JS基础一

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JS基础一

变量类型：typeof()运算符

<!DOCTYPE HTML><html><head><meta charset='utf-8'><title>变量</title><script>var a;//a=12;//alert(typeof a);    //number//a='adsd';//alert(typeof a);//string//a=3.3;//alert(typeof a);//number// a=true;// alert(typeof a);//boolean//alert(typeof b);//undefined// a=document;// alert(typeof a);//object//alert(typeof a);//undefineda=function aaa(){alert(1);}alert(typeof a);//function</script></head></html>

强制类型转换parserInt(),parsetFloat()

<!DOCTYPE HTML><html><head><meta charset='utf-8'><title>变量</title><script>window.onload=function(){var oTxt1=document.getElementById('txt1');var oTxt2=document.getElementById('txt2');var oBtn=document.getElementById('btn');oBtn.onclick=function(){alert(parseInt(oTxt1.value)+parseInt(oTxt2.value));};};</script></head><body><input id='txt1' type="text"><input id='txt2' type="text"><input id='btn' type="button" value="求和"></body></html>

隐示类型转换：===不转换直接比较，==先转换再比较

<!DOCTYPE HTML><html><head><meta charset='utf-8'><title>NaN</title><script>var a=1;var b='1';function aaa(){alert(a===b);}function bbb(){alert(a==b);}aaa();bbb();</script></head></html>

NaN!=NaN

12+NaN=NaN

isNan()判断是否为NaN

<!DOCTYPE HTML><html><head><meta charset='utf-8'><title>变量</title><script>window.onload=function(){var oTxt1=document.getElementById('txt1');var oTxt2=document.getElementById('txt2');var oBtn=document.getElementById('btn');oBtn.onclick=function(){var n1=parseInt(oTxt1.value);var n2=parseInt(oTxt2.value);if(isNaN(n1)&&!isNaN(n2)){alert("第一个数有误！");}else if(isNaN(n2)&&!isNaN(n1)){alert("第二个数有误！");}else if(isNaN(n1)&&isNaN(n2)){alert('两个数字都有误！');}else{ alert(parseInt(oTxt1.value)+parseInt(oTxt2.value));}};};</script></head><body><input id='txt1' type="text"><input id='txt2' type="text"><input id='btn' type="button" value="求和"></body></html>

变量命名规则

类前缀+首字母大写的名字

比如：类前缀 a 数组，o 对象。

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