// \n转换成<br/>
let replaceRegex = /(\n\r|\r\n|\r|\n)/g; 
let str = '123啦啦啦\n哦哦哦234\n'
let newStr = str.replace(replaceRegex, '<br/>');
console.info(newStr);
// 123啦啦啦<br/>哦哦哦234<br/>

//<br/>转换成\n
let text = '你好啊<br/>我很好<br/>好的，下次见'; 
let newText = text.replace(/<br\/>/g, '\n');
console.info(newText);