Reusable knowledge generalized from TIL, organized by category.
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
A rewrite is a proxy, not a redirect, so it passes upstream 3xx responses straight through — why pointing one at an origin that canonicalizes creates an infinite loop, and how to fix it
How injecting real pixel dimensions into markdown body images at build time eliminates both layout shift and fixed-ratio cropping
Why blur or Enter can commit a half-composed Korean character, and how to fix it with composition state rather than assumptions about event order
Supporting old browsers is two different jobs — transpiling new syntax into old syntax, and polyfilling missing APIs with code. A transpiler only rewrites syntax; it does not create runtime functions
When several interactions on one screen stop working at once, it is usually not separate bugs but a single root cause: the client JS never ran, so hydration failed wholesale
Instead of shipping polyfills to every browser, actually detect whether a feature is missing and load them only on the browsers that lack it — making the cost on modern browsers nearly zero