Typescript express middleware

Question

I have a simple auth middleware for express. It checks header and if all cool it calls next()

Now when i am in "DoSomething" "this" is equal to global and not the instance of "Test" and "this.DoSomeThingPrivate" is undefined.

I have tried the

DoSomeThingPrivate :() => void;

this.DoSomeThingPrivate = () => {
...
}

pattern. But also does not work.

import express = require('express');

var app = express();

class Test {    
    constructor() {        
    }

    DoSomething(req:express.Request, res:express.Response, next:Function) :void {       
        this.DoSomeThingPrivate();
    }

    private DoSomeThingPrivate() :void
    {       
    }
}

var test = new Test();

app.use(test.DoSomething);

Relates to this

Any Ideas...

thanks

Yath · Accepted Answer · 2021-06-04 12:54:58Z

21

The following should work fine i.e. use fat arrow for DoSomething not DoSomethingPrivate:

import * as express from 'express';

var app = express();

class Test {    
    constructor() {        
    }

    // important: 
    DoSomething = (req:express.Request, res:express.Response, next:express.NextFunction) => {       
        this.DoSomeThingPrivate();
    }

    private DoSomeThingPrivate() :void
    {       
    }
}

var test = new Test();

app.use(test.DoSomething);

Note: You should not need to use bind. Also https://www.youtube.com/watch?v=KDrWLMUY0R0&hd=1

edited Jun 4, 2021 at 12:54

Yath

35 bronze badges

answered Dec 19, 2014 at 22:37

basarat

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3 Comments

Onur Yıldırım Over a year ago

you can also use type express.NextFunction for next param.

djejaquino Over a year ago

I think this approach is considerably better thant bind.

Ian Liu Rodrigues Over a year ago

You can let the compiler infer the parameters types by typing DoSomething like so: DoSomething: express.RequestHandler = (req, res, next) => ...

johngeorgewright · Accepted Answer · 2014-12-19 13:30:09Z

10

You've passed a reference just to the function itself. The function's instance will be global. You need to bind the function to the instance of test.

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

answered Dec 19, 2014 at 13:30

johngeorgewright

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4 Comments

Pintac Over a year ago

thanks dude that worked. I am still getting use to typescript

johngeorgewright Over a year ago

No worries. That's not a Typescript specific problem though. You'll find that happens generally in JavaScript.

AronVanAmmers Over a year ago

In addition to this answer and @basarat's answer, here's [a great description of 'this' in TypeScript][1] which describes the cause of the problem, the solutions offered here and the tradeoffs when choosing one of them. [1]: github.com/Microsoft/TypeScript/wiki/…

AronVanAmmers Over a year ago

An important argument for the double fat arrow syntax DoSomething = (...params...) => { ...function contents...} versus .bind is that double arrow is typesafe in TypeScript.

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