JavaScript中的Promise.all：如何获得所有承诺的解析价值？ [英] Promise.all in JavaScript: How to get resolve value for all promises?

问题描述

我编写了以下node.js文件：

I wrote the following node.js file:

var csv = require('csv-parser');
var fs = require('fs')
var Promise = require('bluebird');
var filename = "devices.csv";
var devices;

Promise.all(read_csv_file("devices.csv"), read_csv_file("bugs.csv")).then(function(result) {
    console.log(result);
});


function read_csv_file(filename) {
    return new Promise(function (resolve, reject) {
            var result = []
            fs.createReadStream(filename)
                .pipe(csv())
                .on('data', function (data) {
                    result.push(data)
                }).on('end', function () {
                resolve(result);
            });
    })
}

如您所见，我使用 Promise.all 以等待读取csv文件的两个操作。我不明白为什么，但是当我运行代码时，行'console.log（result）'未提交。

As you can see, I use Promise.all in order to wait for both operations of reading the csv files. I don't understand why but when I run the code the line 'console.log(result)' is not committed.

我的第二个问题是我想要 Promise.all.then（）的回调函数接受两个不同的变量，而其中每一个都是相关承诺的结果。

My second question is I want that the callback function of Promise.all.then() accepts two different variables, while each one of them is the result of the relevant promise.

推荐答案

第一个问题

Promise.all 接受一系列承诺

更改：

Promise.all(read_csv_file("devices.csv"), read_csv_file("bugs.csv"))

to（在参数附近添加 [] ）

to (add [] around arguments)

Promise.all([read_csv_file("devices.csv"), read_csv_file("bugs.csv")])
// ---------^-------------------------------------------------------^

第二个问题

Promise.all 为您传递给它的每个承诺解析一系列结果。

The Promise.all resolves with an array of results for each of the promises you passed into it.

这意味着您可以将结果提取为如下变量：

This means you can extract the results into variables like:

Promise.all([read_csv_file("devices.csv"), read_csv_file("bugs.csv")])
  .then(function(results) {
    var first = results[0];  // contents of the first csv file
    var second = results[1]; // contents of the second csv file
  });

您可以使用ES6 + destructuring 以进一步简化代码：

You can use ES6+ destructuring to further simplify the code:

Promise.all([read_csv_file("devices.csv"), read_csv_file("bugs.csv")])
  .then(function([first, second]) {

  });

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