node.js - nodejs，reduce中的异步，await不等待promise解析

Question

我不明白为什么所有的 promise 都会同时得到解决。我读到 await 在 forEach 循环中不能按预期工作，但它也不能与 map 一起工作

const promise = async (ms, name) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log(`resolving ${name}`)
            resolve(name)
        }, ms)
    });
}

async function main() {
    ['foo', 'bar', 'baz'].reduce(async (accum, curr) => {
        const res = await promise(1000, curr)
        return res
    }, Promise.resolve(''))
}

main()

但是在这种情况下，await 工作得很好:

async function main2() {
    const ms = 1000
    await promise(ms, "foo")
    await promise(ms, "bar")
    await promise(ms, "baz")
}

main2()

Answer 1

reduce 回调的每次迭代都将同步初始化。在运行 reduce 回调主体之前，您需要等待上次迭代的累加器(幸运的是，这已经是一个 Promise)解析:

const promise = async (ms, name) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log(`resolving ${name}`)
            resolve(name)
        }, ms)
    });
}

async function main() {
    ['foo', 'bar', 'baz'].reduce(async (accum, curr) => {
        // don't progress further until the last iteration has finished:
        await accum;
        const res = await promise(1000, curr)
        return res
    }, Promise.resolve(''))
}

main()