TypeScript Utility Types

Also Known As

TL;DR

Explanation

TypeScript ships with utility types for common transformations: Partial<T> makes all properties optional; Required<T> makes all required; Readonly<T> makes all readonly; Pick<T,K> selects a subset of properties; Omit<T,K> removes properties; Record<K,V> creates an object type with specific key/value types; ReturnType<F> extracts a function's return type; Parameters<F> extracts parameter types. These eliminate the copy-paste type maintenance problem — derive types from a single source of truth.

Diagram

flowchart LR
    BASE[User type<br/>id name email role]
    BASE -->|Partial| PART[All fields optional<br/>good for update DTOs]
    BASE -->|Required| REQ[All fields required]
    BASE -->|Readonly| RO[All fields readonly]
    BASE -->|Pick name email| PICK[Only name and email]
    BASE -->|Omit role| OMIT[All except role]
    BASE -->|Record string User| REC[Dictionary of Users]
    subgraph Common_Uses
        PART -->|patch endpoints| USE1[UpdateUserDto]
        PICK -->|public API| USE2[PublicProfile]
        RO -->|immutable config| USE3[AppConfig]
    end
style BASE fill:#6e40c9,color:#fff
style PART fill:#1f6feb,color:#fff
style PICK fill:#238636,color:#fff
style RO fill:#d29922,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Manually writing partial or pick types instead of using Partial<T> and Pick<T,K>.
• Using Record<string, any> when a more specific value type is known.
• Not using ReturnType<typeof fn> to infer return types — avoids duplication when the function signature changes.
• Forgetting that utility types are shallow — Partial<T> does not make nested objects partial; use DeepPartial for that.

Code Examples

// Manually duplicated partial type — drifts when User changes:
type User = { id: number; name: string; email: string; role: string };

// Manual copy — must be updated every time User changes:
type UpdateUserDTO = {
    name?: string;
    email?: string;
    role?: string;
};

// Utility types — derived, never drift:
type User = { id: number; name: string; email: string; role: string; password: string };

type UpdateUserDTO = Omit<Partial<User>, 'id' | 'password'>;
// Equivalent to: { name?: string; email?: string; role?: string }
// Automatically includes new User fields, excludes id and password

type UserPublic = Omit<User, 'password'>; // Safe for API responses
type UserKeys = keyof User; // 'id' | 'name' | 'email' | 'role' | 'password'

References

• ↗ https://www.typescriptlang.org/docs/handbook/utility-types.html

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