// 创建字符串
var a = new String('123')
var a = '123'

a.length   // => 3, length属性返回字符串字符的个数
a[0]    // => 1, 索引 求指定索引的字符串
// charAt()方法，也是求指定索引的字符串
a.toUpperCase()  // 将字符串全部转为大写, 
// 例：var str = "hello world"; str.toUpperCase()
a.toLowerCase()  // 将字符串全部转为小写, 
// 例：var str = "HELLO WORLD"; str.toLowerCase()

var s = 'hello world!';
alert(s.indexOf('world')); //返回6
alert(s.indexOf('World')); //返回-1
alert(s.indexOf('o'));  //返回4
alert(s.indexOf('o',6));  //从第六位开始查找，返回7

var str1 = "hello ";
var res = str1.concat(" world","!");
alert(res);  //返回hello world！
alert(str1);  //返回hello

var str = "hello world!";
alert(str.slice(3,7));//返回lo w
alert(str.slice(3));//返回lo world!
alert(str.slice(9,5));//返回""
alert(str.slice(-7,-3));//负数与长度相加，即str.slice(5,9)返回 wor
alert(str.slice(5,9));//返回 wor

var str = "hello world!";
alert(str.substring(3,7));//返回lo w
alert(str.substring(3));//返回lo world!
alert(str.substring(9,5));//返回 wor,即str.substring(5,9),不包含第九项
alert(str.substring(-7,-3));//负数与长度相加，即str.substring(0,0)返回""
alert(str.substring(-7,3));//负数与长度相加，即str.substring(0,3)返回hel

var s = 'hello world!'
alert(s.substr(0, 5));//从索引0开始，截取5个字符串，返回hello
alert(s.substr(7)); //从索引7开始截取，一直到结束，返回orld!
alert(s.substr(-7,3));//负数与长度相加，即str.substr(5,3),返回 wo
alert(s.substr(-7,-3));//负数与长度相加，即str.substr(5,0),返回""

var s = "1,23,45";
var arr1 = s.split(",");
alert(arr1);//返回数组["1","23","45"]
var arr2 = s.split(",",2);
alert(arr2);//返回数组["1","23"]

var str1 = "  hello world ";
var str2 = str1.trim();
alert(str1);//返回"  hello world "
alert(str2);//返回"hello world"