Ein undefiniertes "this" bei Verwendung von Klassen/Async erwarte

Question

Ich habe gerade begonnen zu experimentieren mit Klassen und Async erwarten. Ich benutze Knoten Version 8.9.0 (LTS). Wenn ich console.log(this), bekomme ich undefined anstelle der Referenz auf das Objekt.

subhandler.js

class Handler { 
    constructor(props) { 
    this.defaultRes = { 
     data: successMessage, 
     statusCode: 200 
    }; 
    } 

    async respond(handler, reply, response = this.defaultRes) { 
    console.log(this); // why is `this` undefined???? 
    try { 
     await handler; 
     return reply(response.data).code(response.statusCode) 
    } catch(error) { 
     return reply(error); 
    } 
    } 
} 

class SubHandler extends Handler { 
    constructor(props) { 
    super(props); 
    this.something = 'else'; 
    } 

    makeRequest(request, reply) { 
    console.log(this); // why is `this` undefined!! 
    // in this case, doSomeAsyncRequest is a promise 
    const handler = doSomeAsyncRequest.make(request.params); 
    super.respond(handler, reply, response); 
    } 
} 

module.exports = new SubHandler; 

Innen Hapi Strecke

const SubHandler = require('./subhandler'); 

server.route({ 
    method: 'GET', 
    path: '/', 
    handler: SubHandler.makeRequest, 
    // handler: function (request, reply) { 
    // reply('Hello!'); //leaving here to show example 
    //} 
}); 

Prototype Beispiel

function Example() { 
    this.a = 'a'; 
    this.b = 'b'; 
} 

Example.prototype.fn = function() { 
    console.log(this); // this works here 
} 

const ex = new Example(); 
ex.fn(); 

Answer 1

Wenn y ou wollen this immer in makeRequest auf die Instanz verweisen, bind its context im Konstruktor:

class SubHandler extends Handler { 
    constructor(props) { 
    super(props); 

    this.makeRequest = this.makeRequest.bind(this) 

    this.something = 'else'; 
    } 

    makeRequest(request, reply) { 
    console.log(this); 
    const handler = doSomeAsyncRequest.make(request.params); 
    super.respond(handler, reply, response); 
    } 
}