TypeScript Express Error Function

Question

What are the types of the four parameters for the error handling function when coding in Typescript?

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


        

Answer 1

Custom Error Type

Using any completely gives up the type checking advantage of the Typescript. For err parameter, it's recommended to create your own type or class like following:

// 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
  }
}

And then import it for your error-handling middleware:

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

app.use((err: HttpException, req: Request, res: Response, next: NextFunction) => {
  err.status = 404
  err.message = 'Not Found'
  next(err)
})

We can add any properties and methods to the HttpException class as and when required, like we have added status and message.

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Type Definitions

In case you haven't done already, you need to install type definitions for Node.js and Express.js for your Typescript project. This will make sure the types Request, Response and NextFunction will be recognised and auto-imported.

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

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That's it! Hope that helps.