JSFiddle - React, Tailwind, and code Playground

JavaScript

var someString = "Hello world,\nHe like cats and dogs\nBye bye"
var pos = someString.indexOf('He ');
var nextLineBreak = someString.indexOf('\n', pos);

var firstPart = someString.substring(0, pos);
var secondPart = someString.substring(nextLineBreak +1);

var finalString = firstPart + secondPart;
alert(finalString);