---
name: [pattern-name]
description: [One-line description of when to use this pattern]
---

# [Pattern Title]

## Problem

[What situation triggers the need for this pattern?]

## Solution

[The approach that works]

## Example

[Concrete code example or command sequence]

\`\`\`typescript
// Context: corbat-coco TypeScript ESM
// Problem: [describe]
// Solution:
[code]
\`\`\`

## Why It Works

[Brief explanation of the underlying mechanism]

## Activation

Use this skill when you encounter:
- [trigger 1]
- [trigger 2]

mkdir -p .claude/skills/learned/[pattern-name]
# Then write SKILL.md with Write tool

✅ Learned: [pattern-name]
   Saved to: .claude/skills/learned/[pattern-name]/SKILL.md
   Trigger: [when this will be useful]

---
name: esm-import-js-extension
description: TypeScript ESM imports require .js extension even for .ts source files
---

# ESM Import .js Extension Requirement

## Problem
`Cannot find module './foo'` even though `./foo.ts` exists.

## Solution
TypeScript ESM requires `.js` extension in imports (resolved to `.ts` at compile time):

\`\`\`typescript
// ❌ Fails
import { bar } from "./foo";

// ✅ Works
import { bar } from "./foo.js";
\`\`\`

## Why It Works
Node.js ESM resolver looks for literal file extensions. TypeScript with `moduleResolution: NodeNext`
maps `.js` → `.ts` at compile time. This is intentional, not a bug.

## Activation
Whenever you see "Cannot find module" for a local import in corbat-coco.

/learn                    # extract from this session
/learn the mock pattern   # extract specific thing learned
/learn esm import issue   # extract specific resolved issue