Distinguish Excess Property Checking from Type Checking

Overview

Object literals get special treatment: TypeScript flags unknown properties.

This catches typos and mistakes that structural typing would miss. But it only applies to object literals - understanding this distinction prevents confusion.

When to Use This Skill

• Assigning object literals to typed variables
• Confused why extra properties cause errors sometimes
• Error disappears when using intermediate variable
• Working with optional properties where typos are likely
• Designing interfaces with many optional fields

The Iron Rule