Typescript error when adding property to Error object "Property ... does not exist on type 'Error'"

Question

I'm trying to move to typescript with my Node/Express app. Previously my code was:

//app.js
const error = new Error('Not Found');
error.status = 404;

When I try this:

//app.ts
const error = new Error('Not Found');
error.status = 404; // Property 'status' does not exist on type 'Error'.ts(2339)

I understand from developer.mozilla.org documentation that the Error constructor has the following optional parameters: message, options, fileName, lineNumber - so I guess status shouldn't be permitted? I think I've copied it from a youtube tutorial so I guess it's not actually good practice?

Answer 1

TypeScript does not allow adding unknown properties. There are ways to define objects with arbitrary keys (e.g. Record).

In this case you could create your own subclass to Error which includes the status property.

class StatusError extends Error {
  status: number | undefined;
}

const e = new StatusError('Not found');
e.status = 404;

You could also add it to the constructor, then you can confidently remove the undefined.

class StatusError extends Error {
  constructor(public status: number, message?: string) {
    super(message)
  }
}

const e = new StatusError(404, 'Not found');