Making crop work on one image is the easy part. Making it safe to run on two hundred of somebody else's files is the actual job, and what decided when it shipped was a test list.

The first two tests on that list failed. Not on the crop, which was the point of the list, but on the version of Convertim that had been out since July.

Crop shipped on Monday, in Convertim 2.1. This is how the pipeline is tested, what the crop had to pass before it shipped, and what those two failures taught me about the two hundred tests that had been passing all along.

What was already there

Convertim is a portable image converter for Windows: a desktop app and a CLI, both on top of one conversion engine built on sharp. The desktop app reaches the engine through Electron's main process; the CLI calls it directly. Same functions, same options.

That shared engine is where most of the tests point. There are a little over two hundred, on Node's built-in test runner, in a few layers:

Engine and pipeline.…