Advanced Techniques with Type Guards in TypeScript

Dive deep into advanced TypeScript type guard patterns including custom type guards, discriminated unions, recursive type guards, and generic implementations for building type-safe applications.

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Advanced Techniques with Type Guards

Type guards are one of TypeScript’s most powerful features, yet they often don’t get the attention they deserve. Today, let’s dive deep into some advanced techniques that will level up your type-checking game.

Understanding Custom Type Guards

You might be familiar with typeof and instanceof checks, but custom type guards take things to a whole new level. They allow us to create our own type-narrowing functions that TypeScript fully understands.

Let’s start with a real-world example:

interface User {
  id: string;
  name: string;
  email: string;
}

interface Admin extends User {
  role: 'admin';
  permissions: string[];
}

function isAdmin(user: User | Admin): user is Admin {
  return 'role' in user && user.role === 'admin';
}

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Advanced Pattern: Discriminated Unions with Type Guards

One particularly powerful pattern combines discriminated unions with type guards. This approach gives us compile-time exhaustiveness checking while maintaining runtime type safety.

type Result\<T\> =
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error }
  | { status: 'loading' };

function isSuccess\<T\>(result: Result\<T\>): result is { status: 'success'; data: T } {
  return result.status === 'success';
}

function processResult\<T\>(result: Result\<T\>) {
  if (isSuccess(result)) {
    // TypeScript knows result.data exists here
    console.log(result.data);
  }
}

Recursive Type Guards

When dealing with nested structures, we sometimes need type guards that can handle recursion. Here’s an elegant solution:

interface TreeNode {
  value: string;
  children?: TreeNode[];
}

function isValidTreeNode(node: unknown): node is TreeNode {
  if (typeof node !== 'object' || node === null) return false;

  const candidate = node as TreeNode;
  if (typeof candidate.value !== 'string') return false;

  if (candidate.children !== undefined) {
    if (!Array.isArray(candidate.children)) return false;
    return candidate.children.every(isValidTreeNode);
  }

  return true;
}

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Type Guards with Generics

Combining type guards with generics opens up even more possibilities. Here’s how we can create type-safe validation functions:

type Validator\<T\> = (value: unknown) => value is T;

function validate\<T\>(value: unknown, validator: Validator\<T\>): T {
  if (validator(value)) {
    return value;
  }
  throw new Error('Validation failed');
}

// Example usage
const isString = (value: unknown): value is string => typeof value === 'string';
const validatedString = validate('hello', isString); // type is string

Conclusion

Type guards are more than just simple type checks - they’re a powerful tool for building type-safe applications. By mastering these advanced techniques, you’ll be able to write more robust and maintainable TypeScript code.

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