javascript回调函数只工作一次

Question

我在一个对象中使用了一个回调函数来更新html（xmlhttprequest）。

第一次(onload)，回调工作正常，第二次(timer1)在Post函数下，url属性未定义，控制台无错误。

我的脚本和一些控制台日志。

class PostJSON {
    constructor() {
        this.response = {};
        this.url = "../iot/PostJSON.php";
    }
    callback = function() {
    }

    Post = function(datapost,callback) {
        console.log(this.url);
        console.log(datapost);
        let xhttp = new XMLHttpRequest();
        let myJSON = JSON.stringify(datapost);
        xhttp.open("POST", this.url, true);
        xhttp.setRequestHeader("Content-type", "application/json");
        xhttp.send(myJSON); 
        var instance = this;
        xhttp.onreadystatechange = function() {
            if (this.readyState == 4 && this.status == 200) {
                let json = JSON.parse(this.responseText);
                instance.response = json;
                instance.callback();
            }
        }
    }
}```

const net = new PostJSON();
net.callback = function(){
    console.log(this.response["data"]["datetime"][0]);
    document.getElementById("temp").innerHTML = this.response["data"]["temp"][0] + "°C";
}

window.addEventListener('DOMContentLoaded', net.Post({"id":1}) );

var myTimer1 = setInterval(net.Post, 5000,{"x":1,"id":1});

Answer 1

function PostJSON() {
    this.response = {};
    this.url = "../iot/PostJSON.php";
    this.callback = function() {
    }
    var that = this;
    this.Post = function(datapost,callback) {
        console.log(that.url);
        console.log(datapost);
        let xhttp = new XMLHttpRequest();
        let myJSON = JSON.stringify(datapost);
        xhttp.open("POST", that.url, true);
        xhttp.setRequestHeader("Content-type", "application/json");
        xhttp.send(myJSON); 
        xhttp.onreadystatechange = function() {
            if (this.readyState == 4 && this.status == 200) {
                let json = JSON.parse(this.responseText);
                if (json.hasOwnProperty('infos'))
                    if (json["infos"].hasOwnProperty('infos'))
                        json["infos"]["infos"] = JSON.parse(json["infos"]["infos"]);
                that.response = json;
                that.callback();
            }
        }
    }
}