feat(frontend): extractValue helper for data export

frontend/src/lib/dataExport.js

@@ -0,0 +1,23 @@
+// Pure helpers for exporting daily-parameter data as a date×subject CSV matrix.
+// No React, no I/O — mirrors the crossSubjectChart.js pure-helper pattern.
+
+// Read a single parameter's raw value from one daily status.
+// Returns null when the value is absent. Note: 0 and '' are returned as-is
+// (real values); only genuinely-missing lookups become null.
+export function extractValue(status, param) {
+  if (!status || !param) return null;
+  switch (param.kind) {
+    case 'total':
+      return status.analysis_summary?.total ?? null;
+    case 'success_rate':
+      return status.analysis_summary?.success_rate ?? null;
+    case 'category':
+      return status.analysis_summary?.counts?.[param.category] ?? null;
+    case 'builtin':
+      return status[param.statusKey] ?? null;
+    case 'custom':
+      return status.custom_fields?.[param.fieldId] ?? null;
+    default:
+      return null;
+  }
+}

frontend/tests/dataExport.test.js

@@ -0,0 +1,33 @@
+import { extractValue } from '../src/lib/dataExport';
+
+const status = {
+  vitals: 'ok',
+  custom_fields: { 'f-weight': 250 },
+  analysis_summary: { total: 12, success_rate: 0.83, counts: { Success: 10, Failure: 2 } },
+};
+
+describe('extractValue', () => {
+  it('reads a builtin field via statusKey', () => {
+    expect(extractValue(status, { kind: 'builtin', statusKey: 'vitals' })).toBe('ok');
+  });
+  it('reads a custom field via fieldId', () => {
+    expect(extractValue(status, { kind: 'custom', fieldId: 'f-weight' })).toBe(250);
+  });
+  it('reads session-metric total', () => {
+    expect(extractValue(status, { kind: 'total' })).toBe(12);
+  });
+  it('reads session-metric success_rate as a raw 0-1 fraction', () => {
+    expect(extractValue(status, { kind: 'success_rate' })).toBe(0.83);
+  });
+  it('reads a dynamic count category', () => {
+    expect(extractValue(status, { kind: 'category', category: 'Failure' })).toBe(2);
+  });
+  it('returns null when analysis_summary is absent', () => {
+    expect(extractValue({ custom_fields: {} }, { kind: 'total' })).toBeNull();
+    expect(extractValue({}, { kind: 'category', category: 'Success' })).toBeNull();
+  });
+  it('returns null for a missing custom field', () => {
+    expect(extractValue({ custom_fields: {} }, { kind: 'custom', fieldId: 'nope' })).toBeNull();
+    expect(extractValue({}, { kind: 'builtin', statusKey: 'notes' })).toBeNull();
+  });
+});