using-type-guards

Type guards narrow unknown or any types at runtime, enabling safe property access. Use custom type predicates or built-in guards to validate data before accessing properties, protecting against undefined behavior when working with dynamic values from APIs or user input.

Type guards with type predicates validate API responses before accessing properties. Create a custom guard function that checks the response structure, then use it in a conditional to narrow the type, ensuring type-safe access to validated data.

Write a function with a return type of `is Type` that performs runtime checks on input data. The function returns true when the data matches the expected shape, and TypeScript narrows the type inside subsequent conditional blocks.

Yes. Type guards narrow union types by testing which member the value satisfies. After a guard check passes, TypeScript restricts the type to the matched member, eliminating type errors from accessing union-exclusive properties.

Assertion functions throw on failure and narrow types for the rest of the scope, unlike type guards which use conditionals. Use assertions when invalid data should halt execution; use type guards when you need to branch logic based on the narrowed type.

No. Type guards and assertion functions are built-in TypeScript features requiring no dependencies. You write custom predicates using standard JavaScript logic, though external validation libraries offer pre-built guards for common patterns.