如何使用lodash,underscore或bluebird同步迭代数组 [英] How to iterate an array synchronously using lodash, underscore or bluebird
问题描述
我有一个数组,每个索引都包含文件名。我想一次下载这些文件(同步)。我知道' Async
'模块。但我想知道 Lodash
或 Underscore
或 Bluebird $ c中的任何功能$ c>库支持此功能。
I have an array that contains filenames at each index. I want to download those files one at a time (synchronously). I know about 'Async
' module. But I want to know whether any functions in Lodash
or Underscore
or Bluebird
library supports this functionality.
推荐答案
您可以使用bluebird的 Promise.mapSeries
:
You can use bluebird's Promise.mapSeries
:
var files = [
'file1',
'file2'
];
var result = Promise.mapSeries(files, function(file) {
return downloadFile(file); // << the return must be a promise
});
根据您用来下载文件的内容,您可能需要建立或不承诺。
Depending on what you use to download file, you may have to build a promise or not.
更新1
downloadFile()的例子
仅使用nodejs的函数:
An exemple of a downloadFile()
function using only nodejs:
var http = require('http');
var path = require('path');
var fs = require('fs');
function downloadFile(file) {
console.time('downloaded in');
var name = path.basename(file);
return new Promise(function (resolve, reject) {
http.get(file, function (res) {
res.on('data', function (chunk) {
fs.appendFileSync(name, chunk);
});
res.on('end', function () {
console.timeEnd('downloaded in');
resolve(name);
});
});
});
}
更新2
正如Gorgi Kosev建议的那样,使用循环构建一系列承诺也是有效的:
As Gorgi Kosev suggested, building a chain of promises using a loop works too:
var p = Promise.resolve();
files.forEach(function(file) {
p = p.then(downloadFile.bind(null, file));
});
p.then(_ => console.log('done'));
承诺链只能获得链中最后一个承诺的结果,而 mapSeries()
为您提供一个包含每个承诺结果的数组。
A chain of promise will get you only the result of the last promise in the chain while mapSeries()
gives you an array with the result of each promise.
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