不能调用它在Node.js中的ES6中定义的方法 [英] Cannot call a method within a class it defined it in ES6 in Node.js

问题描述

我正在使用Node.js，Express.js和MongoDB制作应用程序。
我正在使用MVC模式，并且还有路由的单独文件。
我正在尝试我做一个Controller类，其中一个方法调用在其中声明的另一个方法。但我似乎无法做到这一点。我得到无法读取属性的未定义。

I am making an app using Node.js, Express.js and MongoDB. I am using a MVC pattern and also have separate file for routes. I am trying me make a Controller class, in which a method calls another method declared within it. But I cannot seem to be able to do this. I get "Cannot read property '' of undefined".

index.js文件

index.js file

let express = require('express');
let app = express();

let productController = require('../controllers/ProductController');

app.post('/product', productController.create);

http.createServer(app).listen('3000');

ProductController.js文件

ProductController.js file

class ProductController {
  constructor(){}

  create(){
   console.log('Checking if the following logs:');
   this.callme();
  }

 callme(){
  console.log('yes');
 }
}
module.exports = new ProductController();

当我运行这个我收到以下错误消息：

When I run this I get following error message:

Cannot read property 'callme' of undefined

我已经运行了这个代码，没有什么修改，如下所示，它的工作原理。

I have ran this code by itself with little modification as following and it works.

class ProductController {
  constructor(){}
  create(){
    console.log('Checking if the following logs:');
    this.callme();
  }

  callme(){
    console.log('yes');
  }
}
let product = new ProductController();
product.create();

为什么一个工作而不是另一个？
HELP！

Why does one work and not the other? HELP!

推荐答案

您的方法是被反弹到快递内的层类，失去原来上下文。快递处理路由的方法是将每个路径包裹在一个层类中，该类将路由回调分配给自己：

Your method is being rebound to the Layer class within express, losing its original context. The way that express handles routes is by wrapping each one in a Layer class, which assigns the route callback to itself:

this.handle = fn;

这是您的问题出现的地方，此作业自动将功能上下文重新绑定到层。以下是一个简单的示例演示问题：

That is where your problems arise, this assignment automatically rebinds the function context to Layer. Here is a simple example demonstrating the problem:

function Example() { 
   this.message = "I have my own scope"; 
} 
Example.prototype.logThis = function() { 
   console.log(this); 
}

function ReassignedScope(logThisFn) { 
   this.message = "This is my scope now";
   // simulation of what is happening within Express's Layer
   this.logThis = logThisFn; 
}

let example = new Example()
let scopeProblem = new ReassignedScope(example.logThis);

scopeProblem.logThis(); // This is my scope now

其他人已经指出了解决方案，这是明确绑定你的方法到 ProductController 实例：

Others have already pointed out the solution, which is to explicitly bind your method to the ProductController instance:

app.post('/product', productController.create.bind(productController));

这篇关于不能调用它在Node.js中的ES6中定义的方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！