Literal Types & Template Literal Types

Type Inference & Literal Types (const vs. let)

Discover how TypeScript infers types automatically and how let versus const declarations shape wide types versus narrow literal types.

1. Pattern-Based Types

TypeScript allows you to manipulate literal types using string template literal syntax directly in the type system.

2. String Template Literal Types

Just as template literals construct strings at runtime, Template Literal Types construct string types at compile time:

type EventType = "click" | "hover" | "focus";
type Component = "button" | "input" | "card";

// Automatically generates 9 literal string combinations!
type DOMEventName = `${Component}:${EventType}`;
// Inferred as: "button:click" | "button:hover" | "input:click" | ...

const handleEvent = (event: DOMEventName) => {
  console.log("Handling:", event);
};

handleEvent("button:click"); // Valid!
// handleEvent("header:click"); // Error: Type "header:click" is not assignable

3. Advanced Pattern Matching (API Endpoints)

Use template literal types to enforce API route paths or CSS spacing tokens:

type HTTPMethod = "GET" | "POST";
type APIVersion = "v1" | "v2";

type Endpoint = `/api/${APIVersion}/${string}`;

const validRoute: Endpoint = "/api/v1/users";
// const badRoute: Endpoint = "/v1/users"; // Error: Must start with /api/

4. Built-in String Utility Functions

TypeScript provides built-in intrinsic string manipulation type utilities:

• Uppercase<StringType>
• Lowercase<StringType>
• Capitalize<StringType>
• Uncapitalize<StringType>

type Action = "create" | "update";
type GetterName = `get${Capitalize<Action>}`; // "getCreate" | "getUpdate"

5. Common Mistakes

• Combinatorial Explosion: Creating template literal unions from large arrays can produce thousands of string type permutations, slowing down compiler performance.

6. Next Steps

Use template literal types for design system tokens and API route validation. Next, let's explore Type Assertions and as unknown as T!