Be Liberal in What You Accept and Strict in What You Produce

Overview

Accept broad input types, return narrow output types.

This is Postel's Law applied to TypeScript: functions should be flexible about what they accept but precise about what they return. This makes APIs easier to use and types more useful.

When to Use This Skill

• Designing function parameters and return types
• Creating reusable APIs or libraries
• Function returns feel too broad for callers to use
• Want to accept multiple input formats
• Struggling with optional fields that shouldn't be optional in output

The Iron Rule