用作中间件时，此属性的属性未定义

Question

I have a Container class :

// container.js
function Container(clients) {
    this.clients = clients;
}

Container.prototype.test = function (req, res, next) {
    console.log(this.clients['key']);
    next();
};

module.exports = Container; 

And then from another file I create an express server:

// server.js
const Express = require('express');
const BodyParser = require('body-parser');
const Container = require('./container');

var box = new Container({'key': 'secret' });

var app = new Express();
app.use(BodyParser.json());
app.use(box.test);

// some routes...

app.listen(3000);

When I make requests I get this logged to the console:

TypeError: Cannot read property 'key' of undefined

I'm so confused. I've read up on many different things about the improper use of this but I can't see where I'm messing this up?

Any ideas?

Answer 1

When you get a function from the object app.use(box.test) , it loses it's context and this doesn't refer to the object itself. You need to use the bind() function to create a function with attached context, or just use arrow function and inside it call your function on the object.

app.use(() => box.test());

With bind

app.use(box.test.bind(box));