let

块级作用域 重复声明

function outputNum(count){
//块级作用域
  (function(){
    for(var i = 0; i < count; i ++){
      console.log(i)
    }
  })()
  console.log(i)
}
outputNum(5)

const

function _const(key, value) {
	window[key] = value
	Object.defineProperty(window, key, {
		enumerable: false,
		configurable: false,
		get: function () {
			return value
		},
		set: function(newValue) {
			if (newValue !== value) {
				throw TypeError('这是只读变量，不可修改')
			} else {
				return value
			}
		}
	})
}