How to Join two strings in javascript?

Html page:

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

Javascript page:

<script>

function myFunction()

{

    var str1 = "Hello ";

    var str2 = "world!";

    var res = str1.concat(str2);

    document.getElementById("demo").innerHTML = res;

}

</script>

Output:

Hello world!