@babel/plugin-proposal-class-properties

@babel/plugin-proposal-class-properties

一种类的properties的另外一种书写方法，这里的class properties指的是类的实例方法或者实例属性、类的静态方法或者静态属性。特别注意，如果class properties 前面没有添加static，是将属性或者方法添加实例对象上，加上static，是表示类的静态方法或者属性。

 class Bork {
    //Property initializer syntax
    instanceProperty = "bork";
    boundFunction = () => {
      return this.instanceProperty;
    };

    //Static class properties
    static staticProperty = "babelIsCool";
    static staticFunction = function() {
      return Bork.staticProperty;
    };
  }

  let myBork = new Bork;

  //Property initializers are not on the prototype.
  console.log(myBork.__proto__.boundFunction); // > undefined

  //Bound functions are bound to the class instance.
  console.log(myBork.boundFunction.call(undefined)); // > "bork"

  //Static function exists on the class.
  console.log(Bork.staticFunction()); // > "babelIsCool"

安装

npm install --save-dev @babel/plugin-proposal-class-properties

用法（.babelrc或者其它）

{
  "plugins": ["@babel/plugin-proposal-class-properties"]
}

带有选项设置：

{
  "plugins": [
    ["@babel/plugin-proposal-class-properties", { "loose": true }]
  ]
}

options

loose：boolean，默认为false

当loose设置为true，代码被以赋值表达式的形式编译，否则，代码以Object.defineProperty来编译。

比如：

  class Bork {
    static a = 'foo';
    static b;

    x = 'bar';
    y;
  }

如果没有设置loose，或者设置loose为false，则将以Object.defineProperty形式编译：

var Bork = function Bork() {
  babelHelpers.classCallCheck(this, Bork);
  Object.defineProperty(this, "x", {
    configurable: true,
    enumerable: true,
    writable: true,
    value: 'bar'
  });
  Object.defineProperty(this, "y", {
    configurable: true,
    enumerable: true,
    writable: true,
    value: void 0
  });
};

Object.defineProperty(Bork, "a", {
  configurable: true,
  enumerable: true,
  writable: true,
  value: 'foo'
});
Object.defineProperty(Bork, "b", {
  configurable: true,
  enumerable: true,
  writable: true,
  value: void 0
});

如果设置了loose为true，则将会以赋值表达式模式编译：

var Bork = function Bork() {
  babelHelpers.classCallCheck(this, Bork);
  this.x = 'bar';
  this.y = void 0;
};

Bork.a = 'foo';
Bork.b = void 0;