javascript - 局部变量在 javascript 的回调函数中变得未定义

Question

我是一名试图编写 JavaScript 的 Java 程序员，但似乎无法理解调用回调时范围如何变化。

下面的 bot.sendmessage 无法运行。错误日志显示“undefined.bot.sendMessage(formId, resp)”

"use strict";

function Handlers() {
    this.bot = undefined;

    this.registerHandler = (bot)=>{
        this.bot = bot;
        bot.on('message', this._messageCallback);
        bot.on('inline_query', this._inlineCallback)
    }
}

Handlers.prototype = {

    _messageCallback: (msg) => {
        console.log("New Outline Request from: chat=" + msg.chat.id + ", uid=" + msg.from.id);
       
        var fromId = msg.from.id;
        var resp = "Hello there";
        this.bot.sendMessage(fromId, resp);
    },
    _inlineCallback: (msg) => {
        console.log("New Inline Request from: uid=" + msg.from.id);
        /*
         TODO::
         check if user is in data base if not log new entry;
         */
    }
};

module.exports = Handlers;

Answer 1

首先我需要说 Teemu 是正确的。这是 Arrow Functions 的问题我自己对此并不熟悉。如果您想看到一种不使用Arrow Functions来完成同样事情的替代方法看看下面的代码片段。

此代码我稍后会更改为 Immediately-Invoked Function Expression (IIFE)但这是我个人的编程习惯。我不知道你在这里的确切用例。

"use strict";

/**
 * I'm assuming you have this elsewhere?
 * If not I have added it to show how to do it
 */
function bot(){
   /* ... */
}

bot.prototype = {
    sendMessage: function(formId,resp){
        console.log("Form: "+formId+" | Message: "+resp);
    }
};
/**
 * End of my addition
 */

function Handlers() {
    this.bot = new bot();
    
    /* ... */
}

Handlers.prototype = {

    _messageCallback: function(msg){
        /* ... */
        var fromId = "fakeid123";
        var resp = msg;
        this.bot.sendMessage(fromId, resp);
    },
    _inlineCallback: function(msg){
        /* ... */
    }
};

var myModule= new Handlers();
myModule._messageCallback("Here is my test msg.");