Fix the trailing newline by copying the last entry in the contents vs…

… a blank line (#202)

CHANGELOG.md

@@ -3,6 +3,9 @@
 Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ## [UNRELEASED]
+### Fixed
+- Fixes reverting trailing newlines at end of the file [#199](https://github.com/dangoslen/dependabot-changelog-helper/issues/199)
+
 ### Added
 - Adds a new `sectionHeader` option. See the [README](./README.md#sectionheader) for more details.
 

__tests__/changelog-updater.test.ts

@@ -600,6 +600,37 @@ test('updates section with an entry and accounts for multi-line entry', async ()
 - Update \`package\` from v1 to v2 ([#123](https://github.com/owner/repo/pull/123))`)
 })
 
+const CHANGELOG_WITH_ENDING_NEWLINE = `# Changelog
+
+## [v1.0.0]
+### Added
+- Some feature
+
+### Dependencies
+- Update \`other-package\` from beta to alpha
+
+## [v0.9.0]
+
+`
+
+test('should keep the trailing newline', async () => {
+  mockReadStream(CHANGELOG_WITH_ENDING_NEWLINE)
+
+  await updateChangelog(PACKAGE_ENTRY, 'v1.0.0', './CHANGELOG.md', 'Update', 'Dependencies')
+
+  expectWrittenChangelogToBe(`# Changelog
+
+## [v1.0.0]
+### Added
+- Some feature
+
+### Dependencies
+- Update \`other-package\` from beta to alpha
+- Update \`package\` from v1 to v2 ([#123](https://github.com/owner/repo/pull/123))
+
+## [v0.9.0]
+`)
+})
 
 function mockReadStream(changelog: string) {
   fs.createReadStream.mockImplementation((_: PathLike) => {

src/changelog-updater.ts

@@ -177,11 +177,19 @@ function writeEntry(
   changelogEntry: string,
   contents: string[]
 ): void {
-  const length = contents.push('')
+  // Push a copy of the last line to the end of the contents
+  // It will be overwritten when we re-write all the contents
+  const length = contents.push(contents[-1])
+
+  // Copy the contents from the last line up until the line of the entry we want to write
   for (let i = length - 1; i > lineNumber; i--) {
     contents[i] = contents[i - 1]
   }
+
+  // Write the entry
   contents[lineNumber] = changelogEntry
+
+  // Write the contents out, joining with EOL
   fs.writeFileSync(changelogPath, contents.join(EOL))
 }