"Error encountered: Route class unable to reach local function in TypeScript Express application" #codingissues

Question

"Error encountered: Route class unable to reach local function in TypeScript Express application" #codingissues

Experiencing an 'undefined' error for the 'loglogMePleasePlease' function in the code snippet below.

Requesting assistance to resolve this issue.

TypeError: Cannot read property 'logMePleasePlease' of undefined

This error has left me bewildered, causing me to question my coding skills and everything I have done so far.

import {Router, Request, Response, NextFunction} from 'express';
import * as fs from 'fs';
import { db } from '../db/lowDb'

export class employeeRoute {
    router: Router
    constructor() {
        this.router = Router();
        this.init();
    }
    init() {
        this.router.get('/', this.default);
        this.logMePleasePlease('SFDSFSDF');/*This call works fine!*/
    }
    public logMePleasePlease(err){
        console.log(err);
    }
    public default(req: Request, res: Response, next: NextFunction) {
        /*****Error when execution gets to follwing line:
        TypeError: Cannot read property 'logMePleasePlease' of undefined
        ******/
        this.logMePleasePlease('asdad');
        res.send({
            status:'ok',
            message:'employee api home'
        });
    }
}
const employee = new employeeRoute();
export default employee.router;

javascript typescript express

Answer 1

Answer №1

One reason for this issue is how JavaScript handles the binding of this when a function is called.

To better illustrate this, let's examine a few examples and discuss how to resolve it.

class BindingDemo {
  func() {
    console.log(this)
  }
}

const demo = new BindingDemo()
const demoFunc = demo.func

In the above example, we have an object named demo with a method called func, and a variable demoFunc that points to this method.

It's crucial to understand how values are passed in JavaScript, especially while working with functions within classes, where everything is treated as a value. So, when we assign const demoFunc = demo.func, we are directly referencing the BindingDemo.prototype.func value (which is the function declared inside the class).

Consequently, if we run:

demoFunc()

We will notice that it logs undefined instead of the BindingDemo context...

This behavior seems peculiar because when we execute:

demo.func()

We see the expected context!

So, to explain further, this discrepancy arises from invoking the function through the object's prototype technique (in which case this refers to demo) versus calling the function separately (resulting in this being undefined).

The solution is quite straightforward; JavaScript offers the .bind() method to associate a specific this context with a function value.

const boundDemoFunc = demo.func.bind(demo)

By using boundDemoFunc now, we should see our BindingDemo context displayed as intended.

I trust this explanation sheds light on the underlying cause of your observation. To address this in your scenario, simply include the following code snippet in your constructor.

this.router.get('/', this.default.bind(this));

Answer 2

One reason for this issue is how JavaScript handles the binding of this when a function is called.

To better illustrate this, let's examine a few examples and discuss how to resolve it.

class BindingDemo {
  func() {
    console.log(this)
  }
}

const demo = new BindingDemo()
const demoFunc = demo.func

In the above example, we have an object named demo with a method called func, and a variable demoFunc that points to this method.

It's crucial to understand how values are passed in JavaScript, especially while working with functions within classes, where everything is treated as a value. So, when we assign const demoFunc = demo.func, we are directly referencing the BindingDemo.prototype.func value (which is the function declared inside the class).

Consequently, if we run:

demoFunc()

We will notice that it logs undefined instead of the BindingDemo context...

This behavior seems peculiar because when we execute:

demo.func()

We see the expected context!

So, to explain further, this discrepancy arises from invoking the function through the object's prototype technique (in which case this refers to demo) versus calling the function separately (resulting in this being undefined).

The solution is quite straightforward; JavaScript offers the .bind() method to associate a specific this context with a function value.

const boundDemoFunc = demo.func.bind(demo)

By using boundDemoFunc now, we should see our BindingDemo context displayed as intended.

I trust this explanation sheds light on the underlying cause of your observation. To address this in your scenario, simply include the following code snippet in your constructor.

this.router.get('/', this.default.bind(this));

Answer 3

Answer №2

It seems like the issue might be caused by your router interfering with the context of the 'default' function, resulting in it being undefined.

