JSFiddle - React, Tailwind, and code Playground

JavaScript

function concatStrings(stringObj, s2, separator) {
	stringObj.value = stringObj.value + separator + s2;
}

var baseString = "hello";
var stringObj = {value: baseString};
var s2 = "world";
var separator = " ";

concatStrings(stringObj, s2, separator);

window.alert(stringObj.value;