Express TypeScript Error Handling Function

Question

Express TypeScript Error Handling Function

What are the different data types for the four parameters used in the error handling function in Typescript?

app.use((err: ??, req: ??, res: ??, next: ??) => { });

While working in VS Code, I noticed red wiggly lines under all four parameters without any hints provided.

An error message stating "Parameter implicitly has 'any' type" came up, which left me confused. If it is considering it as an any type, is that a valid choice?

typescript express typescript-typings

Answer 1

Answer №1

The function's signature can be found in the DefinitelyTyped repository:

export type ErrorRequestHandler = (err: any, req: Request, res: Response, next: NextFunction) => any;

You have the option to declare the function as a variable of type ErrorRequestHandler or to type the parameters based on that definition.

It's worth noting that the typings for "express-serve-static-core" are imported and re-exported by the typings for "express," where the above definition is located.

import type { ErrorRequestHandler } from "express";

const errorHandler: ErrorRequestHandler = (err, req, res, next) => {};

app.use(errorHandler);

In relation to your inquiry about implicit any, the issue lies in the "implicit" aspect. Explicitly typing as any will resolve the error, but it will also remove typings. Alternatively, you could consider using unknown. Disabling noImplicitAny in your compiler configuration is possible, but it's generally not recommended as it helps prevent various types of bugs.

Answer 2

The function's signature can be found in the DefinitelyTyped repository:

export type ErrorRequestHandler = (err: any, req: Request, res: Response, next: NextFunction) => any;

You have the option to declare the function as a variable of type ErrorRequestHandler or to type the parameters based on that definition.

It's worth noting that the typings for "express-serve-static-core" are imported and re-exported by the typings for "express," where the above definition is located.

import type { ErrorRequestHandler } from "express";

const errorHandler: ErrorRequestHandler = (err, req, res, next) => {};

app.use(errorHandler);

In relation to your inquiry about implicit any, the issue lies in the "implicit" aspect. Explicitly typing as any will resolve the error, but it will also remove typings. Alternatively, you could consider using unknown. Disabling noImplicitAny in your compiler configuration is possible, but it's generally not recommended as it helps prevent various types of bugs.

Answer 3

Answer №2

It's puzzling why Typescript seems to struggle understanding that we are passing an error handler.

After experimenting, here are a few possible solutions:

app.use((err, req, res, next) => {}) // Not working

const errorHandler : ErrorRequestHandler = (err, req, res, next) => {};

app.use(errorHandler); // Works
app.use(((err, req, res, next) => {}) as ErrorRequestHandler); // Works

Answer 4

It's puzzling why Typescript seems to struggle understanding that we are passing an error handler.

After experimenting, here are a few possible solutions:

app.use((err, req, res, next) => {}) // Not working

const errorHandler : ErrorRequestHandler = (err, req, res, next) => {};

app.use(errorHandler); // Works
app.use(((err, req, res, next) => {}) as ErrorRequestHandler); // Works

Answer 5

Answer №3

Custom Error Handling

When using any, the type checking advantage of Typescript is lost. To maintain type safety, consider using the type unknown for the err parameter:

app.use((err: unknown, req: Request, res: Response, next: NextFunction) => { })

Implementation Example

If needed, create a custom error type as shown below:

// HttpException.ts in exceptions directory of your project.

export class HttpException extends Error {
  public status: number
  public message: string
  constructor(status: number, message: string) {
    super(message)
    this.status = status
    this.message = message
  }
}

Then import the custom error where required:

import { HttpException } from './exceptions/HttpException'

app.use((req: Request, res: Response, next: NextFunction) => {
    const err = new HttpException(404, 'Not Found')
    // Handle error here...
    next(err)
})

app.use((err: unknown, req: Request, res: Response, next: NextFunction) => {
    if (err instanceof HttpException) {
        // Perform additional actions for this specific error...
    }
    next(err)
})

Since the type of err is unknown, type checking is required before accessing or modifying it using instanceof to cast it to the correct type (HttpException in this case).

Additional properties and methods can be added to the HttpException class as needed, such as status and message.

Setting up Type Definitions

If not done already, install type definitions for Node.js and Express.js in your Typescript project to ensure proper recognition and auto-importing of types like Request, Response, and NextFunction.

To install type definitions for Node.js:

npm install --save-dev @types/node

To install type definitions for Express.js:

npm install --save-dev @types/express

That concludes the setup. Hope this information is useful.

Answer 6

Custom Error Handling

When using any, the type checking advantage of Typescript is lost. To maintain type safety, consider using the type unknown for the err parameter:

app.use((err: unknown, req: Request, res: Response, next: NextFunction) => { })

Implementation Example

If needed, create a custom error type as shown below:

// HttpException.ts in exceptions directory of your project.

export class HttpException extends Error {
  public status: number
  public message: string
  constructor(status: number, message: string) {
    super(message)
    this.status = status
    this.message = message
  }
}

Then import the custom error where required:

import { HttpException } from './exceptions/HttpException'

app.use((req: Request, res: Response, next: NextFunction) => {
    const err = new HttpException(404, 'Not Found')
    // Handle error here...
    next(err)
})

app.use((err: unknown, req: Request, res: Response, next: NextFunction) => {
    if (err instanceof HttpException) {
        // Perform additional actions for this specific error...
    }
    next(err)
})

Since the type of err is unknown, type checking is required before accessing or modifying it using instanceof to cast it to the correct type (HttpException in this case).

Additional properties and methods can be added to the HttpException class as needed, such as status and message.

Setting up Type Definitions

If not done already, install type definitions for Node.js and Express.js in your Typescript project to ensure proper recognition and auto-importing of types like Request, Response, and NextFunction.

To install type definitions for Node.js:

npm install --save-dev @types/node

To install type definitions for Express.js:

npm install --save-dev @types/express

That concludes the setup. Hope this information is useful.

Answer 7

Answer №4

This is how I am implementing the ErrorRequestHandler:

import type { ErrorRequestHandler } from "express";

type ArgumentTypes<T> = T extends (...args: infer P) => any ? P : never;

type ErrorRequestHandlerArgs = ArgumentTypes<ErrorRequestHandler>;

app.use((...args: ErrorRequestHandlerArgs) => {
  const [err, req, res, next] = args;
  // ...
});

Answer 8

This is how I am implementing the ErrorRequestHandler:

import type { ErrorRequestHandler } from "express";

type ArgumentTypes<T> = T extends (...args: infer P) => any ? P : never;

type ErrorRequestHandlerArgs = ArgumentTypes<ErrorRequestHandler>;

app.use((...args: ErrorRequestHandlerArgs) => {
  const [err, req, res, next] = args;
  // ...
});

Express TypeScript Error Handling Function

Answer №1

Answer №2

Answer №3

Custom Error Handling

Implementation Example

Setting up Type Definitions

Answer №4

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