Type Guards and Parsing Data

Type guards and assertion functions let TypeScript narrow the type of a value at runtime based on checks you perform in code. In practice, this is essential whenever the type you get from the outside world (unknown, any, API responses, user input, etc.) is broader or less trusted than what your business logic actually needs.

For complex data structures (API payloads, nested objects, arrays of records…), we strongly recommend validating and narrowing them with a runtime schema library like zod. For simple checks – e.g. “is this value defined?”, “is it a string?”, “is this array non-empty?”, or narrowing based on element/property type – you can use the narrowland utility library, created and maintained by Applifters. It provides a curated set of boolean type guards (is.*) and assertion type guards (assert.*) that cover the most common narrowing use cases in our codebases.