fix(dashboard): defensive guards prevent 'is not iterable' on partial backend responses (closes #304)

[cristim]

Summary

• getRecommendations() can resolve to null (when apiRequest's JSON-parse catch fires on a 2xx with empty/non-JSON body) or an unexpected object shape — both reach groupRecsByCell() which uses for...of, throwing "X is not iterable" and replacing the entire dashboard with an error banner.
• Added Array.isArray() guard in loadDashboard() to coerce any non-array fulfilled value to [] before it reaches groupRecsByCell().
• Added try/catch soft-fail in renderDashboardSummary() around the savings-range computation so the Potential Monthly Savings card degrades to $0 rather than blanking the dashboard if the guard is ever bypassed.

Fault site (PR #295 — commit e4a52f9)

// Before: no guard — null/object from apiRequest lands directly in groupRecsByCell
const recs = recsResult.status === 'fulfilled'
  ? (recsResult.value as unknown as LocalRecommendation[])
  : [];

Regression tests added

• #304: getRecommendations returning null does not throw "is not iterable"
• #304: getRecommendations returning a non-array object does not throw "is not iterable"
• #304: summaryData missing by_service field does not throw

All 24 dashboard tests pass.

Test plan

• Run cd frontend && npm test -- --testPathPattern=dashboard — all 24 tests pass
• Verify the three new #304 tests are green
• Manually test with a backend returning 5xx for /recommendations — other cards still render, savings card shows $0

Closes #304

Summary by CodeRabbit

Bug Fixes

• Dashboard now gracefully handles unexpected recommendation data formats, preventing application crashes when the API returns null or other malformed response data
• The savings calculation automatically falls back to displaying $0 when encountering unexpected data structures, instead of causing the interface to crash
• Dashboard rendering continues to work correctly even when summary data is missing optional fields

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7c26afe6-759e-47c3-8304-86443550487f

📥 Commits

Reviewing files that changed from the base of the PR and between f83c914 and 46bf10e.

📒 Files selected for processing (2)

• frontend/src/__tests__/dashboard.test.ts
• frontend/src/dashboard.ts

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📝 Walkthrough

Walkthrough

This PR adds defensive error handling to dashboard data loading to prevent crashes when API endpoints return unexpected data shapes (null, non-array objects, missing fields). It guards against iteration errors in recommendations processing and wraps savings calculations in a try/catch fallback to zero.

Changes

Dashboard Defensive Guards

Layer / File(s)	Summary
Core Implementation frontend/src/dashboard.ts	Added Array.isArray guard in loadDashboard (lines 110–121) to handle null or non-array recommendations. Wrapped savings-range computation in renderDashboardSummary (lines 156–170) with try/catch to degrade "Potential Monthly Savings" to $0 on error instead of crashing the entire dashboard.
Regression Tests frontend/src/__tests__/dashboard.test.ts	Added three tests (lines 554–637) covering: getRecommendations returning null, getRecommendations returning a non-array object, and getDashboardSummary missing the by_service field—all verifying safe fallback rendering without error banners.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Possibly related issues

• fix(dashboard): 'Failed to load dashboard: e is not iterable' — defensive guards on iterable fields #304: This PR directly implements the acceptance criteria for defensive guards on iterable fields in dashboard data loading, including Array.isArray checks, soft-fail try/catch handling for savings calculations, and regression test coverage for null/non-array payload scenarios.

Possibly related PRs

• LeanerCloud/CUDly#295: Adds recommendations fetch to dashboard; this PR adds defensive guards around that new recommendations handling.
• LeanerCloud/CUDly#276: Introduces page-level savings range computation; this PR wraps that computation in defensive try/catch.
• LeanerCloud/CUDly#283: Exports groupRecsByCell and pageLevelRange helpers; this PR defensively calls those helpers with error boundaries.

Suggested labels

impact/many

Poem

🐰 When data shapes are wild and strange,
And nulls and objects rearrange,
We guard with care, we catch with grace,
And fallback $0 takes its place!
No more crashes from the sky—
Just safe dashboards, you and I! 🌟

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately summarizes the main change: defensive guards to prevent 'is not iterable' errors on partial backend responses, fixing issue #304.
Linked Issues check	✅ Passed	The PR addresses all coding requirements from issue #304: adds Array.isArray() guards in loadDashboard() and try/catch in renderDashboardSummary(), includes regression tests covering null/non-array recommendations and missing by_service field.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the dashboard iterable error as specified in issue #304; no unrelated modifications detected in test or implementation files.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-304-dashboard-iterable

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[cristim]

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