# sindresorhus/is

Type check values

Repository: https://github.com/sindresorhus/is
Canonical: https://ross.abutalabs.com/products/sindresorhus-is
Language: TypeScript
License: MIT
License Family: permissive
Topics: type-detection, type-check, is, assertions, typescript, node-module, npm-package
Last push: 2026-05-11T13:57:29+00:00

## Health v2 (maintenance only)
Score: 81/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 81, release rhythm 71, longevity 100
- inputs: {"age_days": 3268, "days_push": 114, "days_rel": 114, "gap_med": 78.5, "n_releases_24m": 7}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1791, forks 132 (observed 2026-08-28T04:05:37.005268+00:00)

## What it is
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
- artifact type: library
- maturity: active
- function: type-system, developer-tools
- domain: developer-tools, programming-languages
- platform: cross-platform
- tags: type-checking, type-guards, assertions, typescript, runtime-validation, nodejs

## Member repositories
- sindresorhus/is (main) score 81

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:05:37.005268+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T03:23:07.235565+00:00, confidence not recorded.
  - readme: https://github.com/sindresorhus/is (fetched 2026-08-28T04:05:37.005268+00:00, sha ef56f7b7d635)
- Data as of 2026-08-30T08:39:29.467469+00:00.