Problem Solving & Design 92

The widget solves the stated problem cleanly with strong visual hierarchy, polished light/dark theming, and solid responsive design across three breakpoints.

• +Three summary stats are visually prominent in an accented grid above the table, satisfying the brief's hierarchy requirement
• +Viewport meta tag present, and three breakpoints (820px, 560px) with responsive Tailwind-equivalent custom CSS classes handle mobile/desktop layout adaptation well
• +Light/dark theme toggle with CSS custom properties and `color-scheme` is a thoughtful UX addition beyond the brief
• +Mobile UX additions (Load more, Back to inputs FAB, auto-scroll IntersectionObserver) show strong usability thinking
• +Semantic HTML: `<form>`, `<label for>`, `aria-live='polite'` on results, `aria-label` on form and theme control
• -Desktop pagination instead of a scrollable table is a minor deviation from the brief's explicit 'scrollable if long' instruction

Completeness 88

All required brief features are present and correctly wired: live calculation, three summary stats, payoff table with correct columns, low-payment warning, and no submit button needed.

• +All four required outputs (months, total paid, total interest, breakdown table) are present and rendered in `index.html` wired to live `input` event listeners in `app.js`
• +Low-payment warning (`Payment too low - balance will never be paid off`) is implemented in `setWarning()` and toggled correctly
• +Month-by-month table includes all four required columns: month, interest, principal, remaining balance via `createScheduleRow()`
• +Live recalculation fires on every keystroke via `input` event on all three fields with no submit button
• -Table scrollability relies on pagination/load-more rather than a simple scrollable container; on desktop the table is paginated (10 rows/page) rather than scrollable, which diverges slightly from the brief's 'scrollable if long' spec

Technical Craft 83

Technically sound vanilla JS SPA with well-structured calculation logic, clean separation of concerns, good security hygiene, and notable craft in responsive behaviour and theming.

• +`calculateSummary` uses the correct closed-form amortisation formula (log-based month count, accurate final payment calculation) rather than a loop approximation
• +Separation of concerns is clear: calculation logic, DOM rendering, pagination state, and event wiring are each in distinct functions
• +`createRows` uses `DocumentFragment` for efficient batch DOM insertion, showing performance awareness
• +Security hygiene is clean: no secrets, no `innerHTML` on user input (table data uses `.textContent` via `createScheduleRow` template literals with numeric-only values), no external dependencies
• +`IntersectionObserver`-based auto-load for mobile with graceful fallback for unsupporting browsers shows defensive coding
• -No input sanitisation beyond `Number()` coercion; a non-numeric string returns 0 silently with no user feedback
• -`innerHTML` is used in `createScheduleRow` with interpolated values — while currently safe (all values are formatted numbers), it is a mild pattern risk
• -No automated tests present; logic correctness relies entirely on manual verification