literal-types.md

Literals

Literals are exact values that are JavaScript primitives.

String Literals

You can use a string literal as a type. For example:

let foo: 'Hello';

Here we have created a variable called foo that will only allow the literal value 'Hello' to be assigned to it. This is demonstrated below:

let foo: 'Hello';
foo = 'Bar'; // Error: "Bar" is not assignable to type "Hello"

They are not very useful on their own but can be combined in a type union to create a powerful (and useful) abstraction e.g.:

type CardinalDirection =
    | "North"
    | "East"
    | "South"
    | "West";

function move(distance: number, direction: CardinalDirection) {
    // ...
}

move(1,"North"); // Okay
move(1,"Nurth"); // Error!

Other literal types

TypeScript also supports boolean and number literal types, e.g.:

type OneToFive = 1 | 2 | 3 | 4 | 5;
type Bools = true | false;

Inference

Quite commonly you get an error like Type string is not assignable to type "foo". The following example demonstrates this.

function iTakeFoo(foo: 'foo') { }
const test = {
  someProp: 'foo'
};
iTakeFoo(test.someProp); // Error: Argument of type string is not assignable to parameter of type 'foo'

This is because test is inferred to be of type {someProp: string}. The fix here is to use a simple type assertion to tell TypeScript the literal you want it to infer as shown below:

function iTakeFoo(foo: 'foo') { }
const test = {
  someProp: 'foo' as 'foo'
};
iTakeFoo(test.someProp); // Okay!

or use a type annotation that helps TypeScript infer the correct thing at the point of declaration:

function iTakeFoo(foo: 'foo') { }
type Test = {
  someProp: 'foo',
}
const test: Test = { // Annotate - inferred someProp is always === 'foo'
  someProp: 'foo' 
}; 
iTakeFoo(test.someProp); // Okay!

Use cases

Valid use cases for string literal types are:

String based enums

TypeScript enums are number based. You can use string literals with union types to mock a string based enum as we did in the CardinalDirection example above. You can even generate a Key:Value structure using the following function:

/** Utility function to create a K:V from a list of strings */
function strEnum<T extends string>(o: Array<T>): {[K in T]: K} {
  return o.reduce((res, key) => {
    res[key] = key;
    return res;
  }, Object.create(null));
}

And then generate the literal type union using keyof typeof. Here is a complete example:

/** Utility function to create a K:V from a list of strings */
function strEnum<T extends string>(o: Array<T>): {[K in T]: K} {
  return o.reduce((res, key) => {
    res[key] = key;
    return res;
  }, Object.create(null));
}

/**
  * Sample create a string enum
  */

/** Create a K:V */
const Direction = strEnum([
  'North',
  'South',
  'East',
  'West'
])
/** Create a Type */
type Direction = keyof typeof Direction;

/** 
  * Sample using a string enum
  */
let sample: Direction;

sample = Direction.North; // Okay
sample = 'North'; // Okay
sample = 'AnythingElse'; // ERROR!

Modelling existing JavaScript APIs

E.g. CodeMirror editor has an option readOnly that can either be a boolean or the literal string "nocursor" (effective valid values true,false,"nocursor"). It can be declared as:

readOnly: boolean | 'nocursor';

Discriminated Unions

We will cover this later in the book.