Type Declaration

Definitions

1. A statement that introduces a new named type into the type system — such as an `interface`, `type`, `class`, or `enum` in TypeScript, or a `struct`/`enum` in Rust. In TypeScript specifically, `.d.ts` declaration files provide type information for JavaScript libraries without containing runtime code, enabling type checking against untyped packages.

   In plain English: Formally defining a new named type in your code so the compiler knows what shape the data should have.

   Example: "The library doesn't ship types, so I wrote a type declaration file to get autocomplete and type checking working."

Related Terms

• Declaration File
• Runtime Type Check
• Structural Subtyping
• Type Alias
• Type Annotation
• Type Assertion
• Type Narrowing
• Type Widening
• Union