TypeScript’s Most Useful Features That Most JavaScript Developers Never Touch

Discriminated unions, template literal types, infer keyword, satisfies operator, const type parameters, NoInfer utility type — the TypeScript features that sit between “I know TypeScript” and “TypeScript is doing the work for me.” The upgrade path from competent to dangerous.

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Most developers who use TypeScript use about 30% of it. They annotate variables with types, they write interfaces for their data, they use generics when a library forces them to, and they put as any or as SomeType when the type system pushes back in a way they don’t understand. That’s not TypeScript working for you. That’s TypeScript working alongside you while you occasionally fight it.

The features in this post are the ones that change the relationship. Not because they’re clever or impressive, but because each one represents the type system doing work that you’d otherwise have to do manually — catching a category of bug, eliminating a class of runtime assertion, or making a refactor that previously required touching twenty files happen automatically when…