forEach循环内的XMLHTTPRequest不起作用

Posted 2021-04-16

嗨，我正在编写一个简短的node.js应用程序，每次迭代数组时都会向API发送XMLHTTPRequest。问题是由于异步性质，它会在返回请求之前继续foreach循环。我可能会忽略一些大事，但我已经花了大半个时间试图让我的脑子在这上面。我已经尝试使用等待无济于事，任何解决方案将不胜感激。

提前致谢。

NODE JS应用程序

const mongoose = require("mongoose");
const fs = require("fs");
const ajax = require("./modules/ajax.js");

// Bring in Models
let Dictionary = require("./models/dictionary.js");


//=============================
//     MongoDB connection
//=============================

// Opens connection to database "test"
mongoose.connect("mongodb://localhost/bookCompanion");
let db = mongoose.connection;

// If database test encounters an error, output error to console.
db.on("error", (err)=>{
  console.console.error("Database connection failed.");
});

// Check for connection to the database once.
db.once("open", ()=>{
  console.info("Connected to MongoDB database...");

  fs.readFile("./words.json", "utf8", (err, data)=>{

    if(err){
      console.log(err);
    } else {
      data = JSON.parse(data);
      data.forEach((word, index)=>{

        let search = ajax.get(`LINK TO API?=${word}`);

        search.then((response)=>{

          let newWord = new Dictionary ({
            Word: response.word,
            phonetic: response.phonetic,
            meaning: response.meaning
          }).save();

          console.log(response);

        }).catch((err)=>{
          console.log(err);
        });

      });
    }

  });


});

XMLHTTPRequest模块

// Get Request module utilising promises

const XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;

const get = (url)=>{
  // This function will return a promise, promises use resolve and reject. The resolve is accessed through .then and the reject through the .catch
  return new Promise((resolve, reject)=>{

    // Create new XMLhttp (AJAX) Request
    let xhr = new XMLHttpRequest();
    // Sets up the request, setting it to a GET request, pointing to the website and setting it to asynchronous
    xhr.open("GET", url , true);
    //sends the request
    xhr.send();

    xhr.onload = ()=>{
      if (xhr.status == 200){
        // When loaded pass the response over to the .then method
        resolve(JSON.parse(xhr.responseText));
      } else {
        // if error pass the status code to the .catch method for error handling
        reject(xhr.statusText);
      }
    };

    xhr.onerror = ()=>{
      // if error pass the status code to the .catch method for error handling
      reject(xhr.statusText && xhr.status);
    };

  });
};

module.exports.get = get;

答案

您应该使用promise.all等待所有承诺完成。 Promise.all将一系列承诺作为输入等待，直到所有承诺得到解决。如果您使用promise.all，它会拒绝，您的代码将是这样的。

const mongoose = require("mongoose");
const fs = require("fs");
const ajax = require("./modules/ajax.js");

// Bring in Models
let Dictionary = require("./models/dictionary.js");


//=============================
//     MongoDB connection
//=============================

// Opens connection to database "test"
mongoose.connect("mongodb://localhost/bookCompanion");
let db = mongoose.connection;

// If database test encounters an error, output error to console.
db.on("error", (err) => {
    console.console.error("Database connection failed.");
});

// Check for connection to the database once.
db.once("open", () => {
    console.info("Connected to MongoDB database...");

    fs.readFile("./words.json", "utf8", (err, data) => {

        if (err) {
            console.log(err);
        } else {
            data = JSON.parse(data);
            var promiseArr = []
            Promise.all(promiseArr.push(
                new Promise((resolve, reject) => {

                    let search = ajax.get(`LINK TO API?=${word}`);

                    search.then((response) => {

                        let newWord = new Dictionary({
                            Word: response.word,
                            phonetic: response.phonetic,
                            meaning: response.meaning
                        }).save();

                        console.log(response);
                        resolve();
                    }).catch((err) => {
                        console.log(err);
                        reject();
                    });

                })
            )).then((response) => {
                //whatever you want to do after completion of all the requests
            })
        }

    });


});

另一答案

看起来我的代码在使用较小的数组时工作正常，我遇到的真正问题是处理forEach循环和内存的阻塞性质。我需要遍历的数组包含超过400,000个单词，并且应用程序在forEach循环完成之前耗尽内存并释放调用堆栈以便解析httprequests。

我将非常感谢有关如何创建不阻塞调用堆栈的同步forEach循环的任何信息。