• 字符串的扩展
  includes('str'):判断指定位置有没有字符,返回值是布尔值

let ss = 'asd';
console.log(ss.includes('a'));//true
console.log(ss.includes('e'));//false

startsWith('str',index):判断字符串是不是以指定字符作为开头,index表示查找的结束下标`

let str = 'aasderst';
console.log(str.startsWith('s',2)); //true
console.log(str.startsWith('s',3));//false
console.log(str.startsWith('a'));//true

endsWith('str',num):判断字符串是不是以指定字符作为结尾,num表示查找的个数

let s3 = 'weasfer';
console.log(s3.endsWith('r')); //true
console.log(s3.endsWith('a',3));//true
console.log(s3.endsWith('a',2));//false

repeat(num):将字符串重复几次,num不可以是负数和Infitnity

let s4 = 'asd';
console.log(s4.repeat(2));//asdasd
console.log(s4.repeat(0));console.log(s4.repeat(-0)); //空串
console.log(s4.repeat(0.9));console.log(s4.repeat(-0.5));//空串

padStart('指定字符串长度',补全的字符串):在头部补全字符串

 let s5 = 'llo';
 console.log(s5.padStart(2,'he'));//llo
 console.log(s5.padStart(3,'he'));//llo
 console.log(s5.padStart(5,'he'));//hello
 console.log(s5.padStart(7,'he'));//hehello
 console.log(s5.padStart(8,'he'));//hehehllo

padEnd('指定字符串长度',补全的字符串):在末尾部补全字符串

let s6 = 'llo';
console.log(s6.padEnd(2,'ed'));//llo
console.log(s6.padEnd(3,'ed'));//llo
console.log(s6.padEnd(5,'ed'));//lloed
console.log(s6.padEnd(7,'ed'));//lloeded
console.log(s6.padEnd(8,'ed'));//lloedede

注意:原字符串的长度大于或者等于指定的最小字符串长度,返回原字符串