If you write code, you have seen both camelCase and snake_case. Teams argue about them because languages, databases, and APIs disagree by default. This guide gives clear rules, shows where PascalCase and kebab-case fit, and links you to a free in-browser converter so you can rename identifiers without uploading a file.

What is camelCase?

camelCase starts with a lowercase letter, and each later word starts with an uppercase letter — no spaces or underscores.

Examples: firstName, lastName, getUserProfile, httpResponseCode

What is snake_case?

snake_case uses lowercase letters with words separated by underscores.

Examples: first_name, last_name, get_user_profile, http_response_code

Quick decision: which should you use?

You are writing…	Prefer	Why
JavaScript / TypeScript variables & functions	camelCase	Language convention (ESLint / Prettier ecosystems)
TypeScript / C# / Java types & classes	PascalCase	Types read as nouns; matches mainstream style guides
Python / Ruby	snake_case	PEP 8 and Ruby community defaults
PostgreSQL / MySQL columns	snake_case	SQL identifiers stay readable without quoting
HTTP path segments & CSS custom properties	kebab-case	URLs and CSS already use hyphens
Environment variables	UPPER_SNAKE_CASE	Shell-safe, traditional for config

Rule of thumb: match the language or store you are inside. Convert at the boundary (API JSON ↔ DB columns) instead of inventing a house style that fights every library.

When to use camelCase

Context	Convention	Example
JavaScript variables	camelCase	`userName`
JavaScript functions	camelCase	`calculateTotal()`
TypeScript interfaces	PascalCase	`UserProfile`
Java variables	camelCase	`accountBalance`
C# properties	PascalCase	`FirstName`
Swift properties	camelCase	`viewController`

When to use snake_case

Context	Convention	Example
Python variables	snake_case	`user_name`
Python functions	snake_case	`calculate_total()`
Ruby variables	snake_case	`account_balance`
PostgreSQL columns	snake_case	`created_at`
JSON keys (many backends)	snake_case	`first_name`
Environment variables	UPPER_SNAKE_CASE	`DATABASE_URL`
CSS custom properties	kebab-case	`--primary-color`
URLs / slugs	kebab-case	`/blog/my-post`

Language-by-language examples

JavaScript / TypeScript — camelCase

const firstName = "John";
function getUserProfile() { }
const httpResponseCode = 200;

Python — snake_case

first_name = "John"
def get_user_profile():
    pass
http_response_code = 200

Database columns — snake_case

CREATE TABLE users (
    first_name VARCHAR(100),
    created_at TIMESTAMP,
    is_active BOOLEAN
);

PascalCase and kebab-case (the neighbors)

PascalCase (UserProfile): same word breaks as camelCase, but the first letter is uppercase. Use for classes, React components, and type names.
kebab-case (user-profile): lowercase with hyphens. Use for URL slugs, CSS classes, and HTML data-* style attributes — not for most programming-language identifiers.

If you are stuck renaming a batch of keys, convert once at the edge rather than mixing styles inside one module.

Convert without rewriting by hand

Need camelCase ↔ snake_case (or PascalCase / kebab-case) on a list of names? Use CaseShift — free, runs in your browser, nothing uploaded.

Related: uppercase ↔ lowercase guide when you only need letter case, not word separators.

Related ToolBookly tools

Renamed a batch of JSON keys at an API ↔ DB boundary? Pretty-print and validate the payload locally with JSONly — format, minify, sort keys, and tree-view in your browser with no upload; optional share links live only in the URL fragment and never expire. Need a regex to rewrite identifiers before you convert? Try Regexly — pattern test in the browser, no upload.

Chinese UI: CaseShift 中文

camelCase vs snake_case: When to Use Each (With Quick Conversion)