Typescript: 'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.ts(7009)

Question

The following is the code that works best for displaying custom errors in Chrome Devtools, Node.js, etc. Based on this StackOverflow answer.

function CustomErr (message) {
  var err = new Error(message)
  Object.setPrototypeOf(err, CustomErr.prototype)
  return err
}

CustomErr.prototype = Object.create(Error.prototype, {
  name: { value: 'Custom Error', enumerable: false }
})

However, when I convert it to Typescript:

function CustomErr (message: string) {
  var err = new Error(message)
  Object.setPrototypeOf(err, CustomErr.prototype)
  return err
}

CustomErr.prototype = Object.create(Error.prototype, {
  name: { value: 'Custom Error', enumerable: false }
})

Calling throw new CustomErr("something went wrong") shows this error:

'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.ts(7009)

What can I do to correctly type-annotate my code? If you can find another equivalent code solution, feel free to suggest it, but it MUST have the same behavior in Chrome DevTools (this alone of all solutions I tried displays a custom error name nicely). Thanks!

EDIT: Need to support older browsers, so I can't use ES6 classes. I'd prefer not to transpile classes to ES6 because I'm creating a lightweight library, and a class polyfill alone is 10% of my entire codesize.

So to recap, how can I annotate the code I have now?

Answer 1

You can declare class, but implement it with a function. This way output (the resulting javascript) won't be affected, but typescript will treat the CustomErr as a "newable":

declare class CustomErr extends Error {
    constructor(message: string);
}

function CustomErr(message: string) {
    var err = new Error(message)
    Object.setPrototypeOf(err, CustomErr.prototype)
    return err
}

CustomErr.prototype = Object.create(Error.prototype, {
    name: { value: 'Custom Error', enumerable: false }
})

throw new CustomErr("something went wrong") // no error now

Playground

Answer 2

I was able to implement custom errors in TypeScript by doing

interface Exception {
    code: number;
    message: string;
}

export const Exception = (function (this: Exception, code: number, message: string) {
    this.code = code;
    this.message = message;
} as unknown) as { new (code: number, message: string): Exception };

Then I can use it anywhere in my codebase like

throw new Exception(403, 'A custom error');

Or in an async operation

export const CreateUser = async ({ email, password }: { email: string; password: string }): Promise<IUser> => {
    try {
        // some async operations

        throw new Exception(403, 'Error creating user');
    } catch (error) {
        return Promise.reject(error);
    }
};

Answer 3

I still have no idea how to annotate my code, but just changing throw new CustomErr('err') to throw CustomErr('err') fixed my problem. Though JS allows you to use the new constructor, TypeScript doesn't.