顺序 Promise

Question

所以我试图在不使用异步的情况下按顺序执行我的承诺 下面是我的代码

//promises is an array of function who return a promise
const p = function(promises){
    let result = promises.reduce((prevPromise, promise) => {
        return prevPromise.then(res => {
            return promise.then(Array.prototype.concat.bind(res))
        })
    }, Promise.resolve([]));
    return result;
}

现在说 promises 数组有 2 个函数分别在 5 秒和 10 秒内执行，上面的代码在 10 秒内给出答案，但如果真正的序列执行应该在 15 秒内给出。 请建议。

Answer 1

在我看来。 promises.reduce只是链接了 promise 但没有延迟执行时间。

promise 的执行时间是你创建new Promise()

在then语句中创建新的 promise。

Answer 2

这是因为您正在减少一组承诺，而不是执行返回 promise 的异步操作。

以下面的示例为例，我们有一个delay() function 返回一个 promise，执行异步setTimeout()操作，它解决了超时后的ms延迟。

 // a function that returns a promise that will only resolve // after the setTimeout has finished. const delay = ms => new Promise(resolve => setTimeout( resolve, ms, ms )); // array of milliseconds to execute the delay() function const items = [5000, 10000]; // timer to track the amount of time // passed after all delays are executed console.time('delay'); // Reducing all promises wherein items are the delayed timeout // while also the items that will be added in this reduction const promise = items.reduce((promise, value) => // wait for promise to resolve promise.then(result => // perform async operation delay(value) // add each resolved value.then(item => result + item) ), // default value of reduction Promise.resolve(0) ); promise.then(result => { // Should be the summation of the items array console.log('result', result); // show the time tracker if all of these operations // really finished appropriately. console.timeEnd('delay'); });