使用BabelJS 6访问类的静态属性 [英] Get access to the class static properties using BabelJS 6

问题描述

下面是演示该问题的最小应用程序:

Below is the minimal app which demonstrate the issue:

'use strict';

var _ = require('underscore');

class Model
{
    constructor(value) {
        this._value = value;
    }

    get value() {
        return this._value;
    }

    toJS() {
        return this.value;
    }
}

class ObjectModel extends Model
{
    static properties = {};

    constructor(value) {
        super(_.extend({}, new.target.properties, _.pick(value, _.keys(new.target.properties))));
    }
}

class PostModel extends ObjectModel
{
    static properties = {
        title: 'Hello'
        /* content: '<p>Lorem ipsum dolor sit amet</p>' */
    };
}

console.log(new PostModel({title: 'Nice day', aa: 11, bb: 22}).toJS());

它应该产生{title: 'Nice day'}.相反，它甚至无法编译.我明白了:

It should produce {title: 'Nice day'}. Instead it even not compiles. I get this:

$ babel app.js
SyntaxError: app.js: 'this' is not allowed before super()

我了解为什么要对对象属性执行此操作.但是我不明白为什么还要对类变量执行此操作.

I understand why this was done for object properties. But I can't understand why this also was done for class variables.

在BabelJS 5中，我使用了完成这项工作的技巧:

In BabelJS 5 I used this trick which did the job:

class ObjectModel extends Model
{
    static properties = {};

    constructor(value) {
        if (0) { super(); }
        super(_.extend({}, this.constructor.properties, _.pick(value, _.keys(this.constructor.properties))));
    }
}

在版本6中，它会编译，但是在运行时会产生错误:

In version 6 it compiles, but when running it produces an error:

Uncaught TypeError: Cannot read property 'constructor' of undefined

在调用super之前是否有某种方法可以访问类的静态变量?不能使用init()之类的东西代替constructor.也许创建自定义转换插件?

Is there some way to get access to a class static variables before calling super? Using something like init() instead of a constructor is not an option. Maybe creating custom transformation plugin?

系统详细信息:

$ babel --version
6.2.0 (babel-core 6.2.1)

$ cat .babelrc
{
    "presets": ["es2015", "stage-1"]
}

$ babel-doctor

Babel Doctor
Running sanity checks on your system. This may take a few minutes...

✔ Found config at /path/to/.babelrc
✔ No duplicate babel packages found
✔ All babel packages appear to be up to date
✔ You're on npm >=3.3.0

Everything looks all right!

推荐答案

解决方案如下:

1. 按照 @loganfsmyth :

class ObjectModel extends Model
{
    static properties = {};

    constructor(value) {
        super(_.extend({}, new.target.properties, _.pick(value, _.keys(new.target.properties))));
    }
}

创建一个将所有new.target(ES6)转换为this.constructor(ES5)的编译器:

Create a transpiler which converts all of new.target (ES6) into this.constructor (ES5):

function transpileNewTarget()
{
    return {
        visitor: {
            MetaProperty(path) {
                if (path.isMetaProperty() && path.node.meta.name == 'new' && path.node.property.name == 'target') {
                    path.replaceWithSourceString('this.constructor');
                }
            }
        }
    };
}

这篇关于使用BabelJS 6访问类的静态属性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！