javascript笔记

1.数值转换
NaN 特殊的非数字值
2.字符串的使用
msg = "Hello,"+"world";
msg.length;
last_char = msg.charAt(s.length-1);
sub = msg.substring(1,4);
i = msg.indexOf('a');
3.把数字转为字符串
var n = 100;
var s = n+" bottles of beer on the wall.";
(1)技巧，把数字转为字符串
var n_as_string = n + "";
(2)使用String()函数;
var string_value = String(number);
(3利用toString()方法;
string_value = number.toString();
里面可以传参数，2，8，16代表为进制，如
var n = 17;
binary_string = n.toString(2); //value is "10001"
octal_string = "0"+n.toString(8); //value is "021"
hex_string = "0x"+n.toString(16); //value is "0x11"
(4)toFixed()方法把一个数字转换为字符串
var n = 123456.789;
n.toFixed(0); //1234567
n.toFixed(2); //123456.79
n.toExponential(1); //1.2e+5
4.把字符串转换为数字
var product = "21"*"2" ; //value is number 42
(1) var number = Number(string_value); //它不允许出现任何非空字符
(2) parseInt,parseFloat
parseInt("3 blind mice"); // value is 3
parseFloat("3.14 meters"); // value is 3.14
parseInt("eleven"); //value is NaN
parseFloat("$72.14");