javascript - 解析 JSON 文件时 undefined reference

Question

这个问题已经有答案了:

How do I return the response from an asynchronous call? (42 个回答)

已关闭 8 年前。

我对这件事真的很陌生，所以请耐心等待。我必须解析这样构造的 json 文件:

    {"elements": [{
            "id": 1,
            "name": "my name",
            "description": "my description",
        }, 
        {
            "id": 2,
            "name": "my name 2",
            "description": "my description 2",
        },

我使用 xmlhttprequest 和 JSON.parse 执行以下操作:

//      asynchronous call to open the cards json
        request.open("GET", "../json/stuff.json", true);

//      send request to web server
        request.send();

//      onreadystatechange fires when the request state changes
        request.onreadystatechange = function() {
//            if the readystate is 4 the response from web server is ready
//            if the request status is 200 the status is ok
              if (request.readyState === 4 && request.status === 200 ) {
                 stuff = JSON.parse(request.responseText);
                 console.log("here");
              }
        }

        console.log(stuff[0]);

“request”变量，就像“stuff”一样，在全局范围内定义如下:

var request = new XMLHttpRequest();

我遇到的问题是“stuff”未定义，我不明白为什么。

Answer 1

您正在异步代码中使用同步代码。 request.onreadystatechange 是回调，因此 console.log 在 request.onreadystatechange 之前调用。所以:

request.onreadystatechange = function() {
        if the request status is 200 the status is ok
          if (request.readyState === 4 && request.status === 200 ) {
             stuff = JSON.parse(request.responseText);
             console.log(stuff[0]);
          }
    }