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

Issue

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?

Solution

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.
}; 

Answered By - Titian Cernicova-Dragomir