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  • es64 const

    <!DOCTYPE html>
    <html lang="en">
    <head>
    <meta charset="UTF-8">
    <title>const命令</title>
    <script src="../../../vendor/traceur.js"></script>
    <script src="../../../vendor/bootstrap.js"></script>
    <script type="text/traceur">
        const Pi = 3.1415926535;//只能赋一次值
        console.log(Pi);
        Pi = 3; // Pi is read-only -- Error
        console.log(Pi);
        
        
        const Pi = 3.1415926535;
        console.log(Pi);    //3.1415926535
        console.log(5 * Pi);//15.7079632675
        
        if (true) {
            const Pi = 3.14159265;
        };
        console.log(Pi);    //Pi is not defined
        
        var Pi = 88;
        if (true) {
            console.log(Pi);    //undefined
            const Pi = 3.14159265;
            console.log(s);
        };
    </script>
    </head>
    <body>
    
    </body>
    </html>
    <!DOCTYPE html>
    <html lang="en">
    <head>
    <meta charset="UTF-8">
    <title>const不可重复声明</title>
    <script src="../../../vendor/traceur.js"></script>
    <script src="../../../vendor/bootstrap.js"></script>
    <script type="text/traceur">
        {
            var a   = 100;
            const a = 200;
            console.log(a);//报错,局部不可重复声明
        }
        
        const person = {};//person是地址
        person.name  = "Zhangsan";
        person.age   = 30;
        console.log(person.name);    //Zhangsan
        console.log(person.age);    //30
        console.log(person);        //Object {name: "Zhangsan", age: 30}
        person = {};    //person is read-only
    </script>
    </head>
    <body>
    
    </body>
    </html>
    <!DOCTYPE html>
    <html lang="en">
    <head>
    <meta charset="UTF-8">
    <title>const数组</title>
    <script src="../../../vendor/traceur.js"></script>
    <script src="../../../vendor/bootstrap.js"></script>
    <script type="text/traceur">
        const arr = [];
        console.log(arr);//[]
        console.log(arr.length);//0
        console.log("------");
        arr.push("Hello world!");
        console.log(arr);//["Hello world!"]
        console.log(arr.length);//1
        console.log("------");
        arr.length = 0;//清空数组
        console.log(arr);//[]
        console.log(arr.length);
        console.log("------");
        // 错误用法
        arr = ["Hello Everyone!"];
        
        
        
        const person = Object.freeze({kk:'kk'});//const对象冻结,对象里面的属性都不能改变
        person.name  = "Zhangsan";
        person.age   = 30;
        console.log(person.kk);    //kk
        console.log(person.name);    //undefined
        console.log(person.age);    //undefined
        console.log(person);        //Object {kk: "kk"}
        
        
        
        const person = Object.freeze({//属性只读
            name: "Zhangsan",
            age : 30
        });
        console.log(person.name);    //Zhangsan
        console.log(person.age);    //30
        console.log(person);        //Object
        
        
        
        //冻结函数
        var constantize = (obj) => {
            Object.freeze(obj);
            Object.keys(obj).forEach( (key, value) => {
                if ( typeof obj[key] === 'object' ) {
                    constantize( obj[key] );//key也冻结
                };
            });
        };
    </script>
    </head>
    <body>
    
    </body>
    </html>
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  • 原文地址:https://www.cnblogs.com/yaowen/p/6965250.html
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