sindresorhus/is

Health v2 · maintenance only

Adoption not part of the score

1791 stars · 132 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-30, confidence not recorded

A TypeScript library for checking the type of values at runtime, with type guards and assertions. It provides a simple API like `is.string(value)` and `assert.string(value)` for validating and narrowing types.

Use cases

• check if a value is a string in javascript
• validate function argument types at runtime
• narrow typescript types with type guards
• assert an environment variable is a non-empty string
• detect the type of an unknown value
• validate array items match a type

When to choose

• you need runtime type checks with TypeScript type narrowing
• you want assertions that throw descriptive errors
• you need checks for many built-in types including Map, Promise, and observables

When to avoid

• you need schema validation of object shapes or serialization formats
• you prefer a single validation library like zod for both parsing and checking

Facets

library · maturity active

type-system developer-tools developer-tools programming-languages cross-platform type-checking type-guards assertions typescript runtime-validation nodejs

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