To resolve this, you could try explicitly binding the function to the correct context within the constructor:

this.default.bind(this)

However, this approach can get messy. Have you considered consolidating all routes under a single router instead of creating separate ones for each route? You could pass the main router instance to every route class during initialization.

I've adopted a pattern for creating routes that I picked up from a popular tutorial. While some may debate its heavy use of static methods, it has proven effective for me:

import { NextFunction, Request, Response, Router } from 'express'

export class IndexRoute {

    static CREATE(router: Router) {
        console.log('[IndexRoute::create] Creating route /');

        router.get('/', (req, res, next) => {
            new IndexRoute().index(req, res, next)
        })
    }

    index(req: Request, res: Response, next: NextFunction) {
        console.log('[IndexRoute::index]');
        const data = { status: 'ok' };
        res.status(200).json(data);
    }

}

Answer 4

It seems like the issue might be caused by your router interfering with the context of the 'default' function, resulting in it being undefined.

To resolve this, you could try explicitly binding the function to the correct context within the constructor:

this.default.bind(this)

However, this approach can get messy. Have you considered consolidating all routes under a single router instead of creating separate ones for each route? You could pass the main router instance to every route class during initialization.

I've adopted a pattern for creating routes that I picked up from a popular tutorial. While some may debate its heavy use of static methods, it has proven effective for me:

import { NextFunction, Request, Response, Router } from 'express'

export class IndexRoute {

    static CREATE(router: Router) {
        console.log('[IndexRoute::create] Creating route /');

        router.get('/', (req, res, next) => {
            new IndexRoute().index(req, res, next)
        })
    }

    index(req: Request, res: Response, next: NextFunction) {
        console.log('[IndexRoute::index]');
        const data = { status: 'ok' };
        res.status(200).json(data);
    }

}

Answer 5

Answer №3

If you're looking for an example using classes and the use method on a Router, here's some sample code that might be helpful (you can also find an example at the bottom of the documentation here).

server.ts

import { Test } from './test';
import express from "express";
import compression from "compression";

export class Server {
    private app: express.Application;
    private port: number = 3000;
    private test: Test;

    constructor() {
        // setting up the server
        this.app = express();
        this.app.use(express.urlencoded({ extended: true }));   // required for POST requests
        this.app.use(express.json());   // required for POST requests
        this.app.use(compression());

        // creating instances of classes
        this.test = new Test();

        // specifying routes for the app to use
        this.app.use("/test", this.test.router);

        // starting the server
        this.app.listen(this.port, () => {
            console.log("Node Express server is listening on port " + this.port);
        });
    }

}

test.ts

import express, { Router, Request, Response } from "express";

export class Test {
    public router: Router = express.Router();
    private testVar: string = "Hello World";

    constructor() {
        this.router.get('/', [this.get.bind(this)]);
    }

    private async get(req: Request, res: Response) {
        try {
            // responding with data
            res.status(200).send(this.testVar);
        } catch (error) {
            res.status(500).send(error);
        }
    }

}

Answer 6

If you're looking for an example using classes and the use method on a Router, here's some sample code that might be helpful (you can also find an example at the bottom of the documentation here).

server.ts

import { Test } from './test';
import express from "express";
import compression from "compression";

export class Server {
    private app: express.Application;
    private port: number = 3000;
    private test: Test;

    constructor() {
        // setting up the server
        this.app = express();
        this.app.use(express.urlencoded({ extended: true }));   // required for POST requests
        this.app.use(express.json());   // required for POST requests
        this.app.use(compression());

        // creating instances of classes
        this.test = new Test();

        // specifying routes for the app to use
        this.app.use("/test", this.test.router);

        // starting the server
        this.app.listen(this.port, () => {
            console.log("Node Express server is listening on port " + this.port);
        });
    }

}

test.ts

import express, { Router, Request, Response } from "express";

export class Test {
    public router: Router = express.Router();
    private testVar: string = "Hello World";

    constructor() {
        this.router.get('/', [this.get.bind(this)]);
    }

    private async get(req: Request, res: Response) {
        try {
            // responding with data
            res.status(200).send(this.testVar);
        } catch (error) {
            res.status(500).send(error);
        }
    }

}

"Error encountered: Route class unable to reach local function in TypeScript Express application" #codingissues

Answer №1

Answer №2

Answer №3

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