++a returns the value of an after it has been incremented. It is a pre-increment operator since ++ comes before the operand.
a++ returns the value of a before incrementing. It is a post-increment operator since ++ comes after the operand.
Example
You can try to run the following code to learn the difference between i++ and ++i −
<html>
<body>
<script>
var a =10;
var b =20;
//pre-increment operator
a = ++a;
document.write("++a = "+a);
//post-increment operator
b = b++;
document.write("<br> b++ = "+b);
</script>
</body>
</html>