使用 Node js 从 Web 下载文件并循环

Question

我想使用此代码从网上下载多个文件：

    var fs = require('fs');
    var http = require('http');
    var request = require('request');

    var file;
    for(var i = 1; i <= 5; i++) {
         //CHECK IF REMOTE FILE EXISTS
         request('http://webaddress.com/filename' + i + '.jar', function (err, resp) {

         //IF EXISTS DO
         if (resp.statusCode == 200) {
             //DOWNLOAD DATA AND CREATE A NEW .JAR FILE
             file = fs.createWriteStream('D:\\filename' + i + '.jar');
             http.get('http://webaddress.com/filename' + i + '.jar', function(response) {
                  response.pipe(file);
                  file.on('finish', function() {
                      file.close();
                  });
             });
         }

         //FILE DOES NOT EXIST
         });
     }

我想要的结果是：用文件名filename1-5.jar下载了多个文件。我得到的结果只是一个文件名为 filename5.jar 的文件（或循环中 i var 的最后一个值）。我做错了什么？

Answer 1

就像@Ionut 说你的请求是异步的，所以你需要等待它

let fs = require('fs');
let request = require('request');

let download = (uri, filename) => {
    return new Promise ((resolve, reject) => {
        request.head(uri, function(err, res) {

            if (res.statusCode === 200) {
                request(uri).pipe(fs.createWriteStream(filename)).on('close', resolve);
            } else {
                reject(res.statusCode);
            }
        });
    });
};

let promises = [];

for(let i = 1; i <= 5; i++) {
    promises.push(download('http://webaddress.com/filename' + i + '.jar', 'D:\\filename' + i + '.jar'));
}

Promise.all(promises).then(() => {
    process.exit(0);
}); 

Answer 2

你的请求是异步的，它只会在你的循环完成后执行，因此文件名中的5。一个解决方案是通过创建一个新函数并在循环内调用它来单独威胁您的代码：

var fs = require('fs');
var http = require('http');
var request = require('request');

var file;

function customRequest(i){

  //CHECK IF REMOTE FILE EXISTS
  return request('http://webaddress.com/filename' + i + '.jar', function(err, resp) {

    //IF EXISTS DO
    if (resp.statusCode == 200) {
      //DOWNLOAD DATA AND CREATE A NEW .JAR FILE
      file = fs.createWriteStream('D:\\filename' + i + '.jar');
      http.get('http://webaddress.com/filename' + i + '.jar', function(response) {
        response.pipe(file);
        file.on('finish', function() {
          file.close();
        });
      });
    }

    //FILE DOES NOT EXIST
  });

}

for (var i = 1; i <= 5; i++) {
   customRequest(i)
}