Reusable knowledge generalized from TIL, organized by category.
How to widen the interval (backoff) and mix in randomness (jitter) when retrying transient failures, so retries do not turn into a storm that hammers the other side again, plus the criteria for choosing which errors to retry
A pattern that puts a queue and a worker in front of a slow or one-at-a-time external system, so requests are accepted immediately and processed at the system's own pace, along with how to design acks, fallbacks, and retries
Browsers split even simultaneous input into separate keydown events, which causes the bug where pressing keys together only goes one direction. This explains how to fix it with a short, bounded collection window and fixed-step consumption
Assuming a queue may deliver the same message more than once, this explains how to ack only on success, block duplicate execution with an idempotency key, and report when redelivery is exhausted
How to pick a base point when you follow the import graph backward from changed files to run only the related tests, the fail-closed rule that falls back to a full run when things are uncertain, and why the final gate is kept separate
How to read set, logic, and quantifier notation in specifications and proofs by mapping it 1:1 to TypeScript code and Bend syntax, and the test gaps caused by misreading symbols such as vacuous truth, counterexamples, and reverse implication
Why the same code passes in one repo and fails in another, and the procedure for pinning down your effective tsconfig and lockfile-resolved compiler version once, before you build any type contract
How to move boundary value analysis from runtime tests to the type level, pairing a passing and a rejecting witness on each of the 7 extremes of the type lattice — and the axes types can never cover, like the time axis and side-effect counts
The order to follow: don't store what you can derive, don't copy a union the library already owns, and turn only what's left into a client-state union — plus why actions don't belong inside state
Patterns where the type promises something the implementation never guarantees: the totality Record implies, type predicates that check nothing, ReturnType on wrappers that run later, and excess property checks mistaken for sanitizers
Which of the three layers — assertNever, satisfies Record, pattern matching libraries — to reach for and when, and why a catch-all default branch hides what you missed
The rules that decide whether Dog[] and (x: Dog) => void inherit the Dog-is-an-Animal relationship, and in which direction — plus why method shorthand syntax leaves a hole even with strictFunctionTypes on
How to verify compilation instead of execution so that 'what wrong code does this type actually block' is pinned down in code, plus the @ts-expect-error and mutation checks that tell you whether the test is still alive
Working around the fact that a const declaration can't take type parameters — creating the context inside a generic function and letting the components capture it by closure, plus the trap where calling it during render silently remounts everything
How to keep a list of allowed values in one place and compute the type from it instead of writing the list twice, plus the trap where widening silently removes that protection without a single error
The problem of deciding whether a result is right or wrong, and why tests whose expected values were reverse-engineered from the implementation verify nothing
Why DOM-coordinate highlights break on the next visit, and how quote, context, and position selectors with weighted scoring and confidence thresholds fix it
Planting artificial defects in code to see whether the existing tests fail, exposing the empty assertions and loose checks that coverage hides
How to safely connect extension execution contexts that cannot share memory with message APIs, runtime validation, handler routing, response envelopes, and change notifications
Why a path-comparison main guard silently turns false behind a symlinked path such as an npm global bin, leaving a CLI that exits 0 with no output — and how to diagnose and fix it