Index signatures in TypeScript

Index signature specifies to which format of key:value the object must conform.

While you can specify all properties of an interface

interface IndexedByString {
  a: number,
  b: number,
  c: number
}

You might want to let the key be anything of a particular type.

interface IndexedByString {
  [index:string]:number
}

This way you can use any value conforming to the index type.

let foo: IndexedByString = {
  'a': 1,
  'b': 123,
  'z': 700
}

console.log(foo['z'])