How to change to uppercase or lowercase in Vim

The style guide for the shop's ordering code says constants go in capitals, like TAX_RATE at the top of shop.js. The two below it were typed in lowercase. The letters are right, so there's no reason to type them again.

gU and gu work like d and c. What comes after says how much, so gUw changes up to the next word and gU$ everything to the end of the line. Say the verb twice and it takes the whole line from anywhere on it. gUU puts the line in capitals and guu puts it in lowercase.

g~ flips the case of every letter it covers, capitals to small and small to capitals. That makes g~~ the fix for a line somebody typed with Caps Lock on, where the first letter came out small and everything after it came out big.

~ flips the case of the letter under the cursor and then moves one to the right, which makes it the quickest way to give a word its capital first letter. Press it again and it flips the next letter along. A count flips that many in a row, so 3~ does three. On a digit or a comma it changes nothing and still moves on.

When it's easier to see what you're changing, select it first. v starts a selection that you stretch with any move, and V selects whole lines. Then U puts the selection in capitals and u puts it in lowercase. ~ flips it.

For a whole file, the substitute command can do it. In its replacement, \u makes the next character a capital and \U makes everything after it capitals. \<. matches the first character of each word and & stands for whatever matched, so :%s/\<./\u&/g gives every word on every line a capital first letter. Vim counts an underscore as part of a word, so max_rush becomes Max_rush.