标签:func def lse printing throw 常量 col class log
1. let和var是声明变量的,const是声明常量的
2. let和const不存在变量提升
3. let 和const不允许重复声明
4. 块级作用域
块级作用域存在于
if(true) { let s = 1;
const j = 2 } console.log(s) // 报错 s is not defined
console.log(j) // 报错 j is not defined
5. let和const存在暂时性死区
let s = 1; let Printing = function() { console.log(s); // 报错 s is not defined let s = 2; }
6. 顶层对象中的let 和 const
从es6开始,全局变量(例如:import、class、let、const声明的)将逐步与顶层对象的属性隔离。
//顶层对象中的var var foo = {name: ‘lee‘, age: 18}; cosole.log(foo === window.foo); //true console.log(window.foo); //{name: ‘lee‘, age: 18} //顶层对象中的let和const let obj = {height: 182, weight: 140}; const mt = {msg:'hello’}; console.log(obj === window.obj); //false console.log(window.obj); //undefined console.log(mt === window.mt); //false console.log(window.mt); //undefined
tip:
const声明的常量只是指针不能改变,如果const声明的是引用类型,我们是可以改变其中的值的
const arr = [1,2,3]; arr.push(4); console.log(arr); [1,2,3,4]
如何找到顶层对象
//方法一 (typeof window !== ‘undefined‘ ? window : (typeof process === ‘object‘ && typeof require === ‘function‘ && typeof global === ‘object‘ ) ? global : this) //方法二 var getGlobal = function(){ if(typeof self !== ‘undefined‘) {return self;} if(typeof window !== ‘undefined‘) {return window;} if(typeof global !== ‘undefined‘) {return global;} throw new Error(‘unable to locate global object‘); };
标签:func def lse printing throw 常量 col class log
原文地址:https://www.cnblogs.com/zhangshilei/p/12188401.html