a-0,a*1,a/1都可以:var a="123.45"; alert(a+111.11);//It is a string alert((a-0)+111.11);//It is a number alert((a*1)+111.11);//It is a number alert((a/1)+111.11);//It is a number alert(parseFloat(a)+111.11);//It is a number alert(eval(a)+111.11);//It is a number
a=eval(a);
alert(++a);
alert(a+111.11);//It is a string
alert((a-0)+111.11);//It is a number
alert((a*1)+111.11);//It is a number
alert((a/1)+111.11);//It is a number
alert(parseFloat(a)+111.11);//It is a number
alert(eval(a)+111.11);//It is a number