为一个对象提供一个代用品或占位符，以便控制对它的访问

虚拟代理

//真正的图片
let myImage = (function () {
  let imgNode = document.createElement('img');
  document.body.appendChild(imgNode);
  return {
    setSrc: function (src) {
      imgNode.src = src;
    }
  }
})();
//图片代理
let proxyImage = (function () {
  let img = new Image;
  img.onload = function () {
    myImage.setSrc(this.src);
  }
  return {
    setSrc: function (src) {
      myImage.setSrc('./loading.svg');
      img.src = src;
    }
  }
})();
proxyImage.setSrc('http://www.cdhrsip.com/static/imgs/high-tech/banner.png?version=201512141756');

缓存代理

let mult = function () {
  console.log('开始计算乘积');
  let a = 1;
  for (let i = 0, l = arguments.length; i < l; i++) {
    a = a * arguments[i];
  }
  return a;
};
let proxyMult = (function () {
  let cache = {};
  return function () {
    let args = Array.prototype.join.call(arguments, ',');
    if (args in cache) {
      return cache[args];
    }
    return cache[args] = mult.apply(this, arguments);
  }
})();
console.log(proxyMult(1, 2, 3, 4)); // 输出：24
console.log(proxyMult(1, 2, 3, 4)); // 输出：24