JavaScript装饰器

Posted 2022-01-26 小杰666

JS装饰器是ES7的新语法，浏览器，nodejs不一定支持，需要babel转译。

实验环境

创建一个目录test，结构为：

.
├── .babelrc
├── .npmrc
├── lib
├── package.json
└── src
    └── index.js

babel的装饰器配置：

# .babelrc

    "presets": [
        "@babel/preset-env"
    ],
    "plugins": [
        [
            "@babel/plugin-proposal-decorators",
            
                "legacy": true
            
        ],
        "@babel/plugin-proposal-class-properties"
    ]

依赖包与测试命令：

# package.json

  "name": "testmobx",
  "version": "1.0.0",
  "description": "testmobx",
  "main": "index.js",
  "directories": 
    "lib": "lib"
  ,
  "scripts": 
    "build": "babel src -d lib",
    "test": "node lib/index.js"
  ,
  "author": "jason",
  "license": "MIT",
  "devDependencies": 
    "@babel/core": "^7.15.5",
    "@babel/cli": "^7.15.0",
    "@babel/plugin-proposal-class-properties": "^7.14.5",
    "@babel/plugin-proposal-decorators": "^7.15.4",
    "@babel/preset-env": "^7.15.6"
  

npm源：

# .npmrc
registry=https://registry.npm.taobao.org

安装依赖包：

cd test
yarn

测试代码

源码都在 src/index.js 中。js装饰器可以装饰的对象：类、属性、方法。

// 装饰器：
// 装饰属性（或方法）时：
// target为属性的类对象，key为属性名字符串，descriptor为属性key的属性描述符对象
// 装饰类时：
// target为类，key与descriptor都是undefined

function readOnly(target, key, descriptor) 
    console.log('target:', target, typeof target);
    console.log('key:', key, typeof key);
    console.log('desc:', descriptor, typeof descriptor);
    if (typeof key === 'string')  // 作为属性或方法装饰器时，key为字符串
        if (typeof descriptor.value === 'function')  // 作为方法装饰器
            console.log('~~~ decorator of method');
            return descriptor.value;
         else  // 作为属性装饰器
            return 
                ...descriptor,
                writable: false // 覆盖为false
            
        
     else  // 作为类装饰器
        console.log('~~~ decorator of cls');
        return target;
    


class Oatmeal 
    @readOnly viscosity = 20; // 属性装饰器
    constructor(flavor) 
        this.flavor = flavor;
    


var oatmeal = new Oatmeal('Brown Sugar Cinnamon');
// oatmeal.viscosity = 30; // 已设为只读，修改将报错
console.log(oatmeal);

console.log('\\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~');

@readOnly
class A 
    constructor() 
        this.a = 111;
    
;

var a = new A();
console.log(a.a);

console.log('\\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~');

class B 
    constructor() 
        this.b = 222;
    
    @readOnly
    show() 
        console.log(this.b);
    


var b = new B();
b.show();

执行结果

其中initializer与babel相关，当装饰器给属性装饰时，属性描述符对象中会有initializer，为函数，执行descriptor.initializer()后将返回属性的值20。

参考：
https://segmentfault.com/a/1190000021593373