Dispatches from the frontier of compile-time computation: the techniques behind the archive, pulled apart one at a time.
Type-level programs run inside the compiler, so a careless type can stall your editor or hit a hard wall. The depth limits, union explosions, and the diagnostics that show you where the time goes.
Read articleNarrowing usually evaporates at a function boundary. User-defined type guards and assertion functions carry it across, teaching the compiler what a runtime check proved, with the caveat that it trusts you completely.
Read articleEncode a machine's legal transitions as types, and an illegal one stops compiling. From discriminated-union states to phantom-typed builders whose methods change with each call, here is how to make the type checker enforce a protocol.
Read articleAn annotation validates a value but widens its type. as lies to the compiler and loses safety. The satisfies operator checks a value against a type while keeping the precise type TypeScript inferred.
Read articleThe type system can't add two numbers directly, but it can measure the length of a tuple. That bridge, values to lengths and back, is how Two Sum does arithmetic and how the bracket validator keeps a stack.
Read articleTemplate literal types turn the type checker into a parser. Learn how infer splits strings, how greedy matching against a delimiter works, and why those rules power the Wordle and bracket-validator experiments.
Read articleThe TypeScript type system has no for-loop. Every algorithm in the archive is recursion in disguise. The accumulator pattern and TS 4.5's tail-call elimination make that practical.
Read articleTypeScript is structural, so UserId and PostId are both number and freely interchangeable. Branding adds a phantom tag that makes them distinct, turning validated values and units into types the compiler keeps apart.
Read articleAn array of dogs is assignable to an array of animals, right up until it sinks your program. Covariance, contravariance, and the bivariant hole TypeScript leaves open in method parameters, shown with the assignments that do and do not type-check.
Read articleMapped types transform every property of a type at once. Add key remapping with an as clause and you can rename keys, filter them out, and synthesize new ones from template literals. This is the machinery behind Partial, Pick, and most builder APIs.
Read articleA conditional type over a naked type parameter quietly loops across every member of a union. That behaviour powers Exclude and Extract, breaks in subtle ways, and switches off with a single pair of brackets.
Read articleOne keyword, a dozen jobs. infer extracts return types, peels tuples, constrains what it captures, and produces unions or intersections depending on where it sits. A tour of conditional-type inference past the basics.
Read articleDiscriminated unions and a one-line never check turn whole classes of bugs into compile errors. Model state so invalid combinations cannot be represented, and let exhaustiveness keep your switches honest.
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