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• Outdated information (pre-ES6 patterns presented as current)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

1. For "wrong" examples (marked with ❌):

• Verify they actually produce the wrong/unexpected behavior
• Confirm the explanation of why it's wrong is accurate

1. For "correct" examples (marked with ✓):

• Verify they work as stated
• Confirm they follow current best practices

1. 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/

1. 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

1. Verify API descriptions:

• Compare method signatures with MDN
• Check parameter names and types
• Verify return types
• Confirm edge case behavior

1. Check for deprecated APIs:

• Look for deprecation warnings on MDN
• Flag any deprecated methods being taught as current

1. 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

1. 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

1. Check publication date:

• For time-sensitive topics (async, modules, etc.), prefer recent content
• Flag resources from before 2015 for ES6+ topics

1. 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 var everywhere for ES6+ topics
• Uses callbacks for content about Promises/async
• Teaches jQuery as modern DOM manipulation
• Contains factual errors about JavaScript
• Video is >2 hours without timestamp links
• Content is primarily about another language
• Uses deprecated APIs without noting deprecation

Phase 5: Technical Claims Audit

Review all prose claims about JavaScript behavior.

Claims That Need Verification

| Claim Type | How to Verify | |------------|---------------| | Performance claims | Need benchmarks or caveats | | Browser behavior | Specify which browsers, check MDN | | Historical claims | Verify dates/versions | | "Always" or "never" statements | Check for exceptions | | Comparisons (X vs Y) | Verify both sides accurately |

Red Flags in Technical Claims

• "Always" or "never" without exceptions noted
• Performance claims without benchmarks
• Browser behavior claims without specifying browsers
• Comparisons that oversimplify differences
• Historical claims without dates
• Claims about "how JavaScript works" without spec reference

Examples of Claims to Verify

markdown
❌ "async/await is always better than Promises"
→ Verify: Not always - Promise.all() is better for parallel operations

❌ "JavaScript is an interpreted language"
→ Verify: Modern JS engines use JIT compilation

❌ "Objects are passed by reference"
→ Verify: Technically "passed by sharing" - the reference is passed by value

❌ "=== is faster than =="
→ Verify: Implementation-dependent, not guaranteed by spec

✓ "JavaScript is single-threaded"
→ Verify: Correct for the main thread (Web Workers are separate)

✓ "Promises always resolve asynchronously"
→ Verify: Correct per ECMAScript spec

Common JavaScript Misconceptions

Watch for these misconceptions being stated as fact.

Type System Misconceptions

| Misconception | Reality | How to Verify | |---------------|---------|---------------| | typeof null === "object" is intentional | It's a bug from JS 1.0 that can't be fixed for compatibility | Historical context, TC39 discussions | | JavaScript has no types | JS is dynamically typed, not untyped | ECMAScript spec defines types | | == is always wrong | == null checks both null and undefined, has valid uses | Many style guides allow this pattern | | NaN === NaN is false "by mistake" | It's intentional per IEEE 754 floating point spec | IEEE 754 standard |

Function Misconceptions

| Misconception | Reality | How to Verify | |---------------|---------|---------------| | Arrow functions are just shorter syntax | They have no this, arguments, super, or new.target | MDN, ECMAScript spec | | var is hoisted to function scope with its value | Only declaration is hoisted, not initialization | Code test, MDN | | Closures are a special opt-in feature | All functions in JS are closures | ECMAScript spec | | IIFEs are obsolete | Still useful for one-time initialization | Modern codebases still use them |

Async Misconceptions

| Misconception | Reality | How to Verify | |---------------|---------|---------------| | Promises run in parallel | JS is single-threaded; Promises are async, not parallel | Event loop explanation | | async/await is different from Promises | It's syntactic sugar over Promises | MDN, can await any thenable | | setTimeout(fn, 0) runs immediately | Runs after current execution + microtasks | Event loop, code test | | await pauses the entire program | Only pauses the async function, not the event loop | Code test |

