TypeScript definition
TypeScript is a programming language developed by Microsoft that adds static types to JavaScript. Developers describe the shapes of data and functions, and the TypeScript compiler checks the code for type errors before it runs, then outputs plain JavaScript. It is widely used for large web, Node.js and React Native codebases.
How does TypeScript work?
You write JavaScript with optional type annotations, such as declaring that an order has a string id and a numeric total. TypeScript also infers many types automatically. The compiler checks every use of those values, so passing a string where a number is expected, misspelling a property or forgetting to handle a possibly missing value becomes an error in the editor rather than a bug in production.
Types exist only at build time. The compiler, or faster tools such as esbuild and SWC, strip them away and output ordinary JavaScript that runs in any browser or in Node.js. Deno and Bun run TypeScript directly, and recent Node.js versions can strip types on the fly. Because types disappear at runtime, data from APIs and forms still needs validation.
TypeScript vs JavaScript
Every JavaScript program is valid TypeScript in principle, so the two are not competitors. TypeScript adds a type system and tooling on top. JavaScript is quicker for small scripts and prototypes, with no build step. TypeScript pays off as code grows: editors offer accurate autocomplete, renaming a field updates every usage safely, and new team members can read function signatures instead of guessing what an object contains.
Key TypeScript features
- Interfaces and type aliases describe the shape of objects and API responses.
- Union types and narrowing model values that can be one of several shapes.
- Generics write reusable functions and components that keep type information.
- Literal types restrict values to specific strings or numbers.
- Utility types such as Partial, Pick and Record transform existing types.
- Strict null checks force code to handle missing values.
- Declaration files and DefinitelyTyped add types to plain JavaScript libraries.
Benefits and trade-offs
TypeScript catches a whole class of bugs before code runs, makes large refactors safer and turns types into always-current documentation. The costs are a learning curve, a build step and, in some codebases, complicated types that are hard to read. The any type can quietly switch checking off, so teams usually ban it in reviews and lint rules.
The biggest misconception is that types guarantee runtime safety. If an API returns a different shape than declared, TypeScript cannot know. Schema libraries such as Zod or Valibot validate data at system boundaries and generate matching types, closing that gap.
How to migrate a JavaScript project to TypeScript
Migration works best gradually, alongside normal feature work, rather than as a big-bang rewrite. Nexzem migrates client codebases module by module, typing API boundaries and shared models first, because that is where type errors catch the most real bugs. Feature work never has to pause for the migration.
- Add a tsconfig with allowJs so JavaScript and TypeScript files coexist.
- Convert files one at a time, starting with shared utilities and data models.
- Type API requests and responses early, ideally generated from an OpenAPI or GraphQL schema.
- Turn on strict mode once most files are converted, fixing errors module by module.
- Add type checking to CI so new code cannot introduce errors.