PascalCase vs camelCase

By Case Modify Editorial · Published August 31, 2026

Both glue words without spaces, but the first letter tells you which layer of the stack you are naming.

The one-letter rule

camelCase starts lowercase: userProfileId. PascalCase starts uppercase: UserProfileId. That single difference maps to variables versus types in most modern codebases.

JavaScript and TypeScript

Variables, functions, and object properties → camelCase. Classes, React components, and type aliases → PascalCase. Linters enforce this split so imports read predictably.

JSON and APIs

Many REST APIs document camelCase field names even when databases use snake_case. Document the mapping at the boundary so mobile and web clients do not guess.

Common mistakes

Naming React components in camelCase. Exporting PascalCase JSON keys to JavaScript clients without transformation. Using PascalCase for environment variables — use CONSTANT_CASE instead.