Question

Let’s look at the following Javascript code.

<script type="text/javascript" lang="javascript">
    function test()
    {
        alert('2'+8);
        alert(8-'2');
    }
</script>

---

In the first alert box, it displays the result of concatenation of 2 and 8 which is 28. In the second alert box, however it displays the subtraction of two numbers which is 6. How?

Answer 1

The + operator is overloaded. If any operand is a string, string concatenation is performed. If you have two numbers, addition is performed. The - is not overloaded in such a way and all operands are converted to numbers.

From the specification:

11.6.1 The Addition operator ( + )

(…)
7. If Type(lprim) is String or Type(rprim) is String, then

• Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

8. Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim).
(…)

11.6.2 The Subtraction Operator ( – )

(…)
5. Let lnum be ToNumber(lval).
6. Let rnum be ToNumber(rval).
7. Return the result of applying the subtraction operation to lnum and rnum.
(…)