 1000

// SubTasks should be run as async
// Tasks should be run as sync

const subTaskHandler = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve();
        }, defaultTimeout * 2);
    })
}

const main = async () => {
    await Promise.all(
        mainTasks.map((mainTask, index) => {
            return new Promise(resolve => {
                setTimeout(async () => {
                    for (subTask of subTasks) {
                        await subTaskHandler(defaultTimeout)
                        console.log(`${mainTask} -> ${subTask}: Completed`)
                    }
                    resolve()
                }, defaultTimeout + index * 1000)
            })

        })
    )
    console.log('done')
}

main()

我要同步执行 mainTasks 异步执行 subTasks 最后，我尝试抓住 mainTasks 以及 subTasks 全部完成的时间点。看来我明白了，但我遇到了意想不到的结果。

mainTask1 -> subTask1: Completed
mainTask2 -> subTask1: Completed
mainTask3 -> subTask2: Completed
mainTask1 -> subTask2: Completed
mainTask4 -> subTask3: Completed
mainTask2 -> subTask2: Completed
mainTask3 -> subTask3: Completed
mainTask1 -> subTask3: Completed
mainTask4 -> subTask3: Completed
mainTask2 -> subTask3: Completed
mainTask3 -> subTask3: Completed
mainTask4 -> subTask3: Completed
done

mainTask4 -> subTask1 和 2 在哪里??

Answer 1

与你

for (subTask of subTasks) {

您忘记声明 subTask 变量，因此它是隐含的全局变量 - 它没有块作用域。它在每个 maintask 的每次迭代 and 中被重新分配。所以，异步后await里面的for:

                for (subTask of subTasks) {
                    await subTaskHandler(defaultTimeout)
                    console.log(`${mainTask} -> ${subTask}: Completed`)
                }

在任何 await 解析时，subTask 变量将保留在 最近初始化的子任务 期间分配给它的值（而不是在当前子任务期间分配给它的变量。

更改为：

for (const subTask of subTasks) {

它会按预期工作。

const mainTasks = ['mainTask1', 'mainTask2', 'mainTask3', 'mainTask4']
const subTasks = ['subTask1', 'subTask2', 'subTask3']
const defaultTimeout = 1000

// SubTasks should be run as async
// Tasks should be run as sync

const subTaskHandler = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve();
        }, defaultTimeout * 2);
    })
}

const main = async () => {
    await Promise.all(
        mainTasks.map((mainTask, index) => {
            return new Promise(resolve => {
                setTimeout(async () => {
                    for (const subTask of subTasks) {
                        await subTaskHandler(defaultTimeout)
                        console.log(`${mainTask} -> ${subTask}: Completed`)
                    }
                    resolve()
                }, defaultTimeout + index * 1000)
            })

        })
    )
    console.log('done')
}

main()

您还可以考虑使用严格模式 and/or 一个 linter，以更早地捕获这些错误：

'use strict';

const mainTasks = ['mainTask1', 'mainTask2', 'mainTask3', 'mainTask4']
const subTasks = ['subTask1', 'subTask2', 'subTask3']
const defaultTimeout = 1000

// SubTasks should be run as async
// Tasks should be run as sync

const subTaskHandler = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve();
        }, defaultTimeout * 2);
    })
}

const main = async () => {
    await Promise.all(
        mainTasks.map((mainTask, index) => {
            return new Promise(resolve => {
                setTimeout(async () => {
                    for (subTask of subTasks) {
                        await subTaskHandler(defaultTimeout)
                        console.log(`${mainTask} -> ${subTask}: Completed`)
                    }
                    resolve()
                }, defaultTimeout + index * 1000)
            })

        })
    )
    console.log('done')
}

main()

Error: Unhandled promise rejection: ReferenceError: subTask is not defined

在同步函数中正确执行异步函数

Execute correctly async functions at sync functions

synchronization

asynchronous

node.js

promise