string and variable concatenation in javascript code example

Example 1: string concatenation in js

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

Example 2: how to concatenate strings and variables in javascript

const helloName = name => `Hello ${name}!`

Example 3: concantene number in js

Use "" + 5 + 6 to force it to strings. 
This works with numerical variables too:

var a = 5;
var b = 6;
console.log("" + a + b);

Example 4: string javascript concatenation

var dest = new String("");
    var src = new String("aze");
    ...
    dest += src + src + src + src + src;