isNaN(val) val为数字返回false 不为数字返回true
document.write(isNaN(123));
document.write(isNaN(-1.23));
document.write(isNaN(5-2));
document.write(isNaN(0));
document.write(isNaN("Hello"));
document.write(isNaN("2005/12/12"));
false
false
false
false
true
true