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 `${this.name} has no parser implemented`;
}
}
And then I want to extend like this
class ExtendedModule extends Module {
static name = 'Extended';
static version = {major: 1, minor:0}
static internalParser(data) {
//Some stuff
}
}
But when compiling in node with babel I get
true; if ('value' in descriptor) descriptor.writable = true; Object.defineProp
^
TypeError: Cannot redefine property: name
at Function.defineProperty (native)
Anyone got a clue if this is even possible or just plain wrong?
Trying out ES6 and tried to create a class with static properties and function for parsing. Then I want to extend the base parser for each different type I am parsing. Not sure if I am doing a anti-pattern but I cannot override static properties.
This is my base parser
class Module {
static name = 'Default Module'
static version = {major:10000, minor: 10000}
static checkVersion({majorVersion = 10000, minorVersion = 10000}) {
if(this.version.major !== majorVersion || this.version.minor > minorVersion) {
throw `${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?
Share Improve this question asked Oct 13, 2015 at 12:44 Johan KvintJohan Kvint 9171 gold badge8 silver badges17 bronze badges 5 |2 Answers
Reset to default 11You 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/
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.
Module.nameis"Module"(it's still a named constructor function). You cannot put another.nameon it. – Bergi Commented Oct 13, 2015 at 15:19.nameon it. That's great about Javascript: you can break things that easily, but you still can do. ;) – Thomas Urban Commented May 28, 2017 at 22:16Module. This property isn't writable, right, but configurable. And thus it is still possible to "put another.nameon it". – Thomas Urban Commented May 29, 2017 at 7:29