How to require a specific string in TypeScript interface

Question

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";
}

Try this

export type ReadingTypes = 'some'|'variants'|'of'|'strings';

export interface IReadings {
   param:ReadingTypes
}

Edit: Many thanks for upvotes, but, as time passed and me evolved as a developer :), now in most of the cases I wouldn't recommend this approach anymore. Yes it still valid but the point is the construction above is very similar to enum structure so why not use enum instead (advantages below):

export enum ReadingTypes {
    Some = 'some',
    Variants = 'variants',
    Of = 'of',
    Strings = 'strings',
}
export interface IReadings {
   param: ReadingTypes
}

Advantages: (Yes, might be it is more like IMHO, I understand, but, nonetheless)

1. It is more readable when you see it in the code, for example

if(item.reading === 'some') {
...
}
// vs 
if(item.reading === ReadingTypes.Some) {
...
}

In first case when you read the code you could not catch, from the first glance, that .reading field can only contain few certain params, and not like, any string value.

2. When you write the code you will have better assistance of your editor if you use enums - it is enough to remember name of enum and write it and it will show you all variants of enum. Yeah, with the first type ('some' | 'variants' ... ) it can do so too, but it does it less.. um.. eagerly

TS offers a typing to specific string values, which are called String literal types.

Here is an example of how to use them:

type style =  "collapse" | "expand" | "end-expand" | "none";

interface IOptions {
  indent_size?: number;
  indent_char?: string;
  brace_style1?:  "collapse" | "expand" | "end-expand" | "none";
  brace_style2?:  style;
}

// Ok
let obj1: IOptions = {brace_style1: 'collapse'};

// Compile time error:
// Type '"collapsessss"' is not assignable to type '"collapse" | "expand" | "end-expand" | "none" | undefined'.
let obj2: IOptions = {brace_style1: 'collapsessss'};

Maybe not exactly what you wanted, but Enums seem like a perfect solution for you.

enum BraceStyle {Collapse, Expand, EndExpand, None}

interface IOptions {
  indent_size?: number;
  indent_char?: string;
  brace_style?: BraceStyle
}

Enums are, however, number-based. It means that during runtime a real value for e.g. BraceStyle.Collapse will be 0 in this case. But you can use them with other, even non-typescript scripts, since they compile to objects. This is how BraceStyle will look after compile&run:

{
    0: "Collapse",
    1: "Expand",
    2: "EndExpand",
    3: "None",
    Collapse: 0,
    Expand: 1,
    EndExpand: 2,
    None: 3
}

If you want strings instead, you can use a class with static members, as described here

In TypeScript 2.4 onward you can use String Enums

I favour this approach because it avoids the need to have the same hard coded string in more than one place.

Its possible to make an enum where the values are strings

export enum VISIBILITY {
  PUBLISH = "publish",
  DRAFT = "draft"
}

This enum can then be used as a type on an interface or class

export interface UserOptions  {
  visibility:  VISIBILITY 
}

function keysOf<T>(obj: T, key: keyof T) { return obj[key]; }
interface SomeInterface {
   a: string;
}
const instance: SomeInterface = { a: 'some value'};
let value = keysOf<SomeInterface>(instance, 'b'); // invalid
value =  keysOf<SomeInterface>(instance, 'a'); // valid