It assumes equal repayments, with the first payment being made exactly one month after the loan (or one week for weekly repayments) and gives an answer to two decimal places.
Simple APR calculator
This is a very rough and ready APR calculator, which I wouldn't recommend using for any calculations requiring a high degree of accuracy.
You can re-use the code in this page under the terms of the Mozilla Public Licence v2.0. See the licence notice in the source of the page for more details.