Javascript es6 override static properties

Question

Trying out ES6 and tried to create a class with static properties and function for parsing. Then I want to extend the base parser for each different type I am parsing. Not sure if I am doing a anti-pattern but I cannot override static properties.

This is my base parser

class Module {

  static name = 'Default Module'
  static version = {major:10000, minor: 10000}

  static checkVersion({majorVersion = 10000, minorVersion = 10000}) {
    if(this.version.major !== majorVersion || this.version.minor > minorVersion) {
      throw `${this.name} requires version ${this.version.major}.${this.version.minor} got ${majorVersion}.${minorVersion}`;
    }
  }

  static parse(data) {
    try {
      this.checkVersion(data);
      return this.internalParser(data);

    } catch (e) {
      throw e;
    }
  }

  static internalParser(data) {
    throw `${this.name} has no parser implemented`;
  }
}

And then I want to extend like this

class ExtendedModule extends Module {
  static name = 'Extended';
  static version = {major: 1, minor:0}

  static internalParser(data) {
    //Some stuff
  }
}

But when compiling in node with babel I get

true; if ('value' in descriptor) descriptor.writable = true; Object.defineProp
                                                                    ^
TypeError: Cannot redefine property: name
    at Function.defineProperty (native)

Anyone got a clue if this is even possible or just plain wrong?

Answer 1

You might try using static getter to achieve the initially intended hierarchy in code:

class Module {
    static get name() { return "Default Module"; }
    static get version() { return {major:10000, minor: 10000}; }

    static parse() {
        console.log( this.name );
    }
}

class ExtendedModule extends Module {
    static get name() { return "Extended"; }
    static get version() { return {major:1, minor: 0}; }
}

ExtendedModule.parse();

Using BabelJS this becomes

"use strict";

var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();

function _possibleConstructorReturn(self, call) { if (!self) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return call && (typeof call === "object" || typeof call === "function") ? call : self; }

function _inherits(subClass, superClass) { if (typeof superClass !== "function" && superClass !== null) { throw new TypeError("Super expression must either be null or a function, not " + typeof superClass); } subClass.prototype = Object.create(superClass && superClass.prototype, { constructor: { value: subClass, enumerable: false, writable: true, configurable: true } }); if (superClass) Object.setPrototypeOf ? Object.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass; }

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Module = function () {
    function Module() {
        _classCallCheck(this, Module);
    }

    _createClass(Module, null, [{
        key: "parse",
        value: function parse() {
            console.log(this.name);
        }
    }, {
        key: "name",
        get: function get() {
            return "Default Module";
        }
    }, {
        key: "version",
        get: function get() {
            return { major: 10000, minor: 10000 };
        }
    }]);

    return Module;
}();

var ExtendedModule = function (_Module) {
    _inherits(ExtendedModule, _Module);

    function ExtendedModule() {
        _classCallCheck(this, ExtendedModule);

        return _possibleConstructorReturn(this, (ExtendedModule.__proto__ || Object.getPrototypeOf(ExtendedModule)).apply(this, arguments));
    }

    _createClass(ExtendedModule, null, [{
        key: "name",
        get: function get() {
            return "Extended";
        }
    }, {
        key: "version",
        get: function get() {
            return { major: 1, minor: 0 };
        }
    }]);

    return ExtendedModule;
}(Module);

ExtendedModule.parse();

Running code it is displaying

Extended

on JS console.

• ES5-fiddle: https://jsfiddle.net/dwq698r8/
• ES6-fiddle: https://jsfiddle.net/yd8bf7am/

Answer 2

Classes are functions (in transpiled code), and when you define static properties, they are attached directly to the class constructor function, so:

class Foo {
    static name = 'foo';
}

is the same as doing

function Foo(){}
Object.defineProperty(Foo, 'name', {
    configurable: true,
    writable: true,
    value: 'foo'
});

If you try doing that in your browser, you will get an error, which is exactly what you are seeing. This is because the function already has a property called name and it is Foo. In ES5, the name property was configurable: false, so what you are trying to do will not work, hence the TypeError: Cannot redefine property: name error and you need to rename your static to something else.

In ES6, name is actually configurable: true so what you are trying to do will work eventually, but browsers need to update themselves first.

The bigger question here is why you need to use a class. If you are using all static variables, you might as well just use a module that exports everything directly without the class, and wrap it. It have a module that exports a creation function that you pass an innerParser method or something. Your current code way over-uses classes.