test(scripts): cover commit-msg footer append behavior

Address review feedback: extract a pure appendTicketFooter() from main()
so the footer-writing logic is unit-testable, and add tests for footer
append, existing-footer dedup (TICKET/ISSUE, any case), null-ticket
skip, branch-start anchoring, and global-regex lastIndex safety.

TICKET: WEB-000

Author: mmcshinsky-bitgo

scripts/prepare-commit-msg.js

@@ -17,31 +17,46 @@ const escapedPrefixes = issuePrefixes
   .filter((p) => p !== '#')
   .map((p) => p.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'));
 // Anchored at branch start; case-insensitive so "web-000-desc" matches "WEB-"
-const branchRegex = new RegExp(`^(${escapedPrefixes.join('|')})(\\d+)`, 'i');
+// null when no prefixes are configured — avoids ^()(\d+) matching bare-digit branches.
+const branchRegex =
+  escapedPrefixes.length > 0 ? new RegExp(`^(${escapedPrefixes.join('|')})(\\d+)`, 'i') : null;
 // ex TICKET: WP-1234
 const commitRegex = /(ticket|issue):\s(\S+)/gim;
 
 function extractTicket(branch) {
+  if (!branchRegex) return null;
   const found = branch.match(branchRegex);
   return found ? found[0].toUpperCase() : null;
 }
 
+// Given the existing commit message and a ticket, return the message with a
+// TICKET footer appended, or null when no change is needed (no ticket, or the
+// message already contains a ticket/issue footer). Pure so it is unit-testable.
+function appendTicketFooter(data, ticket) {
+  if (!ticket) {
+    return null;
+  }
+  // Reset lastIndex: commitRegex is global, so .test() is stateful across calls.
+  commitRegex.lastIndex = 0;
+  if (commitRegex.test(data)) {
+    return null;
+  }
+  return `${data}\nTICKET: ${ticket}\n`;
+}
+
 async function main() {
   const commitMsgFilepath = process.argv[2];
   try {
     const branch = (await exec(`git branch --show-current`)).stdout.trim();
     const ticket = extractTicket(branch);
-    if (!ticket) {
-      return;
-    }
     const data = fs.readFileSync(commitMsgFilepath, 'utf8');
-    // Exit if ticket is already in commit footer
-    if (data.match(commitRegex)) {
+    const updated = appendTicketFooter(data, ticket);
+    if (updated === null) {
       return;
     }
-    fs.writeFileSync(commitMsgFilepath, `${data}\nTICKET: ${ticket}\n`);
+    fs.writeFileSync(commitMsgFilepath, updated);
   } catch (e) {}
 }
 
-module.exports = { extractTicket };
+module.exports = { extractTicket, appendTicketFooter };
 if (require.main === module) main();

scripts/tests/prepare-commit-msg.test.js

@@ -1,5 +1,5 @@
 const assert = require('assert');
-const { extractTicket } = require('../prepare-commit-msg');
+const { extractTicket, appendTicketFooter } = require('../prepare-commit-msg');
 
 describe('extractTicket', () => {
   it('extracts uppercase ticket from uppercase branch', () => {
@@ -20,7 +20,39 @@ describe('extractTicket', () => {
     assert.strictEqual(extractTicket('feature-no-ticket'), null);
   });
 
+  it('is anchored at branch start (ignores prefixes mid-name)', () => {
+    assert.strictEqual(extractTicket('feature/WEB-000-x'), null);
+  });
+
+  it('returns null for prefix without digits (WEB-foo)', () => {
+    assert.strictEqual(extractTicket('WEB-foo'), null);
+  });
+
   it('returns null for empty branch', () => {
     assert.strictEqual(extractTicket(''), null);
   });
 });
+
+describe('appendTicketFooter', () => {
+  it('appends a TICKET footer when none is present', () => {
+    assert.strictEqual(appendTicketFooter('feat: add thing', 'WEB-000'), 'feat: add thing\nTICKET: WEB-000\n');
+  });
+
+  it('returns null when ticket is null (branch had no match)', () => {
+    assert.strictEqual(appendTicketFooter('feat: add thing', null), null);
+  });
+
+  it('returns null when a TICKET footer already exists', () => {
+    assert.strictEqual(appendTicketFooter('feat: add thing\n\nTICKET: WEB-000', 'WEB-000'), null);
+  });
+
+  it('returns null when an ISSUE footer already exists (any case)', () => {
+    assert.strictEqual(appendTicketFooter('feat: add thing\n\nissue: WEB-111', 'WEB-000'), null);
+  });
+
+  it('is not stateful across repeated calls (global-regex lastIndex safe)', () => {
+    // Guards against RegExp.lastIndex leaking between .test() invocations.
+    assert.strictEqual(appendTicketFooter('feat: a', 'WEB-000'), 'feat: a\nTICKET: WEB-000\n');
+    assert.strictEqual(appendTicketFooter('feat: b', 'WEB-000'), 'feat: b\nTICKET: WEB-000\n');
+  });
+});