Object Misconceptions

| Misconception | Reality | How to Verify | |---------------|---------|---------------| | Objects are "passed by reference" | References are passed by value ("pass by sharing") | Reassignment test | | const makes objects immutable | const prevents reassignment, not mutation | Code test | | Everything in JavaScript is an object | Primitives are not objects (though they have wrappers) | typeof tests, MDN | | Object.freeze() creates deep immutability | It's shallow - nested objects can still be mutated | Code test |

Performance Misconceptions

| Misconception | Reality | How to Verify | |---------------|---------|---------------| | === is always faster than == | Implementation-dependent, not spec-guaranteed | Benchmarks vary | | for loops are faster than forEach | Modern engines optimize both; depends on use case | Benchmark | | Arrow functions are faster | No performance difference, just different behavior | Benchmark | | Avoiding DOM manipulation is always faster | Sometimes batch mutations are slower than individual | Depends on browser, use case |

Test Integration

Running the project's test suite is a key part of fact-checking.

Test Commands

bash
# Run all tests
npm test

# Run tests in watch mode
npm run test:watch

# Run tests with coverage
npm run test:coverage

# Run tests for specific concept
npm test -- tests/fundamentals/call-stack/
npm test -- tests/fundamentals/primitive-types/
npm test -- tests/fundamentals/value-reference-types/
npm test -- tests/fundamentals/type-coercion/
npm test -- tests/fundamentals/equality-operators/
npm test -- tests/fundamentals/scope-and-closures/

Test Directory Structure

tests/
├── fundamentals/              # Concepts 1-6
│   ├── call-stack/
│   ├── primitive-types/
│   ├── value-reference-types/
│   ├── type-coercion/
│   ├── equality-operators/
│   └── scope-and-closures/
├── functions-execution/       # Concepts 7-8
│   ├── event-loop/
│   └── iife-modules/
└── web-platform/              # Concepts 9-10
    ├── dom/
    └── http-fetch/

When Tests Are Missing

If a concept doesn't have tests:

1. Flag this in the report as "needs test coverage"
2. Manually verify code examples are correct
3. Consider adding tests as a follow-up task

Verification Resources

Primary Sources

| Resource | URL | Use For | |----------|-----|---------| | MDN Web Docs | https://developer.mozilla.org | API docs, guides, compatibility | | ECMAScript Spec | https://tc39.es/ecma262 | Authoritative behavior | | TC39 Proposals | https://github.com/tc39/proposals | New features, stages | | Can I Use | https://caniuse.com | Browser compatibility | | Node.js Docs | https://nodejs.org/docs | Node-specific APIs | | V8 Blog | https://v8.dev/blog | Engine internals |

Project Resources

| Resource | Path | Use For | |----------|------|---------| | Test Suite | /tests/ | Verify code examples | | Concept Pages | /docs/concepts/ | Current content | | Run Tests | npm test | Execute all tests |

Fact Check Report Template

Use this template to document your findings.

markdown
# Fact Check Report: [Concept Name]

**File:** `/docs/concepts/[slug].mdx`
**Date:** YYYY-MM-DD
**Reviewer:** [Name/Claude]
**Overall Status:** ✅ Verified | ⚠️ Minor Issues | ❌ Major Issues

---

## Executive Summary

[2-3 sentence summary of findings. State whether the page is accurate overall and highlight any critical issues.]

**Tests Run:** Yes/No
**Test Results:** X passing, Y failing
**External Links Checked:** X/Y valid

---

## Phase 1: Code Example Verification

| # | Description | Line | Status | Notes |
|---|-------------|------|--------|-------|
| 1 | [Brief description] | XX | ✅/⚠️/❌ | [Notes] |
| 2 | [Brief description] | XX | ✅/⚠️/❌ | [Notes] |
| 3 | [Brief description] | XX | ✅/⚠️/❌ | [Notes] |

### Code Issues Found

#### Issue 1: [Title]

**Location:** Line XX
**Severity:** Critical/Major/Minor
**Current Code:**

// The problematic code

**Problem:** [Explanation of what's wrong]
**Correct Code:**

// The corrected code

---

## Phase 2: MDN/Specification Verification

| Claim | Location | Source | Status | Notes |
|-------|----------|--------|--------|-------|
| [Claim made] | Line XX | MDN/Spec | ✅/⚠️/❌ | [Notes] |

### MDN Link Status

| Link Text | URL | Status |
|-----------|-----|--------|
| [Text] | [URL] | ✅ 200 / ❌ 404 |

### Specification Discrepancies

[If any claims don't match the ECMAScript spec, detail them here]

---

## Phase 3: External Resource Verification

| Resource | Type | Link | Content | Notes |
|----------|------|------|---------|-------|
| [Title] | Article/Video | ✅/❌ | ✅/⚠️/❌ | [Notes] |

### Broken Links

1. **Line XX:** [URL] - 404 Not Found
2. **Line YY:** [URL] - Domain expired

### Content Concerns

1. **[Resource name]:** [Concern - e.g., outdated, wrong language, anti-patterns]

### Description Accuracy

| Resource | Description Accurate? | Notes |
|----------|----------------------|-------|
| [Title] | ✅/❌ | [Notes] |

---

## Phase 4: Technical Claims Audit

| Claim | Location | Verdict | Notes |
|-------|----------|---------|-------|
| "[Claim]" | Line XX | ✅/⚠️/❌ | [Notes] |

### Claims Needing Revision

1. **Line XX:** "[Current claim]"
   - **Issue:** [What's wrong]
   - **Suggested:** "[Revised claim]"

---

## Phase 5: Test Results

**Test File:** `/tests/[category]/[concept]/[concept].test.js`
**Tests Run:** XX
**Passing:** XX
**Failing:** XX

### Failing Tests

| Test Name | Expected | Actual | Related Doc Line |
|-----------|----------|--------|------------------|
| [Test] | [Expected] | [Actual] | Line XX |

### Coverage Gaps

Examples in documentation without corresponding tests:
- [ ] Line XX: [Description of untested example]
- [ ] Line YY: [Description of untested example]

---

## Issues Summary

### Critical (Must Fix Before Publishing)

1. **[Issue title]**
   - Location: Line XX
   - Problem: [Description]
   - Fix: [How to fix]

### Major (Should Fix)

1. **[Issue title]**
   - Location: Line XX
   - Problem: [Description]
   - Fix: [How to fix]

### Minor (Nice to Have)

1. **[Issue title]**
   - Location: Line XX
   - Suggestion: [Improvement]

---

## Recommendations

1. **[Priority 1]:** [Specific actionable recommendation]
2. **[Priority 2]:** [Specific actionable recommendation]
3. **[Priority 3]:** [Specific actionable recommendation]

---

## Verification Checklist

- [ ] All code examples verified for correct output
- [ ] All MDN links checked and valid
- [ ] API descriptions match MDN documentation
- [ ] ECMAScript compliance verified (if applicable)
- [ ] All external resource links accessible
- [ ] Resource descriptions accurately represent content
- [ ] No common JavaScript misconceptions found
- [ ] Technical claims are accurate and nuanced
- [ ] Project tests run and reviewed
- [ ] Report complete and ready for handoff

---

## Sign-off

**Verified by:** [Name/Claude]
**Date:** YYYY-MM-DD
**Recommendation:** ✅ Ready to publish | ⚠️ Fix issues first | ❌ Major revision needed

Quick Reference: Verification Commands

bash
# Run all tests
npm test

# Run specific concept tests
npm test -- tests/fundamentals/call-stack/

# Check for broken links (if you have a link checker)
# Install: npm install -g broken-link-checker
# Run: blc https://developer.mozilla.org/... -ro

# Quick JavaScript REPL for testing
node
> typeof null
'object'
> [1,2,3].map(x => x * 2)
[ 2, 4, 6 ]

Summary

When fact-checking a concept page:

1. Run tests first — npm test catches code errors automatically
2. Verify every code example — Output comments must match reality
3. Check all MDN links — Broken links and incorrect descriptions hurt credibility
4. Verify external resources — Must be accessible, accurate, and JavaScript-focused
5. Audit technical claims — Watch for misconceptions and unsupported statements
6. Document everything — Use the report template for consistent, thorough reviews

Remember: Our readers trust us to teach them correct JavaScript. A single piece of misinformation can create confusion that takes years to unlearn. Take fact-checking seriously.