sindresorhus/ow

Health v2 · maintenance only

Adoption not part of the score

3865 stars · 111 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-29, confidence not recorded

ow is a TypeScript library providing expressive, chainable runtime validation of function arguments and values with descriptive error messages. It includes built-in predicates for many types, custom validation support, and TypeScript type guards.

Use cases

• validate function arguments at runtime
• validate environment variables and config at startup
• assert constructor parameters in classes
• validate CLI arguments and flags
• narrow TypeScript types from unknown values
• validate object shapes with descriptive errors

When to choose

• you need runtime validation of function inputs with clear error messages
• you want self-documenting assertions with a chainable API
• you need TypeScript type guards after validation
• you want dev-only imports stripped from production bundles

When to avoid

• you need complex schema validation for API responses or JSON parsing (use zod)
• you need validation outside the JavaScript/TypeScript ecosystem
• you need zero runtime overhead in production without using the dev-only shim

Facets

library · maturity stable

testing developer-tools developer-tools web-development programming-languages browser cross-platform validation type-checking assertion typescript runtime-validation type-guards nodejs

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