It adds the right operand to the left operand and assigns the result to the left operand.
Example
You can try to run the following code to learn how to work with Addition Assignment Operator −
<html>
<body>
<script>
var a = 33;
var b = 10;
document.write("Value of a => (a += b) => ");
result = (a += b);
document.write(result);
document.write(linebreak);
</script>
</body>
</html>