javascript - learnyounode 练习 9 - 输出显示未定义、理解异步或逻辑的问题

Question

我一直在努力解决 learnyounode 上的 JUGGLING ASYNC 任务。据我所知，我做的事情几乎是正确的，看起来问题出在异步的某个地方，而我自己试图避免bl或其他模块。我在没有其他模块的情况下完成了上一个任务，并希望继续这种趋势。

const http = require('http');
const url = process.argv[2];

let content = [];
let count = 0;

const arguments = process.argv;
let urlArray = arguments.filter(function pullUrls(element, index, array) {
    return index >= 2;
});

function printResults() {
    for (var i = 0; i < 3; i++) {
        console.log(content[i]);
    }
}

function httpGet(index) {
    http.get(urlArray[index], function(response){
        response.on('data', function(data){

            newData = data.toString();
            content[index] = content[index] + newData;
        })
        response.on('end', function(){
            count++;
            if (count === 3){
                printResults();
            }
        })
    })
}

for (var i = 0; i < 3; i++) {
    httpGet(i);
}

常见输出非常接近预期，但在开头包含一个 undefined 字符串，我不明白为什么。

undefinedLets get some ute when we're going parma. Grab us a slab how it'll be bull bar. He's got a massive bushie where stands out like a pot. 
undefinedShe'll be right mongrel heaps as cross as a hit the turps. Stands out like a booze also you little ripper flick. As stands out like ironman when lets throw a bikkie. 
undefinedHe hasn't got a bounce with gutful of struth. Stands out like a aerial pingpong piece of piss built like a battler. 

正如您所看到的，它首先找到数组“未定义”，这是正确的，然后附加到它上面。

我最好的猜测是，content[index] = content[index] + newData; 行以某种方式保留了 let content = [] 的未定义性质在计算出 content[i] 之前。现在我已经把它写出来并仔细考虑了，这可能是一个简单的 JS 问题，我只是忽略了或者不知道我不知道。

任何帮助都会很好。

Answer 1

您获得了正确的输出，content[index] 最初是未定义的，您可以在连接 newData 之前进行未定义的检查。这是经过更改的完整代码。

const http = require('http');
const url = process.argv[2];

let content = [];
let count = 0;

const arguments = process.argv;
let urlArray = arguments.filter(function pullUrls(element, index, array) {
    return index >= 2;
});

function printResults() {
    for (var i = 0; i < 3; i++) {
        console.log(content[i]);
    }
}

function httpGet(index) {
    http.get(urlArray[index], function(response){
        response.on('data', function(data){

            let newData = data.toString();
            //check if undefined
            if(typeof content[index] !== 'undefined'){
              content[index] = content[index] + newData;
            }else{
              //add newData if undefined
              content[index] = newData;
            }

        })
        response.on('end', function(){
            count++;
            if (count === 3){
                printResults();
            }
        })
    })
}

for (var i = 0; i < 3; i++) {
    httpGet(i);
}