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>

 

posted @ 2017-06-08 23:06  无天666  阅读(279)  评论(0)    收藏  举报