A comprehensive JavaScript type checking library with TypeScript support, type guards, and assertions.
`is` is a comprehensive JavaScript type checking library that provides runtime validation for values with full TypeScript integration. It solves the problem of unreliable type checking in JavaScript by offering extensive methods to validate primitives, built-in types, and complex data structures while working correctly across different JavaScript realms.
TypeScript and JavaScript developers who need reliable runtime type validation, especially those building libraries, APIs, or applications where type safety is critical.
Developers choose `is` over alternatives because it offers the most comprehensive type checking coverage, correct behavior across JavaScript realms, seamless TypeScript integration with type guards, and actively maintained code with millions of weekly downloads.
Type check values
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Covers all JavaScript primitives, built-ins like Map/Set, typed arrays, and edge cases like async generators, which many libraries miss, as shown in the extensive API list.
Provides type guards that automatically narrow TypeScript types in conditional blocks and assertion methods that throw descriptive errors, improving type safety without extra boilerplate.
Named exports allow bundlers to include only the specific methods used, reducing bundle size for projects that import a subset of checks, as highlighted in the usage examples.
Avoids unreliable instanceof checks by designing methods to work correctly in iframes, workers, and other JavaScript environments, addressing a common pitfall mentioned in the FAQ.
Boasts millions of weekly downloads and is maintained by reputable developers, ensuring regular updates and reliability for production use.
Limited to type checking only; it lacks built-in support for schema validation, custom formats (e.g., email), or data coercion, requiring manual composition for complex rules.
Generic type parameters are not validated at runtime, only assumed at compile-time, which can lead to mismatches and errors if not handled carefully, as warned in the README.
Even with tree-shaking, importing many methods can increase bundle size, and the library includes code for niche types that might not be used, impacting performance-sensitive environments.