Assertion functions in TypeScript

Assertion functions in TypeScript are a powerful feature that allow developers to express invariants on variables by verifying conditions at the type system level, a capability introduced in TypeScript 3.7. These functions differ from JavaScript-like assertions as they provide enhanced code flow analysis, enabling TypeScript to infer properties about values and ensure type safety. They work by specifying type predicates within their signatures, allowing developers to narrow down types and verify conditions on variable values, such as ensuring non-nullability. While similar to type guards, which return true or false based on type evaluation, assertion functions throw errors if a condition is not met, and can be used without type predicates for more general assertions. This feature adds flexibility to TypeScript, aligning with its nature of being transpiled to JavaScript, and can also be used to express conditions on the actual value of variables, offering a sophisticated way to manage types and improve code reliability.

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