js字符串的方法

var str=‘abcd‘;
var a=str.charAt(0);
console.log(a); //‘a‘
console.log(str); //‘abcd‘

str.charCodeAt(0);   //97

str.indexOf(‘a‘);     //0
str.indexOf(‘e‘);     //-1

str.lastIndexOf(‘b‘);   //1
str.lastIndexOf(‘e‘);   //-1

var str=‘qwe‘;
var str1=‘abc‘;
var str2=str.concat(str1);
console.log(str2);//"qweabc"

var b=s.substr(1,1)
console.log(b);  //‘w‘

var ee=str.substring(1,3);
console.log(ee);  //"bc"

var aa=str.slice(0,3);
console.log（aa）;//‘abc‘

var a=str.split(‘‘);
console.log(a);  //["a", "b", "c", "d"]

var str=‘aaaaee‘;
var reg=/a/g;
str.replace(reg,1);   //"1111ee"

var str=‘aaaa3ed33‘;
var reg=/a/g;
str.match(reg);  //["a", "a", "a", "a"]

 var s = ‘??a‘;

 s.codePointAt(0) // 134071

 s.codePointAt(1) // 57271

var s = ‘??a‘;
s.codePointAt(0).toString(16) // "20bb7"
s.codePointAt(2).toString(16) // "61"不能识别32位的UTF-16字符

var s = ‘??a‘;
for (let ch of s) {
  console.log(ch.codePointAt(0).toString(16));
}
// 20bb7
// 61

String.fromCodePoint(0x20BB7)
// "??"
String.fromCodePoint(0x78, 0x1f680, 0x79) === ‘x\uD83D\uDE80y‘
// true

for (let codePoint of ‘foo‘) {
  console.log(codePoint)
}
// "f"
// "o"
// "o"

‘abc‘.at(0) // "a"
‘??‘.at(0) // "??"

var s = ‘Hello world!‘;
s.startsWith(‘world‘, 6) // true
s.endsWith(‘Hello‘, 5) // true
s.includes(‘Hello‘, 6) // false

‘x‘.repeat(3) // "xxx"
‘hello‘.repeat(2) // "hellohello"
‘na‘.repeat(0) // ""

‘x‘.padStart(5, ‘ab‘) // ‘ababx‘
‘x‘.padStart(4, ‘ab‘) // ‘abax‘

‘x‘.padEnd(5, ‘ab‘) // ‘xabab‘
‘x‘.padEnd(4, ‘ab‘) // ‘xaba‘

‘xxx‘.padStart(2, ‘ab‘) // ‘xxx‘
‘xxx‘.padEnd(2, ‘ab‘) // ‘xxx‘

‘abc‘.padStart(10, ‘0123456789‘)

‘x‘.padStart(4) // ‘   x‘
‘x‘.padEnd(4) // ‘x   ‘