fact-check

name fact-check description 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 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