Applying the Parse-Don't-Validate Principle in TypeScript: Making Illegal States Unrepresentable

Summary

This article explores Alexis King's "Parse, don't validate" principle and applies it to TypeScript development. The author argues that most TypeScript codebases validate data at runtime (e.g., checking `if (user.email)`) rather than parsing it into well-typed, validated structures at system boundaries. The piece examines how TypeScript's type system, while powerful, makes this functional programming principle harder to implement than in languages like Haskell or Rust. The author provides practical strategies for structuring TypeScript code to parse external data into precise types that make illegal states unrepresentable, reducing runtime checks scattered throughout the codebase.

Source

Hacker NewsApplying the Parse-Don't-Validate Principle in TypeScript: Making Illegal States Unrepresentablecekrem.github.io