string concatenation in js

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.var str1 = "Hello ";
var str2 = "world!";
var res = str1.concat(str2);
console.log(res);const str1 = 'Hello';
const str2 = 'World';

console.log(str1.concat(' ', str2));
// expected output: "Hello World"

console.log(str2.concat(', ', str1));
// expected output: "World, Hello"var arr = [1, 2, 3, 4];
var arr2 = [5, 6, 7, 8];

const both = arr.concat(arr2);

console.log(both);
//[1, 2, 3, 4, 5, 6, 7, 8]var aString="";
aString.concat(value1, value2, ... value_n);    var dest = new String("");
    var src = new String("aze");
    var ar = new Array();
    ...
    ar.push(src);
    ar.push(src);
    ...
    dest = ar.join("");