Add ability to prepend to an existing changelog

Fixes #156

README.md

@@ -115,6 +115,10 @@ auto-changelog --issue-url https://www.redmine.org/issues/{id}
 auto-changelog --compare-url https://example.com/repo/compare/{from}...{to}
 ```
 
+#### Add to an existing changelog
+
+If you’d like to keep an existing changelog below your generated one, just add `<!-- auto-changelog-above -->` to your current changelog. The generated changelog will be added above this token, and anything below will remain.
+
 #### Configuration
 
 You can set any option in `package.json` under the `auto-changelog` key, using camelCase options.

src/run.js

@@ -5,7 +5,7 @@ const { fetchRemote } = require('./remote')
 const { fetchTags } = require('./tags')
 const { parseReleases } = require('./releases')
 const { compileTemplate } = require('./template')
-const { parseLimit, readJson, writeFile, fileExists, updateLog, formatBytes } = require('./utils')
+const { parseLimit, readFile, readJson, writeFile, fileExists, updateLog, formatBytes } = require('./utils')
 
 const DEFAULT_OPTIONS = {
   output: 'CHANGELOG.md',
@@ -21,6 +21,7 @@ const DEFAULT_OPTIONS = {
 
 const PACKAGE_FILE = 'package.json'
 const PACKAGE_OPTIONS_KEY = 'auto-changelog'
+const PREPEND_TOKEN = '<!-- auto-changelog-above -->'
 
 async function getOptions (argv) {
   const options = new Command()
@@ -95,12 +96,26 @@ async function run (argv) {
   const onParsed = ({ title }) => log(`Fetched ${title}…`)
   const releases = await parseReleases(tags, remote, latestVersion, options, onParsed)
   const changelog = await compileTemplate(options, { releases })
+  await write(changelog, options, log)
+}
+
+async function write (changelog, options, log) {
   if (options.stdout) {
     process.stdout.write(changelog)
-    process.exit(0)
+    return
   }
-  await writeFile(options.output, changelog)
   const bytes = Buffer.byteLength(changelog, 'utf8')
+  const existing = await fileExists(options.output) && await readFile(options.output, 'utf8')
+  if (existing) {
+    const index = existing.indexOf(PREPEND_TOKEN)
+    if (index !== -1) {
+      const prepended = `${changelog}\n${existing.slice(index)}`
+      await writeFile(options.output, prepended)
+      log(`${formatBytes(bytes)} prepended to ${options.output}\n`)
+      return
+    }
+  }
+  await writeFile(options.output, changelog)
   log(`${formatBytes(bytes)} written to ${options.output}\n`)
 }
 

test/run.js

@@ -141,7 +141,7 @@ describe('run', () => {
   })
 
   it('command line options override options from package.json', async () => {
-    mock('fileExists', () => true)
+    mock('fileExists', path => path === '.auto-changelog')
     mock('readJson', () => ({
       'auto-changelog': {
         output: 'should-not-be-this.md'
@@ -168,7 +168,7 @@ describe('run', () => {
   })
 
   it('command line options override options from .auto-changelog', async () => {
-    mock('fileExists', () => true)
+    mock('fileExists', path => path === '.auto-changelog')
     mock('readJson', (path) => {
       return path === '.auto-changelog' ? { output: 'should-not-be-this.md' } : null
     })