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In the real world, variables and API responses do not always have a single fixed type. TypeScript offers Union Types to handle variability, and Type Narrowing to operate safely on them at runtime.

Union Types

A union type allows a variable to accept values of different types. It is expressed using the vertical bar (|) symbol:

let result: number | string;
result = 42; // Valid
result = 'Error 404'; // Valid

However, when working with a union type, we cannot directly call methods that belong to only one of the types (for example, we cannot call .toUpperCase() if the variable can also be a number). We must first "narrow" the type.

Type Narrowing

Type Narrowing is the process where TypeScript analyzes control flow structures (such as if or switch) to deduce a more specific type for a variable at runtime.

There are several ways to perform narrowing:

1. The typeof Operator

Ideal for distinguishing primitive types:

function printLength(value: string | number) {
  if (typeof value === 'string') {
    // TypeScript knows 'value' is a string here
    console.log(value.length);
  } else {
    // TypeScript knows 'value' is a number here
    console.log(value.toFixed(2));
  }
}

2. The in Operator

Used to check if a specific property exists on an object:

interface Fish {
  swim: () => void;
}
interface Bird {
  fly: () => void;
}

function move(animal: Fish | Bird) {
  if ('swim' in animal) {
    animal.swim(); // Narrowing to Fish
  } else {
    animal.fly(); // Narrowing to Bird
  }
}

Discriminated Unions

The Discriminated Unions pattern consists of creating objects that share a common property with a unique literal value (called a discriminator). TypeScript recognizes this discriminator and automatically narrows the type within conditional blocks.

interface SuccessResponse {
  status: 'success'; // Literal discriminator
  data: string;
}

interface ErrorResponse {
  status: 'error'; // Literal discriminator
  errorMessage: string;
}

type ApiResponse = SuccessResponse | ErrorResponse;

function handleResponse(response: ApiResponse) {
  if (response.status === 'success') {
    console.log('Data received:', response.data);
  } else {
    console.error('An error occurred:', response.errorMessage);
  }
}

Try it yourself

Exercise 1: Union Types

Exercise 2: Basic Type Narrowing

Exercise 3: Narrowing with 'in'

Exercise 4: Discriminated Union Shape