A case that says «do not change me»
CONSTANT_CASE, also called SCREAMING_SNAKE_CASE, uppercases everything with underscores: MAX_RETRY_COUNT. Its purpose is not decorative: in almost every language it signals to the reader that a value must never change at runtime.
Where it is effectively mandatory
Environment variables. On Unix systems the convention is so strong it is nearly a rule: DATABASE_URL, NODE_ENV, API_KEY. A .env file written otherwise will work, but will surprise everyone who reads it.
Frequently asked questions
How does it differ from snake_case?
Only the case: snake_case is lowercase for ordinary variables, CONSTANT_CASE uppercase for constant values.
Does JavaScript require it for const?
No language enforces it technically. It is a readability convention — in JavaScript it is reserved for true global constants, not every const declaration.
Can I use it for filenames?
It is customary for a few root files such as README or LICENSE, but avoid it elsewhere: uppercase causes trouble on case-insensitive systems.