Over Extend

Installation

npm i -S over-extend

Why?

In Babel 5, you could easilly extend built-ins like Error and Array like this:

class CustomArray extends Array {
  constructor () {
    super()

    this.name = this.constructor.name
  }

  getFirst () {
    return this[0]
  }
}

let customArray = new CustomArray()
customArray.push('a')
customArray.push('b')
customArray.push('c')

console.log(customArray.name) // 'CustomArray'
console.log(customArray instanceof CustomArray) // true
console.log(customArray.getFirst()) // 'a'

However, in Babel 6, it doesn't respect the new class and instead treats it like it's the built-in:

console.log(customArray.name) // 'Array'
console.log(customArray instanceof CustomArray) // false
console.log(customArray instanceof Array) // true
console.log(customArray.getFirst()) // throws a method undefined error

This module helps regain that behavior.

Example

Here's an example for how to create custom Error classes:

const OverExtend = require('over-extend')

class HttpError extends OverExtend(Error) {
  constructor () {
    super()

    this.name = this.constructor.name
    Error.captureStackTrace(this, this.constructor)
  }

  getMessage () {
    return this.message
  }
}

class NotFoundError extends HttpError {
  constructor (message) {
    super()

    this.statusCode = 404
    this.message = message || 'Not Found'
  }
}

let userNotFoundError = new NotFoundError('User with that id not found')
console.log(userNotFoundError.name) // 'NotFoundError'
console.log(userNotFoundError.statusCode) // 404

Ready Made Extendables

If you're running Node >= v4, you can use some pre-configured extendable classes.

Note: If you're using Babel 6, you cannot use these readymade examples with the transform-es2015-classes plugin (included in the es-2015 preset) or you will recieve TypeError: Class constructors cannot be invoked without 'new'. To migitgate this, you can install only the plugins that provide functionality that Node v4 does not already provide:

{
  "plugins": [
    "check-es2015-constants",
    "transform-es2015-for-of",
    "transform-es2015-arrow-functions",
    "transform-es2015-parameters",
    "transform-es2015-spread",
    "transform-es2015-destructuring",
    "transform-es2015-modules-commonjs",
    "transform-regenerator"
  ]
}

If you're using npm 3, you can simply install the es-2105 preset and specify the plugins in your .babelrc.

Error

As illustrated in the example error above, in order to get the stack trace of the error, you need to call Error.captureStackTrace. The ExtendableError class in this module does all that magic for you. Check out the source code for the implimentation.

const ExtendableError = require('over-extend/builtins/error')

class ValidationError extends ExtendableError {
  constructor(config) {
    super()
    config = config || {}
    this.message = config.message || 'validation error detected'
    this.errors = config.errors || {}
  }

  getStackTrace () {
    return this.stack
  }

  prettyPrintErrors () {
    let message = 'Errors -'
    
    Object.keys(this.errors).sort().forEach((err) => {
      let error = this.errors[err]
      message += `\n${error.label}: ${error.message},`
    })

    // Cut off trailing comma
    message = message.substring(0, message.length - 1)

    return message
  }
}

//------------------------------

let error = new ValidationError({
  errors: {
    email: {
      label: 'Email',
      message: 'Email not formatted correctly'
    },
    abc: {
      label: 'ABC',
      message: 'You do not know your abcs'
    }
  }
})

// Errors -
// ABC: You do not know your abcs,
// Email: Email not formatted correctly
error.prettyPrintErrors()

Array

If you extend the Array class on your own, you'll have to do extra work in the constructor to make it behave like a normal array when instantiating.

When using ExendableArray, it's recomended that you do not include a constructor in subclass or it may not instantiate correctly. See the source code for more details.

const ExtendableArray = require('over-extend/builtins/array')

class MyArray extends ExtendableArray {
  getFirst () {
    return this[0]
  }

  getLast () {
    return this[this.length - 1]
  }

  reverseEachElement () {
    for (var i = 0; i < this.length; i++) {
      let element = this[i]
      let reversedElement = element.split('').reverse().join('')

      this[i] = reversedElement
    }
  }
}

//------------------------------

let array = new MyArray('abc', '123')

array.reverseEachElement()

array // [ 'cba', '321' ]
array.getFirst() // 'cba'
array.getLast() // '321'

Credits

This was largely adapted from https://phabricator.babeljs.io/T3083

Testing

The builtins tests require Node v4 or higher.

npm t