Ask Claude to Add Type Annotations to Your Untyped Code for Better Safety and Autocompletion

Adding types to a codebase that grew up without them is tedious but valuable — better autocompletion, earlier bug detection, and self-documenting code. Claude can do the grunt work by reading how your code is actually used and inferring the right types.

> add TypeScript types to all the functions in src/utils/ —
> infer types from usage, don't use 'any'

Claude reads each function, traces how it's called throughout the codebase, looks at what values get passed in and returned, and adds precise type annotations — not just slapping any on everything but actually figuring out the real types.

This works across languages:

> add Python type hints to the functions in app/services/
> — use modern syntax (str | None instead of Optional[str])

> add PHPDoc type annotations to all methods in app/Repositories/
> — include @param, @return, and @throws

> add JSDoc types to the functions in lib/ so VS Code
> can provide autocompletion without switching to TypeScript

Claude infers types that a human would have to trace manually:

• Return types — reads every return path to determine what the function actually returns
• Parameter types — checks call sites to see what values are passed in
• Nullable types — detects when values can be null based on conditional checks in the code
• Generic types — recognizes collections and maps the element types from usage
• Union types — identifies when a parameter accepts multiple types

For gradual TypeScript migration, Claude can convert one file at a time:

> convert src/utils/formatting.js to TypeScript — rename to .ts,
> add types, and update the imports in files that reference it

Claude renames the file, adds types, updates import paths across the codebase, and fixes any type errors that surface — all in one pass.

Types are documentation that the compiler enforces — let Claude add them so you get the safety without the tedium.

via Claude Code