The rounding bug that almost shipped in a payroll calculator

I've been building PayTimeHub, a set of free payroll and pay calculators — paycheck withholding, raise calculator, hours-to-decimal, time and a half. Money math looks trivial until you actually ship it, and one bug almost got through that's worth writing up.

The setup

Every calculator on the site takes a dollar amount, does some arithmetic on it, and shows the result back to the user. A raise calculator multiplies a salary by a percentage. An overtime calculator multiplies an hourly rate by 1.5. Simple stuff, until you remember that JavaScript does floating-point math.

Try this in a console:

0.1 + 0.2