Interfaces

Inline

function foo(bar: { bazz: string }) {
  console.log(bar.bazz)
}

// Note the semicolon.
function foo(): { a: string; b?: number } {
}

Explicit

Define an interface then use it later.

interface Bar {
  bazz: string
}

function foo(bar: Bar) {
  console.log(bar.bazz)
}

You can define in the file where it is used as above, or export it in a .d.ts type definitions file.

export interface Bar {
  bazz: string
}

Set an optional key:

interface Bar {
  bazz?: string
}

Make it read-only.

interface Bar {
  readonly bazz: string
}

interface Bar {
  bazz: string[] // array
  buzz: { [key: string]: string } // dictionary
}

You can extract a type from an interface.

interface Building {
  room: {
    door: string,
    walls: string[],
  };
}

// string[]
type Walls = Building['room']['walls'];