Hacktoberfest 5 has begon, and as my first contribution I wanted to make
a tiny library with errors.

Whenever I start a new Javascript-based project, whether that’s on the server,
or if I’m writing an API client with Fetch, I often find myself doing the
same thing over and over again, which is to define a simple set of exceptions
representing HTTP errors, like this:

class NotFound extends Error {
   httpStatus = 404;

A bit fed up with this, I decided to make a small that’s just a
of errors for Typescript, along with some tiny utilities.


import { NotFound } from '@curveball/http-errors';
throw new NotFound(' not found');

The idea is that the interface is really just this:

export interface HttpError extends Error {
  httpStatus: number;

Which means that any error with a httpStatus property automatically follows
this pattern, and generic middlewares can be written to listen for them.

It comes with a simple utility function to see if the Error conforms with
this pattern:

import { isHttpError } from '@curveball/http-errors';

const myError = new Error('Custom error');
myError.httpStatus = 500;

console.log(isHttpError(myError)); // true


A great idea for emitting errors from a HTTP API is to use the
application/problem+json format, defined in RFC7807. The package also
contains a few utilities to help with these:

export interface HttpProblem extends HttpError {

  type: string | null;
  title: string;
  detail: string | null;
  instance: string | null;


Every standard exception that ships with this package also implements this
interface. Most properties (except title) default to NULL as they are likely
application specific.

Hypothetically, someone could write a library that emits an error with
HttpProblem properties, without having to depend on this package and a generic
application middleware can magically turn this into the right format.

A few HTTP responses require or suggest including extra HTTP headers with more
information about the error. For example, the 405 Method Not Allowed response
should include an Allow header, and 503 Service Unavailable should have a
Retry-After header. The built-in errors support these:

import { MethodNotAllowed, ServiceUnavailable } from '@curveball/http-errors';

try {
  throw new MethodNotAllowed('Not allowed', ['GET', 'PUT']);
} catch (e) {

try {
  throw new ServiceUnavailable('Not  on sundays', 3600*24);
} catch (e) {

I intend to add a few more like this. The UnsupportedMediaType class could for
example include a list of -types that are supported. This might not
correlate directly to a HTTP response header, but someone could use this to
for example automatically populate a JSON response body.

I hope this is useful to anyone else. By doing this once in the right way, I
hope to never have to do this again.


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