I'm doing propety-based test since years for frontend stuff. The hardest part is, that there is so much between the test inputs and the application under test, that I find 50% of the time problems with the frontend test frameworks/libs and not in our code.
I'm a huge fan of property based testing, I've built some runners before, and I think it can be great for UI things too so very happy to see this coming around more.
Something I couldn't see was how those examples actually work, there are no actions specified. Do they watch a user, default to randomly hitting the keyboard, neither and you need to specify some actions to take?
What about rerunning things?
Is there shrinking?
edit - a suggestion for examples, have a basic UI hosted on a static page which is broken in a way the test can find. Like a thing with a button that triggers a notification and doesn't actually have a limit of 5 notifications.
How effective is property based testing in practice? I would assume it has no trouble uncovering things like missing null checks or an inverted condition because you can cover edge cases like null, -1, 0, 1, 2^n - 1 with relatively few test cases and exhaustively test booleans. But beyond that, if I have a handful of integers, dates, or strings, then the state space is just enormous and it seems all but impossible to me that blindly trying random inputs will ever find any interesting input. If I have a condition like (state == "disallowed") or (limit == 4096) when it should have been 4095, what are the odds that a random input will ever pass this condition and test the code behind it?
Microsoft had a remotely similar tool named Pex [1] but instead of randomly generating inputs, it instrumented the code to also enable executing the code symbolically and then used their Z3 theorem proofer to systematically find inputs to make all encountered conditions either true or false and with that incrementally explore all possible execution paths. If I remember correctly, it then generated a unit test for each discovered input with the corresponding output and you could then judge if the output is what you expected.
Hey, yeah the default specification includes a set of action generators that are picked from randomly. If you write a custom spec you can define your own action generators and their weights.
Rerunning things: nothing built for that yet, but I do have some design ideas. Repros are notoriously shaky in testing like this (unless run against a deterministic app, or inside Antithesis), but I think Bombadil should offer best-effort repros if it can at least detect and warn when things diverge.
Shrinking: also nothing there yet. I'm experimenting with a state machine inference model as an aid to shrinking. It connects to the prior point about shaky repros, but I'm cautiously optimistic. Because the speed of browser testing isn't great, shrinking is also hard to do within reasonable time bounds.
Hey Oskar ~ great project and looks promising. I would be curious to hear what is still work-in-progress for Bombadil.
It's helpful to know what the tool maintainers see as upcoming or incomplete work. It also saves a consultant like me a lot of time to evaluate new tools for clients if I also know the limitations before diving in. Maybe a section in the manual for "What Bombadil can't do".
I especially like that it's a single executable according to the docs.
Recently evaluated other testing tools/frameworks and if you're not already running the npm-dependencyhell-shitshow for your projects, most tools will pull in at least 100 dependencies.
I might be old fashioned but that's just too much for my taste. I love single-use tools with limited scope like e.g. esbuild or now this.
All I can think of is "the Token Ring had no power over him" but then I realized that "token ring" has a completely different meaning now in the age of AI.
My kingdom for a way to stop this godforsaken industry from stripping Tolkien's fiction for parts.
We can go strip Shakespeare instead.
I'm doing propety-based test since years for frontend stuff. The hardest part is, that there is so much between the test inputs and the application under test, that I find 50% of the time problems with the frontend test frameworks/libs and not in our code.
Interesting. What kind of properties are you checking?
I'm a huge fan of property based testing, I've built some runners before, and I think it can be great for UI things too so very happy to see this coming around more.
Something I couldn't see was how those examples actually work, there are no actions specified. Do they watch a user, default to randomly hitting the keyboard, neither and you need to specify some actions to take?
What about rerunning things?
Is there shrinking?
edit - a suggestion for examples, have a basic UI hosted on a static page which is broken in a way the test can find. Like a thing with a button that triggers a notification and doesn't actually have a limit of 5 notifications.
How effective is property based testing in practice? I would assume it has no trouble uncovering things like missing null checks or an inverted condition because you can cover edge cases like null, -1, 0, 1, 2^n - 1 with relatively few test cases and exhaustively test booleans. But beyond that, if I have a handful of integers, dates, or strings, then the state space is just enormous and it seems all but impossible to me that blindly trying random inputs will ever find any interesting input. If I have a condition like (state == "disallowed") or (limit == 4096) when it should have been 4095, what are the odds that a random input will ever pass this condition and test the code behind it?
Microsoft had a remotely similar tool named Pex [1] but instead of randomly generating inputs, it instrumented the code to also enable executing the code symbolically and then used their Z3 theorem proofer to systematically find inputs to make all encountered conditions either true or false and with that incrementally explore all possible execution paths. If I remember correctly, it then generated a unit test for each discovered input with the corresponding output and you could then judge if the output is what you expected.
[1] https://www.microsoft.com/en-us/research/publication/pex-whi...
Hey, yeah the default specification includes a set of action generators that are picked from randomly. If you write a custom spec you can define your own action generators and their weights.
Rerunning things: nothing built for that yet, but I do have some design ideas. Repros are notoriously shaky in testing like this (unless run against a deterministic app, or inside Antithesis), but I think Bombadil should offer best-effort repros if it can at least detect and warn when things diverge.
Shrinking: also nothing there yet. I'm experimenting with a state machine inference model as an aid to shrinking. It connects to the prior point about shaky repros, but I'm cautiously optimistic. Because the speed of browser testing isn't great, shrinking is also hard to do within reasonable time bounds.
Thanks for the questions and feedback!
Hey Oskar ~ great project and looks promising. I would be curious to hear what is still work-in-progress for Bombadil.
It's helpful to know what the tool maintainers see as upcoming or incomplete work. It also saves a consultant like me a lot of time to evaluate new tools for clients if I also know the limitations before diving in. Maybe a section in the manual for "What Bombadil can't do".
Great work!
I especially like that it's a single executable according to the docs.
Recently evaluated other testing tools/frameworks and if you're not already running the npm-dependencyhell-shitshow for your projects, most tools will pull in at least 100 dependencies.
I might be old fashioned but that's just too much for my taste. I love single-use tools with limited scope like e.g. esbuild or now this.
Will give this a try, soon.
Glad you noticed! I've been putting quite some energy into keeping things this way. VERY worth it, IMO.
"Bombadil" means that I'll probably skip most of these tests.
Struggling to understand what this is or how it works.
Author here, happy to answer questions about Bombadil! :)
All I can think of is "the Token Ring had no power over him" but then I realized that "token ring" has a completely different meaning now in the age of AI.
Nice name, now who is he?
btw, some background on the project: https://wickstrom.tech/2026-01-28-there-and-back-again-from-...
Bombadillo Crocodillo
Ok I will see myself out
(Yes I know it's actually from the Tolkien book)