Think of Types as Sets of Values

Overview

A type is a set of possible values. Assignability means subset.

Understanding types as sets helps you reason about unions, intersections, extends, and never. This mental model makes TypeScript's behavior intuitive.

When to Use This Skill

• Confused why A & B has MORE properties than A or B
• Don't understand why extends means "subset"
• Reasoning about union and intersection types
• Working with never or unknown types
• Debugging "not assignable to" errors

The Iron Rule