闭包问题中函数立即执行

Question

var Counter = (function() {
  var privateCounter = 0;
  function changeBy(val) {
    privateCounter += val;
  }
  return {
    increment: function() {
      changeBy(1);
    },
    decrement: function() {
      changeBy(-1);
    },
    value: function() {
      return privateCounter;
    }
  }   
})();

console.log(Counter.value()); /* logs 0 */
Counter.increment();
Counter.increment();
console.log(Counter.value()); /* logs 2 */
Counter.decrement();
console.log(Counter.value()); /* logs 1 */

请问为什么Counter（）要立即执行才能运行正确？

Answer 1

如果不立即执行，要达到你上面代码的效果，应该就要这样写了。 不立即执行的话只是声明了个函数。

var Counter = function() {
  var privateCounter = 0;
  function changeBy(val) {
    privateCounter += val;
  }
  return {
    increment: function() {
      changeBy(1);
    },
    decrement: function() {
      changeBy(-1);
    },
    value: function() {
      return privateCounter;
    }
  }   
}
var obj = Counter();
console.log(obj.value());
obj.increment();
obj.increment();
console.log(obj.value());
obj.decrement();
console.log(obj.value());