how to concatenate strings javascript

var str1 = "Hello ";
var str2 = "world!";
var res = str1.concat(str2);
// does not change the existing strings, but
// returns a new string containing the text
// of the joined strings.let string1 = "Hello"
let string2 = "World"

let finalString = string1 + ", " + string2 + "!" // Hello, World!