Train Case Converter
Train-Case writes Hello-World-Example. Train case is what HTTP headers look like on the wire: Content-Type, X-Request-Id, Accept-Encoding. HTTP/1.1 header names are case-insensitive and HTTP/2 lower-cases them entirely, but Train-Case remains the conventional spelling in documentation, specs and code that constructs headers by hand.
Conversions
| Input | Train-Case |
|---|---|
hello world example | Hello-World-Example |
XMLHttpRequest | Xml-Http-Request |
user_first_name | User-First-Name |
Total Order Count | Total-Order-Count |
XMLHttpRequest becomes Xml-Http-Request. HTTP header names in the wild are inconsistent about this — ETag and WWW-Authenticate both violate a strict Train-Case rule and are correct as written.The Same Text in Other Cases
| Case | Result |
|---|---|
| camelCase | helloWorldExample |
| PascalCase | HelloWorldExample |
| snake_case | hello_world_example |
| SCREAMING_SNAKE_CASE | HELLO_WORLD_EXAMPLE |
| kebab-case | hello-world-example |
| dot.case | hello.world.example |
Where Train-Case Fits
Train-Case is what HTTP headers look like in specifications and documentation — Content-Type, X-Request-Id, Accept-Encoding — and it appears in hyphenated titles and some file naming schemes. It is the only common convention that combines capitalisation with hyphens.
Converting in Code
``javascript
const train = (s) =>
words(s)
.map((w) => w[0].toUpperCase() + w.slice(1).toLowerCase())
.join('-');
`
Header names are case-insensitive per RFC 7230 and HTTP/2 lower-cases them on the wire, so Train-Case is a documentation convention rather than a requirement. Code that compares header names case-sensitively is broken regardless of how they are written.
Converting at the API Boundary
The recurring friction is that JavaScript uses camelCase and Python, Ruby, Go and SQL use
snake_case. The fix is to convert in exactly one place — the client that talks to the API —
rather than letting both conventions into the same codebase.
`javascript
const toCamel = (s) => s.replace(/_([a-z])/g, (_, c) => c.toUpperCase());
const toSnake = (s) => s.replace(/[A-Z]/g, (c) => '_' + c.toLowerCase());
// Recursively, for a whole payload
const convertKeys = (value, fn) =>
Array.isArray(value)
? value.map((v) => convertKeys(v, fn))
: value && typeof value === 'object'
? Object.fromEntries(Object.entries(value).map(([k, v]) => [fn(k), convertKeys(v, fn)]))
: value;
`
Two things to watch: keys that are user data rather than field names must not be
converted, and the round trip is not always lossless — userID → user_id → userId
changes the original.
Where Each Convention Is Mandatory
Not stylistic — these will break if you deviate:
| Context | Requirement |
|---|---|
| Environment variables | Uppercase with underscores; POSIX reserves lowercase |
| Custom HTML elements | Must contain a hyphen |
| Python modules | Cannot contain hyphens; the import statement will not parse |
| SQL identifiers | Folded to one case unless quoted, so camelCase does not survive |
| React components | Must start uppercase, or JSX treats it as an HTML tag |
Renaming Safely
A find-and-replace across a codebase will hit strings, comments and unrelated identifiers.
Use your language server's rename symbol instead — it understands scope. For a bulk rename
across files, restrict the pattern with word boundaries and review the diff before
committing:
`bash
grep -rn '\buserId\b' src/ # look first
``