JavaScript初学学习笔记2：JavaScript隐式类型转化详解

傷城~ 2023-02-28 08:55 98阅读 0赞

## 前言 ##

上一篇初步介绍了显显式类型转化  
这一篇的隐式类型转化是承接上一篇的  
[JavaScript初学 学习笔记1：JavaScript变量，数据类型以及显式类型转换][JavaScript_ _1_JavaScript]  
隐式类型转化背后，内部调用的都是显式转化的方法

## 隐式类型转化 ##

### isNaN() ###

这个看看就懂了，其中内部进行了Number()的调用

console.log(isNaN(undefined));
    console.log(isNaN(NaN));
    console.log(isNaN("a"));
    		
    console.log(isNaN(null));
    console.log(isNaN(false));
    onsole.log(isNaN(true));
    console.log(isNaN(123));
    console.log(isNaN("123"));

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70]

### “++”"- -""+""-" ###

加加，减减，一元正负运算  
内部调用也是Number()

var a = "123";
    a ++;
    console.log(typeof(a) + " : " + a);
    
    var b = "abc";
    b --;
    console.log(typeof(b) + " : " + b);
    
    var c = null;
    c ++;
    console.log(typeof(c) + " : " + c);
    
    var d = "a";
    d = +d;
    console.log(typeof(d) + " : " + d);
    
    var e = "123";
    e = -e;
    console.log(typeof(e) + " : " + e);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 1]

### “+”加号 ###

加号有一点比较特殊，当加号两边有一侧是字符串的话，那么表达式得到的就是字符串，也就是隐式调用了String()

var a = "a" + 1;
    console.log(typeof(a) + " : " + a); 
    
    var b = 1 + "";
    console.log(typeof(b) + " : " + b);
    
    var c = null + "abc";
    console.log(typeof(c) + " : " + c);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 2]

### “-” “\*” “/” “%” ###

减号，乘号，除号，mod，他们隐式调用的也是Number()

var a = "a" * 1;
    console.log(typeof(a) + " : " + a); 
    
    var b = 1 - "";
    console.log(typeof(b) + " : " + b);
    
    var c = "2" / null;
    console.log(typeof(c) + " : " + c);
    
    var d = "1" - 1;
    console.log(typeof(d) + " : " + d);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 3]

### && || ！ if() ###

与， 或， 非 ，if() 条件判断，均是调用给了Boolean()  
这里就不做演示了

### “>” “<” “>=” “<=” ###

比较大小的判断，若比较符两边有一个是数字，就会进行Number转化,同时返回boolean类型的值

var a = "a" > 1;
    console.log(typeof(a) + " : " + a); 
    
    var b = 1 < "";
    console.log(typeof(b) + " : " + b);
    
    var c = "2" > null;
    console.log(typeof(c) + " : " + c);
    
    var d = "1" > 1;
    console.log(typeof(d) + " : " + d);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 4]

### “==” “!=” ###

var a = "1" == 1;
    console.log(typeof(a) + " : " + a); 
    
    var b = 0 == "";
    console.log(typeof(b) + " : " + b);
    
    var c = 0 != null;
    console.log(typeof(c) + " : " + c);
    
    var d = 0 == false;
    console.log(typeof(d) + " : " + d);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 5]  
补充最特殊的：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 6]

null == undefined 结果为true  
null === undefined 结果为false  
NaN == NaN 结果为false

最后，再来给你们看看一个东西吧  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xlb3dhaGFoYQ_size_16_color_FFFFFF_t_70 7]  
这个是为什么，就请自己思考了

Ps：若有错误或者不足，欢迎向我指出

[JavaScript_ _1_JavaScript]: https://blog.csdn.net/leowahaha/article/details/107436394
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