fact-check

Verify technical accuracy of JavaScript concept pages by checking code examples, MDN/ECMAScript compliance, and external resources to prevent misinformation

# Skill: JavaScript Fact Checker

Use this skill to verify the technical accuracy of concept documentation pages for the 33 JavaScript Concepts project. This ensures we're not spreading misinformation about JavaScript.

## When to Use

- Before publishing a new concept page
- After significant edits to existing content
- When reviewing community contributions
- When updating pages with new JavaScript features
- Periodic accuracy audits of existing content

## What We're Protecting Against

- Incorrect JavaScript behavior claims
- Outdated information (pre-ES6 patterns presented as current)
- Code examples that don't produce stated outputs
- Broken or misleading external resource links
- Common misconceptions stated as fact
- Browser-specific behavior presented as universal
- Inaccurate API descriptions

---

## Fact-Checking Methodology

Follow these five phases in order for a complete fact check.

### Phase 1: Code Example Verification

Every code example in the concept page must be verified for accuracy.

#### Step-by-Step Process

1. **Identify all code blocks** in the document
2. **For each code block:**
   - Read the code and any output comments (e.g., `// "string"`)
   - Mentally execute the code or test in a JavaScript environment
   - Verify the output matches what's stated in comments
   - Check that variable names and logic are correct

3. **For "wrong" examples (marked with ❌):**
   - Verify they actually produce the wrong/unexpected behavior
   - Confirm the explanation of why it's wrong is accurate

4. **For "correct" examples (marked with ✓):**
   - Verify they work as stated
   - Confirm they follow current best practices

5. **Run project tests:**
   ```bash
   # Run all tests
   npm test
   
   # Run tests for a specific concept
   npm test -- tests/fundamentals/call-stack/
   npm test -- tests/fundamentals/primitive-types/
   ```

6. **Check test coverage:**
   - Look in `/tests/{category}/{concept-name}/`
   - Verify tests exist for major code examples
   - Flag examples without test coverage

#### Code Verification Checklist

| Check | How to Verify |
|-------|---------------|
| `console.log` outputs match comments | Run code or trace mentally |
| Variables are correctly named/used | Read through logic |
| Functions return expected values | Trace execution |
| Async code resolves in stated order | Understand event loop |
| Error examples actually throw | Test in try/catch |
| Array/object methods return correct types | Check MDN |
| `typeof` results are accurate | Test common cases |
| Strict mode behavior noted if relevant | Check if example depends on it |

#### Common Output Mistakes to Catch

```javascript
// Watch for these common mistakes:

// 1. typeof null
typeof null        // "object" (not "null"!)

// 2. Array methods that return new arrays vs mutate
const arr = [1, 2, 3]
arr.push(4)        // Returns 4 (length), not the array!
arr.map(x => x*2)  // Returns NEW array, doesn't mutate

// 3. Promise resolution order
Promise.resolve().then(() => console.log('micro'))
setTimeout(() => console.log('macro'), 0)
console.log('sync')
// Output: sync, micro, macro (NOT sync, macro, micro)

// 4. Comparison results
[] == false        // true
[] === false       // false
![]                // false (empty array is truthy!)

// 5. this binding
const obj = {
  name: 'Alice',
  greet: () => console.log(this.name)  // undefined! Arrow has no this
}
```

---

### Phase 2: MDN Documentation Verification

All claims about JavaScript APIs, methods, and behavior should align with MDN documentation.

#### Step-by-Step Process

1. **Check all MDN links:**
   - Click each MDN link in the document
   - Verify the link returns 200 (not 404)
   - Confirm the linked page matches what's being referenced

2. **Verify API descriptions:**
   - Compare method signatures with MDN
   - Check parameter names and types
   - Verify return types
   - Confirm edge case behavior

3. **Check for deprecated APIs:**
   - Look for deprecation warnings on MDN
   - Flag any deprecated methods being taught as current

4. **Verify browser compatibility claims:**
   - Cross-reference with MDN compatibility tables
   - Check Can I Use for broader support data

#### MDN Link Patterns

| Content Type | MDN URL Pattern |
|--------------|-----------------|
| Web APIs | `https://developer.mozilla.org/en-US/docs/Web/API/{APIName}` |
| Global Objects | `https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/{Object}` |
| Statements | `https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/{Statement}` |
| Operators | `https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/{Operator}` |
| HTTP | `https://developer.mozilla.org/en-US/docs/Web/HTTP` |

#### What to Verify Against MDN

| Claim Type | What to Check |
|------------|---------------|
| Method signature | Parameters, optional params, return type |
| Return value | Exact type and possible values |
| Side effects | Does it mutate? What does it affect? |
| Exceptions | What errors can it throw? |
| Browser support | Compatibility tables |
| Deprecation status | Any deprecation warnings? |

---

### Phase 3: ECMAScript Specification Compliance

For nuanced JavaScript behavior, verify against the ECMAScript specification.

#### When to Check the Spec

- Edge cases and unusual behavior
- Claims about "how JavaScript works internally"
- Type coercion rules
- Operator precedence
- Execution order guarantees
- Claims using words like "always", "never", "guaranteed"

#### How to Navigate the Spec

The ECMAScript specification is at: https://tc39.es/ecma262/

| Concept | Spec Section |
|---------|--------------|
| Type coercion | Abstract Operations (7.1) |
| Equality | Abstract Equality Comparison (7.2.14), Strict Equality (7.2.15) |
| typeof | The typeof Operator (13.5.3) |
| Objects | Ordinary and Exotic Objects' Behaviours (10) |
| Functions | ECMAScript Function Objects (10.2) |
| this binding | ResolveThisBinding (9.4.4) |
| Promises | Promise Objects (27.2) |
| Iteration | Iteration (27.1) |

#### Spec Verification Examples

```javascript
// Claim: "typeof null returns 'object' due to a bug"
// Spec says: typeof null → "object" (Table 41)
// Historical context: This is a known quirk from JS 1.0
// Verdict: ✓ Correct, though calling it a "bug" is slightly informal

// Claim: "Promises always resolve asynchronously"
// Spec says: Promise reaction jobs are enqueued (27.2.1.3.2)
// Verdict: ✓ Correct - even resolved promises schedule microtasks

// Claim: "=== is faster than =="
// Spec says: Nothing about performance
// Verdict: ⚠️ Needs nuance - this is implementation-dependent
```

---

### Phase 4: External Resource Verification

All external links (articles, videos, courses) must be verified.

#### Step-by-Step Process

1. **Check link accessibility:**
   - Click each external link
   - Verify it loads (not 404, not paywalled)
   - Note any redirects to different URLs

2. **Verify content accuracy:**
   - Skim the resource for obvious errors
   - Check it's JavaScript-focused (not C#, Python, Java)
   - Verify it's not teaching anti-patterns

3. **Check publication date:**
   - For time-sensitive topics (async, modules, etc.), prefer recent content
   - Flag resources from before 2015 for ES6+ topics

4. **Verify description accuracy:**
   - Does our description match what the resource actually covers?
   - Is the description specific (not generic)?

#### External Resource Checklist

| Check | Pass Criteria |
|-------|---------------|
| Link works | Returns 200, content loads |
| Not paywalled | Free to access (or clearly marked) |
| JavaScript-focused | Not primarily about other languages |
| Not outdated | Post-2015 for modern JS topics |
| Accurate description | Our description matches actual content |
| No anti-patterns | Doesn't teach bad practices |
| Reputable source | From known/trusted creators |

#### Red Flags in External Resources

- Uses `va