Basically, concatenation is the process of joining or combining the two entities. We can concatenate two variables , variables and string , functions etc.
'string1' + 'string2';
<script type="text/javascript">
var mystring = "Hello to the";
var concatenation = mystring+"CodingPk Easy JavaScript Tutorials ";
</script>
<script type="text/javascript">
var mystring = "Reforms";
var concatenation = "A reformer is one who brings "+mystring+" in the society .";
alert(concatenation);
</script>