Typescript 4.1: Template literal types, computed property names, and more

type Foo = 'a' | 'b' | 'c';
// Bar = 'a-template' | 'b-template' | 'c-template'
type Bar = `${Foo}-template`;
In addition, a number of new utility types have been introduced:
type A = Uppercase<'MixedCase'>; // 'MIXEDCASE'
type B = Lowercase<'MixedCase'>; // 'mixedcase'
type C = Capitalize<'MixedCase'>; // 'MixedCase'
type D = Uncapitalize<'MixedCase'>; // 'mixedCase'
Combining the above with the as keyword makes creating Redux-like functionality with strict types a breeze.
type State = {
    foo: string;
    bar: number;
 * Actions = {
 *   setFoo: (state: State, value: string) => void;
 *   setBar: (state: State, value: number) => void;
type Actions = {
  [K in keyof State & string as `set${Capitalize<K>}`]: (state: State, value: State[K]) => void;