TypeScript patterns that prevent real bugs — discriminated unions, branded types, parse-don't-validate, and the type-level idioms that turn a refactor from a leap of faith into a compile-time confirmation.
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A handful of users reported that clicking a notification took them to the wrong record. The IDs were off by one or two at the very end, always on the biggest accounts. The backend was sending the correct 64-bit ID every time. Our frontend was quietly rounding it the instant it parsed the JSON, because in JavaScript every number is a float.
Covariance, contravariance, and the method-vs-function loophole that lets an unsound callback slip past the type checker even with strict on.
A type annotation checks a value but throws away what it knows about it. `satisfies` checks against a type while keeping the narrow, literal type the value actually has.
UserId, OrderId, Email — all of them strings to the compiler, all of them disasters when accidentally swapped. Branded types are five lines of code that prevent the whole class.
Optional fields are how TypeScript codebases drift into nullable-everywhere chaos. Discriminated unions are how they get rescued.