js有六大数据类型:String,Number,Boolean,Null,Undefined,Object。其中前五项数据类型被称作原始类型。
typeof 操作符返回值有"string","number","boolean","undefined","object","function"。其中Null类型的返回值是"object",而Function作为Object类型的子类，返回的结果却是"function"
非数值类型转换为数值类型规则
//空字符串被转换为0console.log(Number(""));//0//字符串只包含数字,返回十进制数console.log(Number("19"));//19//字符串是十六进制形式，返回相等的十进制console.log(Number("0x19"));//25//字符串是浮点形式，返回浮点数console.log(Number("12.5"));//12.5//字符串是浮点形式，返回浮点数console.log(Number("1.234e2"));//123.4//除以上的字符串形式都返回NaNconsole.log(Number("a")); //NaN//Boolean true 返回1console.log(Number(true));//1//Boolean false 返回0console.log(Number(false));//0//数字形式，返回原参数console.log(Number(23));//23//Null类型返回0console.log(Number(null));//0//Undefined类型返回NaNconsole.log(Number(undefined));//NaN ????????对象类型转换有特殊的规则
将对象类型强转为Number类型时，先调用对象的valueOf()方法,如果方法的返回值是原始类型，那么就Number(valueOf的返回值),如果方法的返回类型是Object类型，那么会进一步调用对象的toString()方法。toString()方法的返回值如果是原始类型，则Number(toString的返回值)，如果toString()方法的返回值是Object类型，那么最终报错了
<script> ???var obj = { ???????name:"pmx", ???????age:20, ???????valueOf:function(){ ???????????console.log("value of"); ???????????return this.name; ???????}, ???????toString:function(){ ???????????console.log(‘to string‘); ???????????return this.name; ???????} ???}</script>Number(‘pmx‘)的结果为NaN
<script>var obj = { ???name:"pmx", ???age:20, ???valueOf:function(){ ???????console.log("value of"); ???????return null ???}, ???toString:function(){ ???????console.log(‘to string‘); ???????return this.name; ???}}</script>Number(null)的结果是0
<script>var obj = { ???name:"pmx", ???age:20, ???valueOf:function(){ ???????console.log("value of"); ???????return {} ???}, ???toString:function(){ ???????console.log(‘to string‘); ???????return this.name; ???}}</script>valueOf()返回的是非原始类型，那么调用obj的toString()方法，Number(‘pmx‘)结果为NaN
<script>var obj = { ???name:"pmx", ???age:20, ???valueOf:function(){ ???????console.log("value of"); ???????return {} ???}, ???toString:function(){ ???????console.log(‘to string‘); ???????return {}; ???}}</script>toString()的返回结果是Object类型，最后Number强制转换失败，报错了
<script>var obj = { ???name:"pmx", ???age:20, ???valueOf:function(){ ???????console.log("value of"); ???????return {} ???}, ???toString:function(){ ???????console.log(‘to string‘); ???????return { ???????????a:10, ???????????valueOf:function(){ ???????????????return 10; ???????????}, ???????????toString:function(){ ???????????????return ‘a‘ ???????????} ???????}; ???}}</script>toString()方法返回的对象中自定义了valueOf()方法和toString()方法，仍然报错，说明数值强转不会递归进行
js强转
原文地址：https://www.cnblogs.com/bibiafa/p/9376476.html