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Custom Errors in JavaScript

Custom errors are useful when you want to track code with context-specific information. There are several ways to define a custom error in JavaScript. But since inheritance is not a first-citizen feature in JS (until ES2015); there's also so much discussion on how to implement a proper custom error.

See this SO Question, this and this answers, that leads to this discussion (with @mbrowne) and finally, to this module you're viewing.

This module tests various implementations for custom errors, widely used by developers; such as methods making use of Error.call(), Object.create(), new Error(), Object.setPrototypeOf(), extends (ES2015) and util.inherits (Node). See /errors directory for included implementations.

What we expect from a custom error implementation is:

  • e.constructor.name MUST be set to custom error name.
  • E.prototype MUST inherit from Error.prototype.
  • e.message MUST be set.
  • e MUST be an instance of Error.
  • e MUST be an instance of CustomError.
  • e.stack MUST be set and should have line-tracable info.
  • e.tostring() MUST return error name and/or message.
  • ({}).toString.call(e) SHOULD output "error".
  • and some more...

Test

Run npm start which will output test results to console for the current Node.js environment. And will also generate an HTML file, which will display results for the used browser.

Better, you can directly view this page for testing your current browser.

Screenshot Capture of a test on Chrome 55 browser.

CustomError: The One

So; with some research, discussions and these tests I think this is the one closest to ES2015's extends Error. If you think this is a bit over-kill, this should be quite enough.

License

MIT.