Getting undefined on private class method

I have a class that is having weird behavior. I don't usually work in TypeScript, so it might be how I'm using it. I'm using nodejs, TypeScript, and express for the application. When I run the method getLibs on the controller class I get the error:

const data = this.transformRequest(req);
[1]                               ^
[1]
[1] TypeError: Cannot read properties of undefined (reading 'transformRequest')

The class is:

import { NextFunction, Request, Response } from "express";
import { Controller } from "../../common/Controller";
import { Service } from "../../common/Service";
import { AdLibService } from "./AdLibService";
import { AdLibValidator } from "./AdLibValidator";
import { ErrroHandler } from "../../errors/ErrorHandler";
import { AdLibProps } from "../../ts/types/AdLibProps";
export class AdLibController extends Controller {
constructor() {
super();
this.service = new AdLibService();
this.validator = new AdLibValidator();
}
private transformRequest(req: Request): AdLibProps {
const page: number = parseInt((req.query.page as string) ?? 0);
const pagination: number = parseInt((req.query.pagination as string) ?? 0);
const time = req.query.timestamp;
const data: AdLibProps = {
timestamp: new Date(time as string),
pagination,
page,
};
return data;
}
public async getLibs(req: Request, res: Response, next: NextFunction) {
const data: AdLibProps = this.transformRequest(req);
if (!this.validator.validate(data)) {
return next({
status: 400,
message: `These properties are not valid: ${this.validator.getInvalidPropertiesAsString()}`,
});
}
try {
const foundAdLibs = await this.service.getLibs(
new Date(req.query.timestamp as string),
data.page,
data.pagination
);
return AdLibController.sendResponse(res, foundAdLibs, 200);
} catch (e: unknown) {
const error = ErrroHandler.ensureError(e);
return next({
status: 400,
message: error.message,
});
}
}
private service: AdLibService;
private validator: AdLibValidator;
}

import { Response } from "express";
export abstract class Controller {
public static sendResponse(
res: Response,
data: any,
status: number
): Response {
return res.status(status).json(data);
}
}

import { AdLibProps } from "../../ts/types/AdLibProps";
export class AdLibValidator {
constructor() {
this.invalidProperties = [];
}
public validate(data: AdLibProps): boolean {
if (!this.isValidTimeStamp(data.timestamp)) {
this.addToInvalidProperties("timestamp");
}
if (!this.isValidPage(data.page)) {
this.addToInvalidProperties("page");
}
if (this.isValidPagination(data.pagination)) {
this.addToInvalidProperties("pagination");
}
return this.getInvalidProperties.length === 0;
}
private isValidTimeStamp(timestamp: Date): boolean {
const currentTime = new Date();
if (currentTime < timestamp) {
return false;
}
return true;
}
private isValidPage(page: number): boolean {
return true;
}
private isValidPagination(pagination: number): boolean {
return true;
}
public addToInvalidProperties(property: string): void {
this.invalidProperties.push(property);
}
public getInvalidPropertiesAsString(): string {
return this.invalidProperties.join(", ");
}
public getInvalidProperties(): string[] {
return this.invalidProperties;
}
private invalidProperties: string[];
}

import express, { Router } from "express";
import { RouterErrorHandler } from "../../errors/RouterErrorHandler";
import { AdLibController } from "./AdLibController";
export class AdLibRouter {
public static init(): Router {
const controller = new AdLibController();
AdLibRouter.router
.route("/")
.get(controller.getLibs)
.all(RouterErrorHandler.methodNotAllowed);
return AdLibRouter.router;
}
private static router: Router = express.Router();
}

I'm also having the same issue with using the validator and service properties in the class. It works however when I refactor the properties and method to static methods. I prefer to keep them non-static and just fix this weird behavior.

I think when express calls the route handler, it doesn't maintain the context of the class instance and this keyword inside the getLibs method refers to something else (or undefined).<br>you can explicitly bind the getLibs method to the class instance in the constructor. you can also bind the transformRequest method if you plan to use it elsewhere in the class.

export class AdLibController extends Controller {
constructor() {
super();
this.service = new AdLibService();
this.validator = new AdLibValidator();
// explicitly bind the methods to the class instance
this.getLibs = this.getLibs.bind(this);
this.transformRequest = this.transformRequest.bind(this);
}
// rest of the class remains the same...
}