meet it

Helper types change the letters of a literal type.

TypeScript has four built-in helpers for literal string types. is "TEA", and Lowercase does the reverse. is "Tea", changing only the first letter, and Uncapitalize turns it back.

They shine inside template literal types. If your fields are "name" | "email", then set${Capitalize<Field>} is "setName" | "setEmail". Add a field and its setter name follows.

type Field = "name" | "email";
type Setter = `set${Capitalize<Field>}`;
type Shout = Uppercase<"stop">;
const setters: Setter[] = ["setName", "setEmail"];
const sign: Shout = "STOP";
console.log(setters, sign);

These helpers only work on types. At runtime you still need .toUpperCase().

guess first

One small letter. What happens?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

type Field = "name" | "email";
type Setter = `set${Capitalize<Field>}`;
const call: Setter = "setname";
console.log(call);

your turn

Your turn. Name the traffic lights.

Choose the helper that makes every letter a capital, then list both light names that fit the pattern.

Your code

press run to see what happens

fix it

The pet getters will not match. Fix the type.

Broken code to fix

press run to see what happens

Read the error. Which strings does Getter allow right now?
get${Pet} gives "getcat" and "getdog", all small letters.
On line 2, use Capitalize<Pet> inside the pattern.

nailed it

your type names change case on command. :D

Uppercase, Lowercase, Capitalize and Uncapitalize work on literal types.
Combine them with patterns, like set${Capitalize<Field>}.
They change types only; values still need string methods.

new words in your collection

Uppercase
Capitalize

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