Custom error class in TypeScript

Question

I'd like to create my own error class in TypeScript, extending core Error to provide better error handling and customized reporting. For example, I want to create an HttpRequestError class with url, response and body passed into its constructor, which reponds with Http request to http://example.com failed with status code 500 and message: Something went wrong and proper stack trace.

How to extend core Error class in TypeScript? I've already found post in SO: How do I extend a host object (e.g. Error) in TypeScript but this solution doesn't work for me. I use TypeScript 1.5.3

Any ideas?

In what way do those answers not help you? You can't extend the Error class yet. That's coming in 1.6. — David Sherret
– David Sherret, Commented Jul 25, 2015 at 14:45
@DavidSherret I have some compilation errors which, as I see, haven't been reported by tsc in earlier versions. — Kuba T
– Kuba T, Commented Jul 25, 2015 at 19:09

bfontaine · Accepted Answer · 2024-01-04 10:43:34Z

232

TypeScript 2.1 had a breaking changes regarding Extending built-ins like Error.

From the TypeScript breaking changes documentation

class FooError extends Error {
    constructor(msg: string) {
        super(msg);

        // Set the prototype explicitly.
        Object.setPrototypeOf(this, FooError.prototype);
    }

    sayHello() {
        return "hello " + this.message;
    }
}

Then you can use:

let error = new FooError("Something really bad went wrong");
if(error instanceof FooError){
   console.log(error.sayHello());
}

edited Jan 4, 2024 at 10:43

bfontaine

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answered Jan 2, 2017 at 15:07

ziv

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7 Comments

Benny Neugebauer Over a year ago

It's worth mentioning that Object.setPrototypeOf needs to be called immediately after any super(...) calls.

Searene Over a year ago

Why do we need to add Object.setPrototypeOf(this, FooError.prototype);?

Westy92 Over a year ago

I needed to make sure tsconfig.json had "target": "es6".

ChetPrickles Over a year ago

@Searene If you click the "TypeScript breaking changes documentation" link it explains why.

Simon Over a year ago

The link does not work anymore.

|

thoughtrepo · Accepted Answer · 2015-07-25 15:45:34Z

31

Until 1.6 rolls around, I've just been making my own extendable classes.

class BaseError {
    constructor () {
        Error.apply(this, arguments);
    }
}

BaseError.prototype = new Error();

class HttpRequestError extends BaseError {
    constructor (public status: number, public message: string) {
        super();    
    }
}

var error = new HttpRequestError(500, 'Server Error');

console.log(
    error,
    // True
    error instanceof HttpRequestError,
    // True
    error instanceof Error
);

answered Jul 25, 2015 at 15:45

thoughtrepo

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2 Comments

Kuba T Over a year ago

Yeah, my solutions are familiar, the only thing I wondered about is how to extend core classes in the same way as project's ones. Saddly, I don't see any date when TS 1.6 could be released. So, well, I think your solution is the closest to my expectations so far, thanks! :)

Estus Flask Over a year ago

BaseError cannot define methods with class syntax, this way they will be replaced with BaseError.prototype = new Error().

j1n3l0 · Accepted Answer · 2022-06-07 16:51:40Z

24

This does not appear to be an issue in the current latest version of typescript (4.7.3):

import { expect } from 'chai';
import { describe } from 'mocha';

class MyError extends Error {}

describe('Custom error class "MyError"', () => {
  it('should be an instance of "MyError"', () => {
    try {
      throw new MyError();
    } catch (e) {
      expect(e).to.be.an.instanceOf(MyError);
    }
  });
});

I am unsure what version changed the behaviour but this test fails on 2.1.

answered Jun 7, 2022 at 16:51

j1n3l0

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3 Comments

Anentropic Over a year ago

I concur, similar test works for me on Node 18 and Typescript 4.9. I also could not find the issue or changelog where this behaviour started working again, only the original announcement that it would break back in TS 2.2.

Daniel Wolf Over a year ago

The code certainly seems to work fine without the Object.setPrototypeOf call. It would be great though if someone could find an official reference regarding when and how this became obsolete.

Matthew Scharley Over a year ago

This is the real modern answer. It became unnecessary with modern ES targets which use the class syntax in compiled output code rather than transpiling it into manual constructors.

Benny Neugebauer · Accepted Answer · 2016-07-09 15:42:39Z

23

I am using TypeScript 1.8 and this is how I use custom error classes:

UnexpectedInput.ts

class UnexpectedInput extends Error {

  public static UNSUPPORTED_TYPE: string = "Please provide a 'String', 'Uint8Array' or 'Array'.";

  constructor(public message?: string) {
    super(message);
    this.name = "UnexpectedInput";
    this.stack = (<any> new Error()).stack;
  }

}

export default UnexpectedInput;

MyApp.ts

import UnexpectedInput from "./UnexpectedInput";

...

throw new UnexpectedInput(UnexpectedInput.UNSUPPORTED_TYPE);

For TypeScript versions older than 1.8, you need to declare Error:

export declare class Error {
  public message: string;
  public name: string;
  public stack: string;
  constructor(message?: string);
}

edited Jul 9, 2016 at 15:42

answered Jul 9, 2016 at 14:35

Benny Neugebauer

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2 Comments

StewartArmbrecht Over a year ago

You should be careful with this approach. I thought I read that calling the stack property is EXPENSIVE and should be avoided in your code. I think you might be adding a significant amount of overhead for you custom errors. From the documentation "The string representing the stack trace is lazily generated when the error.stack property is accessed."

DarkNeuron Over a year ago

I don't understand why this is necessary in the first place: this.stack = (<any> new Error()).stack; Should be inherited from the Error class, yes?

Peter · Accepted Answer · 2020-02-20 15:22:48Z

19

For Typescript 3.7.5 this code provided a custom error class that also captured the correct stack information. Note instanceof does not work so I use name instead

// based on https://gunargessner.com/subclassing-exception

// example usage
try {
  throw new DataError('Boom')
} catch(error) {
  console.log(error.name === 'DataError') // true
  console.log(error instanceof DataError) // false
  console.log(error instanceof Error) // true
}

class DataError {
  constructor(message: string) {
    const error = Error(message);

    // set immutable object properties
    Object.defineProperty(error, 'message', {
      get() {
        return message;
      }
    });
    Object.defineProperty(error, 'name', {
      get() {
        return 'DataError';
      }
    });
    // capture where error occured
    Error.captureStackTrace(error, DataError);
    return error;
  }
}

There are some other alternatives and a discussion of the reasons.

answered Feb 20, 2020 at 15:22

Peter

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1 Comment

Johnathon Sullinger Over a year ago

You can get the name without having to define a property on it with the name explicitly hard-coded via error.constructor.name. This will return the Type name for you.

Hugo Gresse · Accepted Answer · 2020-11-11 11:03:50Z

10

There is a neat library for this at https://www.npmjs.com/package/ts-custom-error

ts-custom-error allow you to create error custom Error very easily:

import { CustomError } from 'ts-custom-error'
 
class HttpError extends CustomError {
    public constructor(
        public code: number,
        message?: string,
    ) {
        super(message)
    }
}

usage:

new HttpError(404, 'Not found')

answered Nov 11, 2020 at 11:03

Hugo Gresse

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