Yes, and it works because it specifically idenfifies the values as integers (numbers). Otherwise, javascript can add two STRINGS together in other contexts, as you experienced.
For example when I program using VB.net, I use value1 + value2 to create my program title - it adds the version string to the program name. Your script was simply adding two 'text' values together. Specifying the values as integers solves this.
edited original code post: replaced 'summing' with 'number', my mistake.
cr3ative
Thanks, for the explanation, and i already replaced 'summing' with 'number'.