Union Types, Intersection Types and Type Guards

Union and intersection types are two of TypeScript's most powerful features for composing types. Combined with type guards, they allow you to write code that is both flexible and safe. This lesson covers how to create, narrow, and combine types effectively.

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Union Types

A union type describes a value that can be one of several types. Use the pipe | operator:

type StringOrNumber = string | number;

function formatValue(value: StringOrNumber): string {
  if (typeof value === 'string') {
    return value.toUpperCase();
  }
  return value.toFixed(2);
}

formatValue('hello');  // 'HELLO'
formatValue(3.14159);  // '3.14'

Union types are not limited to two members:

type Status = 'idle' | 'loading' | 'success' | 'error';
type PrimitiveValue = string | number | boolean | null;

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Type Narrowing with Type Guards

When you have a union type, you must narrow it before accessing type-specific properties. TypeScript recognises several built-in type guards.

typeof

The typeof operator works for primitive types: