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• Outdated information (pre-ES6 patterns presented as current)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

Identify all code blocks in the document

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

For "wrong" examples (marked with ❌):

• Verify they actually produce the wrong/unexpected behavior

• Confirm the explanation of why it's wrong is accurate

For "correct" examples (marked with ✓):

• Verify they work as stated

• Confirm they follow current best practices

Run project tests:

Run all tests

npm test

Run tests for a specific concept

npm test -- tests/fundamentals/call-stack/ npm test -- tests/fundamentals/primitive-types/

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

// 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

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

Verify API descriptions:

• Compare method signatures with MDN

• Check parameter names and types

• Verify return types

• Confirm edge case behavior

Check for deprecated APIs:

• Look for deprecation warnings on MDN

• Flag any deprecated methods being taught as current

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

// 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

Check link accessibility:

• Click each external link

• Verify it loads (not 404, not paywalled)

• Note any redirects to different URLs

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

Check publication date:

• For time-sensitive topics (async, modules, etc.), prefer recent content

• Flag resources from before 2015 for ES6+ topics

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

❌ "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

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:

• Flag this in the report as "needs test coverage"

• Manually verify code examples are correct

• 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.

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

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

- Line XX: [URL] - 404 Not Found

- Line YY: [URL] - Domain expired

Content Concerns

- [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

- 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)

- [Issue title]

- Location: Line XX

- Problem: [Description]

- Fix: [How to fix]

Major (Should Fix)

- [Issue title]

- Location: Line XX

- Problem: [Description]

- Fix: [How to fix]

Minor (Nice to Have)

- [Issue title]

- Location: Line XX

- Suggestion: [Improvement]

Recommendations

- [Priority 1]: [Specific actionable recommendation]

- [Priority 2]: [Specific actionable recommendation]

- [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:

- Run tests first — npm test
 catches code errors automatically

- Verify every code example — Output comments must match reality

- Check all MDN links — Broken links and incorrect descriptions hurt credibility

- Verify external resources — Must be accessible, accurate, and JavaScript-focused

- Audit technical claims — Watch for misconceptions and unsupported statements

- 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.