TypeScript Template Literal Types: Building Expressive, Type-Safe APIs

Template literal types landed in TypeScript 4.1 and most teams use them for one or two tricks before moving on. That's leaving most of their power on the table. Used systematically, they let you encode protocol-level contracts — event buses, route registries, permission systems, CSS token naming conventions — directly into the type system. Zero runtime overhead, full IDE autocomplete, errors caught at compile time.

String-heavy APIs are the hardest to keep type-safe. Event emitters, route handlers, CSS-in-JS helpers — they all accept strings, and without template literal types, those strings are either string (useless) or a hand-written union (fragile and verbose). The hand-written union is the real problem: it drifts the moment a domain concept is renamed.