Literal Types

Part of the Introduction To TypeScript section of Coddy's JavaScript journey — lesson 33 of 73.

Literal types let you constrain a variable to exact, specific values rather than broad categories. Instead of accepting any string, you can limit it to only "left", "right", "up", or "down".

A literal type uses the exact value as the type itself.

You can create literal types for strings, numbers, or booleans:

let direction: "left" = "left";  // Only "left" is allowed
let count: 42 = 42;              // Only the number 42 is allowed
let isReady: true = true;        // Only true is allowed

Literal types become powerful when combined with union types, creating a controlled set of allowed values. This is perfect for scenarios like directions, status codes, or configuration options:

type Direction = "left" | "right" | "up" | "down";
type Status = "pending" | "approved" | "rejected";

let playerMove: Direction = "up";    // Valid
let orderStatus: Status = "pending"; // Valid
// let invalid: Direction = "diagonal"; // Error!

This approach gives you the flexibility of multiple options while maintaining strict type safety. TypeScript will catch typos and invalid values at compile time, making your code more reliable than using general string types.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Challenge

Easy

Create a type alias named Direction for the literal types "left" | "right" | "up" | "down".

Create a type alias named GameState for the literal types "menu" | "playing" | "paused" | "gameover".

Create a type alias named Difficulty for the literal types "easy" | "medium" | "hard".

Now declare the following variables using your type aliases:

Declare a variable named playerDirection of type Direction and assign it the value "up"
Declare a variable named currentState of type GameState and assign it the value "playing"
Declare a variable named selectedDifficulty of type Difficulty and assign it the value "medium"

Create a function named movePlayer that accepts a parameter called direction of type Direction and returns a string. The function should return a message in the format "Moving [direction]".

Create another function named updateGameState that accepts a parameter called state of type GameState and returns a string. The function should return a message in the format "Game state: [state]".

Call your functions with the variables you created and print the results:

Call movePlayer with playerDirection and print the result
Call updateGameState with currentState and print the result
Print the value of selectedDifficulty

Print each result on a separate line in the order specified above.

Cheat sheet

Literal types constrain variables to exact, specific values rather than broad categories:

let direction: "left" = "left";  // Only "left" is allowed
let count: 42 = 42;              // Only the number 42 is allowed
let isReady: true = true;        // Only true is allowed

Combine literal types with union types to create controlled sets of allowed values:

type Direction = "left" | "right" | "up" | "down";
type Status = "pending" | "approved" | "rejected";

let playerMove: Direction = "up";    // Valid
let orderStatus: Status = "pending"; // Valid

This provides type safety by catching typos and invalid values at compile time while maintaining flexibility with multiple options.

Try it yourself

// TODO: Write your code here
// Create type aliases for Direction, GameState, and Difficulty
// Declare variables using your type aliases
// Create the movePlayer and updateGameState functions
// Call the functions and print the results

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Literal Types

Cheat sheet

All lessons in Introduction To TypeScript