javascript基础知识梳理-显示类型转换

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<!DOCTYPE html><html><head><meta charset="UTF-8"><title>Insert title here</title><script type="text/javascript">//当使用一个值为undefined的变量时,如果用var关键字定义了,但是没有初始化,用这个变量没问题//但是如果没有用var关键字定义,直接使用就会报错(只测试了google浏览器)//但是如果访问一个对象的某个不存在的属性,就不会报错。if (this.o1) {console.log("o1为true");} else {console.log("o1为false");}//显示类型转换函数,String(),Boolean(),Number(),Object(),转换规则与隐式类型转换完全相同//将null通过String函数转换成stringvar result = String(null);console.log("将null转换成string,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (null == result));//将undefined通过String函数转换成stringresult = String(undefined);console.log("将undefined转换成string,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:"+ (undefined == result));//将NaN通过String函数转换成stringresult = String(NaN);console.log("将NaN转换成string,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (NaN == result));//将true通过String函数转换成stringresult = String(true);console.log("将true转换成string,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (true == result));//将null通过Number函数转换成numberresult = Number(null);console.log("将null转换成number,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (null == result));//将undefined通过Number函数转换成numberresult = Number(undefined);console.log("将undefined转换成number,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:"+ (undefined == result));//将true通过Number函数转换成numberresult = Number(true);console.log("将true转换成number,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (true == result));//将"123"通过Number函数转换成numberresult = Number("123");console.log("将'123'转换成number,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + ("123" == result));//将"12a"通过Number函数转换成numberresult = Number("12a");console.log("将'12a'转换成number,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + ("12a" == result));//将null通过Boolean函数转换成booleanresult = Boolean(null);console.log("将null转换成boolean,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (null == result));//将undefined通过Boolean函数转换成booleanresult = Boolean(undefined);console.log("将undefined转换成boolean,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:"+ (undefined == result));//将null通过Object函数转换成object,最后会返回一个空的对象{}result = Object(null);console.log("将null转换成object,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (null == result));//将undefined通过Object函数转换成object,最后会返回一个空的对象{}result = Object(undefined);console.log("将undefined转换成object,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:"+ (undefined == result));//将123通过Object函数转换成object,其实就相当于result = new Number(123);result = Object(123);console.log("将123转换成object,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (123 == result));//将true通过Object函数转换成object,其实就相当于result = new Boolean(true);result = Object(true);console.log("将true转换成object,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + (true == result));//将"123"通过Object函数转换成object,其实就相当于result = new String("123");result = Object("123");console.log("将'123'转换成object,转换后类型为:" + (typeof result) + ",值为:" + result + ",转换前后的值是否相等:" + ("123" == result));</script></head><body></body></html>
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