Immediately after the last post about summing a range of numbers, a talented friend of mine, Gábor, offered an even shorter, cleverer solution:

What I especially like about it is that instead of relying on esoteric JavaScript functions to do the job, it’s done with reasoning and plain maths, which makes it clearer and easier to read too.

The Maths.abs() call is only needed because in the original problem, the arguments could be in either order; the logic is still the same though, you just need to make sure the range is positive. In short, it’s (a+b) × (a-b+1) ÷ 2.