TypeScript's literal types allow developers to specify exact values for variables, function parameters, or properties, enhancing type safety by ensuring variables can only hold predefined values.

• Allow variables to have specific, exact values.
• Enhance code reliability by restricting permissible values.

Types of literal types

Here are the different types of literal types:

1. String Literal Types

String literal types allow a variable to accept only a specific set of string values.

type Direction = "Up" | "Down" | "Left" | "Right";

let move: Direction;

move = "Up";
// move = "Forward";

Output:

Up
Error: Type '"Forward"' is not assignable to type 'Direction'

In this example:

• The Direction type can only be one of the specified string literals: "Up", "Down", "Left", or "Right".
• Assigning any value outside this set results in a compile-time error.

2. Numeric Literal Types

Numeric literal types restrict a variable to a specific set of numeric values.

type DiceRoll = 1 | 2 | 3 | 4 | 5 | 6;

function rollDice(): DiceRoll {
  return 4; // Valid
  // return 7; Error
}

console.log(rollDice());

Output:

4
Error: Type '7' is not assignable to type 'DiceRoll'

In this example:

• The DiceRoll type allows only the numbers 1 through 6.
• Returning a number outside this range causes a compile-time error.

3. Boolean Literal Types

Boolean literal types constrain a variable to the boolean values true or false.

type Success = true;

function operation(): Success {
    return true; // Valid return value
    // return false; // Error
}
console.log(operation());

Output:

true
Error: Type 'false' is not assignable to type 'Success'

In this example:

• The Success type is strictly true.
• Returning false would result in a compile-time error.