How do I convert the entire current line to lowercase or uppercase in one command?

Explanation

Vim's case operators gu (lowercase) and gU (uppercase) follow the same doubling convention as dd and yy: repeating the operator letter applies it to the whole current line. guu lowercases the line; gUU uppercases it — no motion or text-object needed.

How it works

• guu — convert the current line to all lowercase
• gUU — convert the current line to all UPPERCASE
• g~~ — toggle the case of every character on the current line
• These are shorthand forms; gul / gUl + $ would achieve the same result but are more verbose
• A count prefix affects multiple lines: 3guu lowercases 3 lines starting from the current

Example

Before:

Const API_BASE_URL = 'https://Api.Example.Com';

After guu:

const api_base_url = 'https://api.example.com';

After gUU (from the original):

CONST API_BASE_URL = 'HTTPS://API.EXAMPLE.COM';

Tips

• For finer granularity: guiw lowercases the current word, gUiw uppercases it
• guu is especially handy when normalizing ALL-CAPS constants or accidentally-shifted lines
• In Visual mode, select a range of lines and press u (lowercase) or U (uppercase) to apply case conversion to the entire selection
• The case operators respect multibyte characters in most cases, though behavior depends on the locale and Vim's encoding setting