feat(rn-changelog-generator): Prepend changelogs to the supplied file (…

…#1105)

.changeset/tasty-birds-give.md

@@ -0,0 +1,5 @@
+---
+"@rnx-kit/rn-changelog-generator": minor
+---
+
+prepend the newly generated changelog to the supplied --changelog file

packages/rn-changelog-generator/README.md

@@ -67,7 +67,7 @@ Generate a changelog for `react-native` commits between versions 0.65.0 and
 0.66.0:
 
 ```sh
-npx rn-changelog-generator --base v0.65.0 --compare v0.66.0 --repo ../../../react-native --changelog ../../../react-native/CHANGELOG.md > NEW_CHANGES.md
+npx @rnx-kit/rn-changelog-generator --base v0.65.0 --compare v0.66.0 --repo ../../../react-native --changelog ../../../react-native/CHANGELOG.md
 ```
 
 As explained above, you will need to have a local clone of `react-native`, which

packages/rn-changelog-generator/src/generator.ts

@@ -514,8 +514,26 @@ function handler(argv: GenerateArgs) {
     .then(async () => {
       const existingChangelogData = fs.readFileSync(argv.changelog, "utf-8");
       const base = await getOffsetBaseCommit(gitDir, argv.base, argv.compare);
-      const data = await run({ ...argv, base, gitDir, existingChangelogData });
-      return console.log(data);
+      const newChangeLogData = await run({
+        ...argv,
+        base,
+        gitDir,
+        existingChangelogData,
+      });
+
+      const changelogHeader = "# Changelog";
+      fs.writeFileSync(argv.changelog, changelogHeader, {
+        encoding: "utf8",
+        flag: "w",
+      });
+      fs.appendFileSync(argv.changelog, newChangeLogData);
+      fs.appendFileSync(
+        argv.changelog,
+        existingChangelogData.substring(
+          existingChangelogData.indexOf(changelogHeader) +
+            changelogHeader.length
+        )
+      );
     })
     .catch((e) => {
       console.error(e);