How to understand syntax in TypeScript interface [key: string]

Question

I'm having trouble deciphering a TypeScript syntax I found within an interface declaration here.

interface FormattingOptions {
    tabSize: number;
    insertSpaces: boolean;
    [key: string]: boolean | number | string;
}

Can someone explain me the third parameter of this interface? The one containing [key: string] ...? How is this type of syntax called?

Answer 1

It's an index signature. It means that besides the known properties of the interface, any other properties of type boolean, number or string can be present

interface FormattingOptions {
    tabSize: number;
    insertSpaces: boolean;
    [key: string]: boolean | number | string;
}

let f: FormattingOptions = {
  tabSize: 1,
  insertSpaces: true,
  other: '' // witout the  index signature this would be invalid.
};