Merging Interfaces in Typescript

The simplest, and perhaps most common, type of declaration merging is interface merging. At the most basic level, the merge mechanically joins the members of both declarations into a single interface with the same name.

interface Box {
  height: number;
  width: number;
}
interface Box {
  scale: number;
}
let box: Box = { height: 5, width: 6, scale: 10 };