TypeScript-All-In-One(A.R)

Literals

Literals are exact values that are JavaScript primitives.

A Literal Type is a single element set from a collective type, it defines a very exact value that is a JavaScript primitive.

Literal Types in TypeScript are numbers, strings, and booleans.

Example of literals:

const a = 'a'; // String literal type
const b = 1; // Numeric literal type
const c = true; // Boolean literal type

String, Numeric, and Boolean Literal Types are used in the union, type guard, and type aliases. In the following example you can see a type alias union, O can be the only value specified and not any other string:

type O = 'a' | 'b' | 'c';

Template Literal Types

Template Literal Types allow to manipulate string value at type level and generate new string types based on existing ones. They are useful to create more expressive and precise types from string-based operations.

type Department = 'engineering' | 'hr';
type Language = 'english' | 'spanish';
type Id = `${Department}-${Language}-id`; // "engineering-english-id" | "engineering-spanish-id" | "hr-english-id" | "hr-spanish-id"

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;