JavaScript动态代理的各种用法详解

const target = {};
const handler = {
  get: function(target, prop) {
    return prop in target ? target[prop] : 37;
  }
};

const proxy = new Proxy(target, handler);
proxy.a = 1;
console.log(proxy.a); // 1
console.log(proxy.b); // 37 (handler中定义的默认值)

const emptyObject = new Proxy({}, {
  get(target, prop) {
    return undefined; // 所有属性访问返回undefined
  },
  set(target, prop, value) {
    return false; // 阻止所有赋值操作
  },
  has(target, prop) {
    return false; // 所有in操作返回false
  }
});

console.log(emptyObject.anyProperty); // undefined
emptyObject.a = 1; // 静默失败
console.log('a' in emptyObject); // false

const person = {
  name: 'John',
  age: 30
};

const proxy = new Proxy(person, {
  get(target, prop) {
    if (prop === 'age') {
      return `Age is ${target[prop]}`;
    }
    return target[prop] || `Property "${prop}" does not exist`;
  }
});

console.log(proxy.name); // "John"
console.log(proxy.age); // "Age is 30"
console.log(proxy.job); // "Property "job" does not exist"

const handler = {
  // 拦截属性读取
  get(target, prop, receiver) {
    console.log(`Getting ${prop}`);
    return Reflect.get(...arguments);
  },
  
  // 拦截属性设置
  set(target, prop, value, receiver) {
    console.log(`Setting ${prop} to ${value}`);
    return Reflect.set(...arguments);
  },
  
  // 拦截in操作符
  has(target, prop) {
    console.log(`Checking if ${prop} exists`);
    return Reflect.has(...arguments);
  },
  
  // 拦截delete操作
  deleteProperty(target, prop) {
    console.log(`Deleting ${prop}`);
    return Reflect.deleteProperty(...arguments);
  },
  
  // 拦截Object.keys等操作
  ownKeys(target) {
    console.log('Getting own keys');
    return Reflect.ownKeys(...arguments);
  },
  
  // 拦截函数调用（当代理目标是函数时）
  apply(target, thisArg, argumentsList) {
    console.log('Function called with', argumentsList);
    return Reflect.apply(...arguments);
  },
  
  // 拦截new操作符
  construct(target, argumentsList, newTarget) {
    console.log('Constructor called with', argumentsList);
    return Reflect.construct(...arguments);
  }
};

const obj = new Proxy({}, handler);
obj.a = 1; // 日志: Setting a to 1
console.log('a' in obj); // 日志: Checking if a exists
delete obj.a; // 日志: Deleting a

const validator = {
  set(target, prop, value) {
    if (prop === 'age') {
      if (typeof value !== 'number' || value <= 0) {
        throw new TypeError('Age must be a positive number');
      }
    }
    target[prop] = value;
    return true;
  }
};

const person = new Proxy({}, validator);
person.age = 30; // 正常
person.age = 'thirty'; // 抛出TypeError

const autoFiller = {
  get(target, prop) {
    if (!(prop in target)) {
      target[prop] = {};
    }
    return target[prop];
  }
};

const obj = new Proxy({}, autoFiller);
obj.a.b.c = 'value'; // 自动创建嵌套结构
console.log(obj); // { a: { b: { c: 'value' } } }

function createNegativeArray(array) {
  return new Proxy(array, {
    get(target, prop, receiver) {
      if (typeof prop === 'string') {
        const index = parseInt(prop, 10);
        if (index < 0) {
          prop = target.length + index;
        }
      }
      return Reflect.get(target, prop, receiver);
    }
  });
}

const array = createNegativeArray([1, 2, 3, 4, 5]);
console.log(array[-1]); // 5
console.log(array[-2]); // 4

const chainable = {
  get(target, prop) {
    if (prop in target) {
      return target[prop];
    }
    
    return function(...args) {
      console.log(`Called ${prop} with args: ${args}`);
      return target; // 返回自身以实现链式调用
    };
  }
};

const api = new Proxy({}, chainable);
api.method1().method2(1, 2).method3('a', 'b');
// 输出:
// Called method1 with args: 
// Called method2 with args: 1,2
// Called method3 with args: a,b