npm

Need private packages and team management tools?Check out npm Orgs. »

babel-plugin-transform-class-extended-hook

1.0.3 • Public • Published

babel-plugin-transform-class-extended-hook

Babel plugin that transforms subclass declarations to call superclass.extended afterwards (if present).

Example

Before:

class Apple extends Fruit {
  tastiness() {
    return 7;
  }
}

After:

var Apple = function () {
  // Declare the Apple class as an anonymous class _Apple.
  var _Apple = class extends Fruit {
    tastiness() {
      return 7;
    }
  }
 
  // Set _Apple's name property to 'Apple'.
  Object.defineProperty(_Apple, "name", { value: "Apple", configurable: true });
 
  // If Fruit has a property called extended,
  if ("extended" in Fruit) {
    // and it's a function,
    if (typeof Fruit.extended == 'function') {
      // call it and save the returned value.
      var _Apple2 = Fruit.extended(_Apple);
 
      // If Fruit.extended returned a value,
      if (_Apple2 !== undefined) {
        // if it's a function, define its name property as 'Apple', if it didn't have that already.
        if (typeof _Apple2 == 'function' && _Apple2.name !== "Apple") {
          Object.defineProperty(_Apple2, "name", { value: "Apple", configurable: true });
        }
 
        // Use the returned value as the class instead of the declared one
        _Apple = _Apple2;
      }
    // If Fruit.extended is present but not a function,
    } else {
      // complain about it
      throw new TypeError("Attempted to call extended, but it was not a function");
    }
  }
 
  // Return the class so it gets set as the variable
  return _Apple;
}();

NOTE: Actual implementation uses a helper function so this logic isn't repeated every single class declaration.

What?

Every class declaration with a superClass gets transformed into an expression that:

  • Creates the child class
  • calls SuperClass.extended(ChildClass) if present
  • evaluates to the return value of SuperClass.extended(ChildClass) if any, otherwise the created child class

Why?

This lets you hook class inheritance:

function register(klass){ ... } // Add to a map, wire things up, etc
 
class RegisteredItem {
  static extended(child) {
    console.log(`A new registered item class was created: ${child.name}`);
    register(child);
  }
}

It also lets you transform classes at inheritance time:

class Polyfill {
  // Whether we need to shim the native behavior
  static needsToBeShimmed() {
    return true;
  }
 
  // If we don't need to shim the native behavior, then
  // this is the native class that should be used instead
  static nativeClass() {
    return null;
  }
 
  static extended(child) {
    let { needsToBeShimmed, nativeClass } = child;
 
    if (!needsToBeShimmed()) {
      // If we return a value from extended, it will be used
      // as the value of the class declaration instead of child
      return nativeClass();
    }
  }
}
 
class Promise extends Polyfill {
  static needsToBeShimmed() {
    return !window.Promise;
  }
 
  static nativeClass() {
    return window.Promise;
  }
 
  constructor(func) {
    ...
  }
}
 
// Promise now refers to either window.Promise or the class defined by
// the Promise class declaration above, depending on if needsToBeShimmed()
// evaluated to true or false

Installation

$ npm install --save babel-plugin-transform-class-extended-hook

Usage

Via .babelrc (Recommended)

.babelrc

{
  "plugins": ["transform-class-extended-hook"]
}

Via CLI

$ babel --plugins transform-class-extended-hook script.js

Via Node API

require('babel-core').transform('code', {
  plugins: ['transform-class-extended-hook']
});

Thanks

install

npm i babel-plugin-transform-class-extended-hook

Downloadsweekly downloads

2

version

1.0.3

license

MIT

homepage

github.com

repository

Gitgithub

last publish

collaborators

  • avatar
Report a vulnerability