Writing
Notes from the daemon.
Engineering posts on Jest internals, the compatibility surface we have to honor, and the benchmark methodology behind the numbers on the home page.
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How we benchmarked: methodology + caveats
The 100x number in the README is from a specific workload, on a specific machine, with a specific definition of cold and warm. This post explains exactly what we measured, where the speedup is largest, where it shrinks, and what would be misleading to report.
benchmarksmethodology -
Drop-in compatibility: the contract that makes adoption tolerable
rjest's only interesting feature for the first hour is that it does not break anything. We explain the compatibility contract — config, CLI, matchers, mocks, output shape — and why we treat any deviation as a bug we have to fix, not a tradeoff we get to negotiate.
compatibilityengineering -
Why warm Jest runs are slow (it's not the test code)
On a real codebase, the time Jest takes between npx jest and the first PASS rarely has anything to do with assertions. We break down the hidden cost — config, transform, worker boot, JIT — and explain where rjest amortizes each one.
engineeringperformance