Template literal-types

type Greeting = `Hello, ${string}!`;
const a: Greeting = 'Hello, Alice!';  // OK
const b: Greeting = 'Hi, Alice!';     // Error

type Direction = 'top' | 'right' | 'bottom' | 'left';
type Property = 'margin' | 'padding';

type Side = `${Property}-${Direction}`;
// 'margin-top' | 'margin-right' | ... | 'padding-left' (8 strings)

type EventName = 'click' | 'focus' | 'blur' | 'change';
type HandlerName = `on${Capitalize<EventName>}`;
// 'onClick' | 'onFocus' | 'onBlur' | 'onChange'

type Handlers = { [K in HandlerName]?: (e: Event) => void };
// { onClick?: ...; onFocus?: ...; onBlur?: ...; onChange?: ... }

type Hi = Capitalize<'hello'>;    // 'Hello'
type Yell = Uppercase<'hello'>;   // 'HELLO'
type Quiet = Lowercase<'HELLO'>;  // 'hello'
type Lower = Uncapitalize<'Hi'>;  // 'hi'

type Px = `${number}px`;
type Hex = `#${string}`;
type Pct = `${number}%`;

type Style = { padding: Px; color: Hex; width: Pct };

const s: Style = { padding: '8px', color: '#fff', width: '50%' }; // OK
const bad: Style = { padding: '8', color: 'white', width: '50' }; // Error

type ExtractName<T> = T extends `Hello, ${infer Name}!` ? Name : never;

type Result = ExtractName<'Hello, Alice!'>;  // 'Alice'

type PathParams<T extends string> =
  T extends `${string}/:${infer Param}/${infer Rest}`
    ? { [K in Param | keyof PathParams<`/${Rest}`>]: string }
    : T extends `${string}/:${infer Param}`
    ? { [K in Param]: string }
    : {};

type P = PathParams<'/users/:userId/posts/:postId'>;
// { userId: string; postId: string }

type Spacing = 0 | 1 | 2 | 4 | 8 | 16;
type Side = 't' | 'r' | 'b' | 'l';
type TwSpacing = `m${Side}-${Spacing}` | `p${Side}-${Spacing}`;
// 'mt-0' | 'mt-1' | ... | 'pl-16'

function Box(props: { className: TwSpacing }) { /* ... */ }