utility-types.md

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Utility Types

TypeScript includes several utility types that help you transform and manipulate existing types. These built-in tools make type operations easier and more consistent.

Partial<T>

Makes all properties optional:

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

// All fields are optional
type PartialUser = Partial<User>;

// Valid
const updateUser: PartialUser = {
    name: "John"  // email and id can be omitted
};

Pick<T, K> and Omit<T, K>

Select or remove specific properties:

interface Article {
    title: string;
    content: string;
    tags: string[];
    publishDate: Date;
}

// Only these properties
type ArticlePreview = Pick<Article, 'title' | 'tags'>;

// Everything except these
type DraftArticle = Omit<Article, 'publishDate'>;

Record<K, T>

Creates a type with specified keys and value types:

type UserRoles = Record<string, string[]>;

const roles: UserRoles = {
    admin: ['read', 'write', 'delete'],
    user: ['read'],
    editor: ['read', 'write']
};

Required<T> and Readonly<T>

Make properties required or read-only:

interface Config {
    cache?: boolean;
    timeout?: number;
    retries?: number;
}

// All properties required
type RequiredConfig = Required<Config>;

// All properties read-only
type ReadonlyConfig = Readonly<Config>;

const config: ReadonlyConfig = {
    cache: true,
    timeout: 1000
};
// config.cache = false;  // Error: cannot modify readonly property

Extract<T, U> and Exclude<T, U>

Work with union types:

type Status = 'pending' | 'active' | 'closed' | 'deleted';

// Only these values
type ActiveStatus = Extract<Status, 'pending' | 'active'>;
// Result: 'pending' | 'active'

// Everything except these
type NonDeletedStatus = Exclude<Status, 'deleted'>;
// Result: 'pending' | 'active' | 'closed'

ReturnType<T>

Gets the return type of a function:

function createUser(name: string, age: number) {
    return { id: Date.now(), name, age };
}

type User = ReturnType<typeof createUser>;
// Result: { id: number, name: string, age: number }

Best Practices

1. Use utility types to avoid repeating type definitions
2. Combine utility types for complex transformations
3. Consider creating custom utility types for project-specific needs
4. Document complex type transformations

Practice Exercise

Create a type system for a task management application using utility types:

1. Define a base Task interface
2. Create types for task creation (omitting ID and dates)
3. Create types for task updates (all fields optional)
4. Create a read-only type for completed tasks