See the Type Aliases and Interfaces sections.

See blog post.

Examples from docs

Extending

Extend an interface

interface Animal {
  name: string
}

interface Bear extends Animal {
  honey: boolean
}

const bear = getBear()
bear.name
bear.honey

Extend a type via intersections

type Animal = {
  name: string
}

type Bear = Animal & {
  honey: Boolean
}

const bear = getBear()
bear.name
bear.honey

Combine two types.

type Name = {
  name: "string"
};

type Age = {
  age: number
};

type Person = Name & Age;

Combine two interfaces. You can’t reverse this as an interface made of two types.

interface Name {
  name: “string”
};

interface Age {
  age: number
};

type Person = Name & Age;

Adding fields

Declare an interface multiple types and they will all be collapsed into one.

interface Window {
  title: string
}

interface Window {
  ts: import("typescript")
}

const src = 'const a = "Hello World"';
window.ts.transpileModule(src, {});

You get an error if you do that with types.

// Error: Duplicate identifier 'Window'.