Typescript Magician

For every TypeScript challenge:

• Explain the type theory behind the problem
• Provide multiple solution approaches when applicable
• Show before/after type representations
• Include comprehensive type tests
• Ensure full IntelliSense support

Quick Examples

Eliminating any with generics

Before

function getProperty(obj: any, key: string): any {
  return obj[key];
}

After

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}
// getProperty({ name: "Alice" }, "name") → inferred as string ✓

Narrowing an unknown API response

Before

async function fetchUser(): Promise<any> {
  const res = await fetch("/api/user");
  return res.json();
}

After

interface User { id: number; name: string }

function isUser(value: unknown): value is User {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value &&
    "name" in value
  );
}

async function fetchUser(): Promise<User> {
  const res = await fetch("/api/user");
  const data: unknown = await res.json();
  if (!isUser(data)) throw new Error("Invalid user shape");
  return data;
}

Reference

Read individual rule files for detailed explanations and code examples:

Core Patterns

• rules/as-const-typeof.md - Deriving types from runtime values using as const and typeof
• rules/array-index-access.md - Accessing array element types using [number] indexing
• rules/utility-types.md - Built-in utility types: Parameters, ReturnType, Awaited, Omit, Partial, Record

Advanced Generics

• rules/generics-basics.md - Fundamentals of generic types, constraints, and inference
• rules/builder-pattern.md - Type-safe builder pattern with chainable methods
• rules/deep-inference.md - Achieving deep type inference with F.Narrow and const type parameters

Type-Level Programming

• rules/conditional-types.md - Conditional types for type-level if/else logic
• rules/infer-keyword.md - Using infer to extract types within conditional types
• rules/template-literal-types.md - String manipulation at the type level
• rules/mapped-types.md - Creating new types by transforming existing type properties

Type Safety Patterns

• rules/opaque-types.md - Brand types and opaque types for type-safe identifiers
• rules/type-narrowing.md - Narrowing types through control flow analysis
• rules/function-overloads.md - Using function overloads for complex function signatures

Debugging

• rules/error-diagnosis.md - Strategies for diagnosing and understanding TypeScript type errors