add a basic test for updateChangelog

src/prepare.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect, vi } from 'vitest';
+
+import { updateChangelog } from './prepare.js';
+
+const mocks = vi.hoisted(() => {
+  return {
+    readFileSync: vi.fn().mockImplementation(() => ''),
+    writeFileSync: vi.fn().mockImplementation(() => ''),
+  };
+});
+
+vi.mock('node:fs', () => {
+  return mocks;
+});
+
+describe('prepare', function () {
+  describe('updateChangelog', function () {
+    it('updates changelog correctly', function () {
+      mocks.readFileSync.mockReturnValue(`# A Totally ficticious Changelog
+
+## Some Old version
+
+- added some features
+- spent a lot of time trying to figure out releases (if only there was a tool to help with that)
+`);
+      updateChangelog(
+        `## v1.0.0
+
+- added release-plan (how did I live without it!?)
+- releasing initial working version`,
+        new Map([['thing', { newVersion: 'v1.0.0', impact: 'major' }]]) as any,
+      );
+      const [, newChangelog] = mocks.writeFileSync.mock.lastCall;
+      expect(newChangelog).to.eq(`# A Totally ficticious Changelog
+## v1.0.0
+
+thing v1.0.0 (major)
+
+- added release-plan (how did I live without it!?)
+- releasing initial working version
+
+## Some Old version
+
+- added some features
+- spent a lot of time trying to figure out releases (if only there was a tool to help with that)
+`);
+    });
+  });
+});

src/prepare.ts

@@ -1,5 +1,5 @@
 import { parseChangeLogOrExit } from './change-parser.js';
-import { readFileSync, writeFileSync } from 'fs';
+import { readFileSync, writeFileSync } from 'node:fs';
 import type { Solution } from './plan.js';
 import { planVersionBumps, saveSolution } from './plan.js';
 import fsExtra from 'fs-extra';
@@ -8,7 +8,7 @@ const { readJSONSync, writeJSONSync } = fsExtra;
 
 const changelogPreamblePattern = /#.*Changelog.*$/;
 
-function updateChangelog(
+export function updateChangelog(
   newChangelogContent: string,
   solution: Solution,
 ): string {