1.charAt

• 返回指定位置字符，a.charAt(1)等价于a[1]

var a = "123abc";
console.log(a.charAt(1));//2
console.log(a[1]);//2

2.slice

• 返回一个子字符串，不改变原字符串

var a = "123abc";
console.log(a.slice(1, 4));//23a
console.log(a.slice(1));//23abc
console.log(a.slice(-5, -1));//23ab

3.substring

• 与slice方法相似，若第二个参数大于第一个参数，会自动更换两个参数的位置，参数不为负

var a = "123abc";
console.log(a.substring(1, 4));//23a
console.log(a.substring(1));//23abc
console.log(a.substring(4, 1));//23a

4.substr

• 与slice方法相似，第一个参数为起始位置，第二个参数为截取长度。若第二个参数为负，自动转为0，返回空字符串

var a = "123abc";
console.log(a.substr(1, 4));//23a
console.log(a.substr(1));//23abc
console.log(a.substr(4, 1));//23a
console.log(a.substr(1, -1));//空字符串

5.indexOf

• 返回查询字符串第一次出现的位置，没有返回-1

var a = "123abc";
console.log(a.indexOf('a'));//3
console.log(a.indexOf("f"));//-1

6.trim

• 去除字符串两端空格，返回新字符串，不改变原字符串

var a = " 123abc ";
console.log(a);// 123abc 
console.log(a.trim());//123abc

7.toLowerCase

• 将字符串全部转为小写，返回新字符串，不改变原字符串

var a = "ABCDEFG";
console.log(a.toLowerCase());//abcdefg

8.toUpperCase

• 将字符串全部转为大写，返回新字符串，不改变原字符串

var a = "abcdefg";
console.log(a.toUpperCase());//ABCDEFG

9.replace

• 替换匹配的子字符串，一般情况下只替换第一个匹配

var a = "123 abc abc";
console.log(a.replace("abc", "efg"));//123 efg abc

10.split

• 按照给定规则分割字符串，返回一个由子字符串组成的数组

var a = "1*2*3*a*b*c";
console.log(a.split("*"));//["1", "2", "3", "a", "b", "c"]

11.charCodeAt

• 返回字符串指定位置的Unicode码点（十进制）

var a = "123abc";
console.log(a.charCodeAt(3));//97

12.fromCharCode

• 返回码点所表示的字符

console.log(String.fromCharCode(97));//a

13.match

• 将匹配的结果放入数组中，并返回

var a = "123abc456def";
console.log(a.match(/123/));//["123"]
console.log(a.match(/\d/g));//["1", "2", "3", "4", "5", "6"]

14.search

• 查找符合正则的字串的位置，只返回第一个匹配的位置

var a = "123abc456def";
console.log(a.search(/\d/));//0
console.log(a.search(/[a-z]/));//3

15.includes

• 返回布尔值，表示是否找到了参数字符串

var a = "123abc456def";
console.log(a.includes("123"));//true
console.log(a.includes("dfa"));//false

16.startsWith & endsWith

• 返回布尔值，表示参数字符串是否在原字符串的头部或尾部

var a = "123abc456def";
console.log(a.startsWith("123"));//true
console.log(a.endsWith("deg"));//false

17.repeat

• 将字符串复制n次

var a = "123abc";
console.log(a.repeat(2));//123abc123abc

18.padStart & padEnd

• 补全字符串，第一个参数为最小长度（需大于字符串长度），第二个参数为用来补全的字符，若不写第二个参数，默认以空格补全

var a = "123abc";
console.log(a.padStart(9, "+"));//+++123abc
console.log(a.padEnd(9, "-"));//123abc---