How to require a specific string in TypeScript interface

Issue

I'm creating a TypeScript definition file for a 3rd party js library. One of the methods allows for an options object, and one of the properties of the options object accepts a string from the list: "collapse", "expand", "end-expand", and "none".

I have an interface for the options object:

interface IOptions {
  indent_size?: number;
  indent_char?: string;
  brace_style?: // "collapse" | "expand" | "end-expand" | "none"
}

Can the interface enforce this, so if you include an IOptions object with the brace_style property, it will only allow a string that is in the acceptable list?

Solution

This was released in version 1.8 as "string literal types"

What's New in Typescript - String Literal Types

Example from the page:

interface AnimationOptions {
  deltaX: number;
  deltaY: number;
  easing: "ease-in" | "ease-out" | "ease-in-out";
}

Answered By - Ryan Q