Stefan Judis • 4/4/2026

The scope of type guards and assertion functions (#tilPost)

This article discusses the scope of TypeScript type guards versus assertion functions. It shows how a standard type guard narrows a type only within a conditional block, while an assertion function (using `asserts value is User`) permanently narrows the type for the rest of the current scope if it doesn't throw. Includes practical examples with JSON.parse and unknown data sources.

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#TypeScript #type narrowing #type guards