TypeScript Exclude Utility Type

`Exclude< T, U >` creates a new union type by removing types assignable to `U` from `T`. It filters out unwanted union members.

Basic Example: Removing Certain Event Types

type AllEvents = "click" | "hover" | "scroll" | "resize";
  
  // Remove 'scroll' and 'resize' events
  type UserEvents = Exclude<AllEvents, "scroll" | "resize">;
  
  const event1: UserEvents = "click"; // OK
  const event2: UserEvents = "hover"; // OK
  // const event3: UserEvents = "scroll"; // Error: Type '"scroll"' is not assignable

Removing `scroll` and `resize` events leaves only user interaction events.

Excluding Nullable Types

type Primitive = string | number | boolean | null | undefined;
  
  // Exclude null and undefined
  type NonNullablePrimitive = Exclude<Primitive, null | undefined>;
  
  const value1: NonNullablePrimitive = "hello"; // OK
  const value2: NonNullablePrimitive = 42; // OK
  // const value3: NonNullablePrimitive = null; // Error
  // const value4: NonNullablePrimitive = undefined; // Error

Removing `null` and `undefined` ensures values are always defined and valid.

Usage in State Management

type Status = "success" | "error" | "pending";
  
  // Exclude 'pending' status to create 'final' statuses
  type FinalStatus = Exclude<Status, "pending">;
  
  function handleStatus(status: FinalStatus) {
    if (status === "success") {
      console.log("Operation succeeded");
    } else {
      console.log("Operation failed");
    }
  }
  
  handleStatus("success"); // OK
  handleStatus("error");   // OK
  // handleStatus("pending"); // Error

Excluding `pending` state defines a type with only final results, simplifying function logic.

Why Use Exclude?

`Exclude` helps create precise and controlled types by filtering out unwanted union members.