CASE STUDY 02 / FAMILY FINANCE · WEB + MOBILE
A family budgeting project spanning web and Expo mobile experiences. Transactions, bills, savings goals, and household collaboration bring access rules and financial correctness into the same workflow.
Website, web app, and iPhone listing checked November 6, 2026. Links open in a new tab.
Preview the web app. Budget figures are sample data.
Open NestBudget
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Income, spending, budgets, and a savings goal in one view. Sample data.
Compare planned income and category spending. Sample data.
01 / SYSTEM VIEW
A simplified view to support a technical conversation.
Household budgets and financial workflows
Native sign-in and shared financial logic
Household roles · input validation · version checks
Transactions, bills, and savings goals
Integer cents and recurring dates
Conceptual view of the project. The reviewed financial model tests verify defined behaviors; they are not an end-to-end test of mobile synchronization or the deployed API.
02 / ENGINEERING DECISIONS
Implementation choices paired with the tradeoffs they introduce.
The selected tests check exact decimal parsing and reject fractional-cent inputs, negative amounts, and non-finite values.
Validation rules depend on the field: a zero budget limit can be valid even when a zero transaction is not.
The bill model checks for an existing payment before adding another transaction and next occurrence.
Model-level idempotency does not alone prove protection against concurrent API requests; storage behavior needs a separate check.
Monthly totals isolate dates and currencies. Savings contributions are tested separately from spendable balance.
A clear summary depends on consistent definitions across every client, export, and reporting screen.
03 / REVIEW SCENARIOS
Illustrative explanations of selected test definitions. These controls do not call the application or execute its tests.
A household member records payment for a bill, then repeats the same action.
The selected model test expects no additional transaction and only one next recurring occurrence.
Review concurrency and persistence separately before extending this result to real API retries.
A monthly bill is paid with a January 31 due date.
The selected non-leap-year example expects the next due date to clamp to the last valid day in February.
Test leap years, time zones, and notification scheduling as separate boundaries.
A household role is Viewer, or the role value is unknown.
The selected permission helper test grants edits to Owner and Editor, and denies Viewer and unknown roles.
The corresponding API must enforce the same rule; hiding an edit button is not a security boundary.
04 / TRACEABLE EVIDENCE
Selected source reviewed on November 6, 2026.
Source reviewed · tests not reruntests/finance.test.tsa67c022b5d48These statements summarize the test definitions at this repository snapshot. They do not report a passing run, complete coverage, or an independent security audit. Application source stays private.
WHAT I WOULD EXAMINE NEXT
A fuller production review would test concurrent saves, expired mobile sessions, household membership changes, and recovery after interrupted synchronization.