Most validation libraries like Zod create deep clones of your data during validation, which can impact performance in high-throughput applications. I built decode-kit to take a different approach: assertion-based validation that validates and narrows TypeScript types in-place, without any copying or transformation. Here's what the API looks like in practice:

import { object, string, number, validate } from "decode-kit";

// Example of untrusted data (e.g., from an API) const input: unknown = { id: 123, name: "Alice" };

// Validate the data (throws if validation fails) validate(input, object({ id: number(), name: string() }));

// `input` is now typed as { id: number; name: string } console.log(input.id, input.name);

When validation fails, decode-kit takes an equally thoughtful approach. Rather than being prescriptive about error formatting, it exposes a structured error system with an AST-like path that precisely indicates where validation failed. It does include a sensible default error message for debugging, but you can also traverse the error path to build whatever error handling approach fits your application - from simple logging to sophisticated user-facing messages.

The library also follows a fail-fast approach, immediately throwing when validation fails, which provides both better performance and clearer error messages by focusing on the first issue encountered.

I'd love to hear your thoughts and feedback on this approach.