变量的作用域和闭包

• 什么是闭包?

• 闭包是一个函数,用于捕获作用域内的外部绑定。

  //  * @param FACTOR
  //  * @returns {Function}
  //factor是createScaleFunction函数内的变量,却能在该函数返回后仍然使用
  //它从高阶函数中return实现"越狱"
  function createScaleFunction(FACTOR) {
      var factor = FACTOR;
      return function (array) {
          return array.map(function (item) {
              return item * factor;
          })
      }
  }
  var scale10 = createScaleFunction(10);
  console.log(scale10([1, 2, 3, 4, 5]));  //10, 20, 30, 40, 50
  var scale2 = createScaleFunction(2);
  console.log(scale2([1, 2, 3, 4, 5]));   //2,4,6,8,10

  // * @returns {Function}

  //利用闭包封装私有数据
  //实现一个自增计数器
  function increment() {
      var counter = 0;
      return function () {
          return ++counter;
      }
  }
  // console.log(counter);  //error,访问不到
  var counter = increment();
  console.log(counter()); //1
  console.log(counter()); //2
  console.log(counter()); //3

  // * 实现bind函数

  function.prototype.bind = function () {
      var args = [].slice.apply(arguments);
      var context = args.shift();
      var self = this;   //保存原函数
      return function () {
          var args2 = args.concat([].slice.apply(arguments));
          return self.apply(context, args2);
      }
  }
  var func = function (a, b, c, d) {
      console.log(this.name);        //21cm
      console.log([a, b, c, d]);     //3,4,1,2
  }.bind({name: '21cm'}, 3, 4);
  func(1, 2);