# StandardError.js

StandardError.js is a tiny JavaScript library that simplifies creating subclasses of `Error` for custom error classes with the correct `name` and `stack` property. Saves you from writing a few lines of boilerplate.

### Tour

- Create custom error classes and add new behavior to them while keeping the standard `Error` behavior in tact.
- Add extra properties to the error by just passing in an object.
- StandardError.js sets the error's stack trace correctly, even if your error class subclasses/inherits from StandardError.
  Just inheriting from `Error` with `Object.create` breaks the stack trace.
- Every `StandardError` instance is also an instance of `Error`.
- Serializes all expected properties when passing it to `JSON.stringify`.
  Did you know that the default `Error` object serializes to an empty object (`{}`)?
- Works both in Node.js and browsers and sets the stack trace via `Error.captureStackTrace` where available.

## Installing

    npm install standard-error

## Using

Just require StandardError.js and either use it directly or inherit from it for your custom error class.

### Throwing StandardError

Like `Error`, `StandardError` takes a message argument, but in addition to that, you may give it an object with other properties to be set:

    var StandardError = require("standard-error")
    throw new StandardError("Not Found", {code: 404})

The thrown instance of `StandardError` will then have both the `message` and the `code` property.
It'll also also have a `name` property set to `"StandardError"`.

You can skip the explicit `message` argument and give everything as an object of properties:

    new StandardError({message: "Not Found", code: 404})

Note: All properties besides `stack` will be enumerable for easier serialization with `JSON.stringify`. That includes the `name` property which will be set from the constructor's name (defaults to `"StandardError"`).

### Subclassing and inheriting from StandardError

The real benefit of StandardError.js comes from subclassing it to create new error classes and adding custom behavior to them.

Let's create an `HttpError` that we can instantiate with the HTTP status code (`new HttpError(404)`) and have it set the message automatically based on that:

    var Http = require("http")
    var StandardError = require("standard-error")

    function HttpError(code, msg) {
      StandardError.call(this, msg || Http.STATUS_CODES[code], {code: code})
    }

    HttpError.prototype = Object.create(StandardError.prototype, {
      constructor: {value: HttpError, configurable: true, writable: true}
    })

Note that you must set the `constructor` property like in the above example. First, that's the proper way to subclass in JavaScript and second, StandardError.js depends on that to know which functions to skip in the stack trace.

#### Name

StandardError.js finds out the name (`err.name`) of your subclassed error from its constructor function. However, if you minify your code, you can also set or change it explicitly:

    ChildError.prototype.name = "FallacyError"

### Adding behavior to your subclass of StandardError

Now that you've inherited, you can, for example, customize stringifying by overwriting `toString` on your subclass. To get `new HttpError(404)` to print itself as `404 Not Found`:

    HttpError.prototype.toString = function() {
      return this.code + " " + this.message
    }

## License

StandardError.js is released under a Lesser GNU Affero General Public License, which in summary means:

- You can use this program for no cost.
- You can use this program for both personal and commercial reasons.
- You do not have to share your own program's code which uses this program.
- You have to share modifications (e.g. bug-fixes) you've made to this program.

For more convoluted language, see the `LICENSE` file.

## About

Andri Möll typed this and the code.
Monday Calendar supported the engineering work.

If you find StandardError.js needs improving, please don't hesitate to type to me now at andri@dot.ee or create an issue online.