声明提前和函数声明/变量声明权重

傷城~ 2022-01-21 12:01 1236阅读 0赞

1.同名的变量声明，JavaScript采用的是忽略原则，后声明的会被忽略，变量声明和赋值操作只有声明会被提前，默认为undefined。

console.log(a);
    var a = 1;
    console.log(a);
    var a = 2;
    
    undefined
    1

2.同名的函数声明JavaScript采用的是覆盖原则，后声明的会覆盖前面的。

console.log(a())
    function a(){console.log(1)};
    function a(){console.log(2)};
    
    2

3.同名的函数声明和变量声明，函数声明会提升到变量声明之前，且赋值之后，不再指向函数。

console.log(a);
    var a = 1;
    console.log(a);
    function a(){console.log(1)};
    console.log(a);
    
    ƒ a(){console.log(1)}
    1
    1

通过上面的说明，我们来看一道题：

参考文章：[https://www.cnblogs.com/bugda/p/6036282.html][https_www.cnblogs.com_bugda_p_6036282.html]

var a=100;
    function a(){
    　　console.log(a);
    }
    a();

这个串代码执行完会报错 : a is not a function

上面的代码等同于 :

var a=function (){
    　　console.log(a);
    }
    var a=100;
    a();

相当于给a重新赋值了,所以会报错.

再看一个例子：

第二个难题是处理重载函数标识符的问题。
    console.log('---------------------重载函数标识符--------------------');
    //fun指向一个函数
    if (typeof fun === 'function') {
      console.log('We access the function.');
    }
    //定义变量fun并赋值为数字3
    var fun = 3;
    //fun指向一个数字
    if (typeof fun === 'number') {
      console.log('Now we access the number.');
    }
    //函数声明
    function fun() {}
     
    //fun仍然指向数字
    if (typeof fun === 'number') {
      console.log('Still a number.');
    }

输出结果：

![在这里插入图片描述][2019061009395169.png]

更多可以参考文章：  
[https://blog.csdn.net/u014465934/article/details/88934610][https_blog.csdn.net_u014465934_article_details_88934610]  
[https://blog.csdn.net/u014465934/article/details/83986685][https_blog.csdn.net_u014465934_article_details_83986685]

[https://segmentfault.com/a/1190000014831240][https_segmentfault.com_a_1190000014831240]  
[https://www.cnblogs.com/isaboy/p/javascript\_function.html][https_www.cnblogs.com_isaboy_p_javascript_function.html]

[https://blog.csdn.net/zhangying1994/article/details/85267091][https_blog.csdn.net_zhangying1994_article_details_85267091]

[https_www.cnblogs.com_bugda_p_6036282.html]: https://www.cnblogs.com/bugda/p/6036282.html
[2019061009395169.png]: /images/20220121/e5d884df59864656bd189856acac1a2e.png
[https_blog.csdn.net_u014465934_article_details_88934610]: https://blog.csdn.net/u014465934/article/details/88934610
[https_blog.csdn.net_u014465934_article_details_83986685]: https://blog.csdn.net/u014465934/article/details/83986685
[https_segmentfault.com_a_1190000014831240]: https://segmentfault.com/a/1190000014831240
[https_www.cnblogs.com_isaboy_p_javascript_function.html]: https://www.cnblogs.com/isaboy/p/javascript_function.html
[https_blog.csdn.net_zhangying1994_article_details_85267091]: https://blog.csdn.net/zhangying1994/article/details/85267091