es6 const

2022-10-28 12:41:31
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <body>
        <script src="js/browser.min.js" type="text/javascript" charset="utf-8"></script>
        <script type="text/javascript">
            'use strict';
            //const用来声明常量
            const PI = 3.1415926;

            //const PI = 3;    //报错
            //var PI;    //报错
            //const foo;    //必须立即初始化

            //const只对声明的块级作用域内有效
            if(true){
                const MAX = 6;
            }
            //alert(MAX);    //MAX is not defined

            var message = "hello";
            var age = 25;
            //不能重复声明变量
            //const message = "aaa";    //报错
            //let age = 26;    //报错

            //如果是对象或者数组，只能保证地址不变，但是变量的值可以变
            const foo = {};

            foo.prop = 123;
            console.log(foo.prop);    //123

            //foo = {}//地址改变了，所以报错了

            //如果真的想冻结一个对象，可以使用Object.freeze
            const foo2 = Object.freeze({});
            //foo2.prop = 123;    //报错

            //对象彻底冻结的方法，即属性也冻结
            var constantize = (obj) =>{
                Object.freeze(obj);
                Object.keys(obj).forEach(key , value) =>{
                    if(typeof obj[key] === 'object'){
                        constantize(obj[key]);
                    }
                }
            }
            //没理解这上面写的啥玩意

        </script>
    </body>
</html>
码农公寓

相关文章
