- introduce new API allowing to write changelog directly to result file

- introduce test case for writing a file

src/main.ts

@@ -1,5 +1,9 @@
 import * as core from '@actions/core'
-import {retrieveRepositoryPath, resolveConfiguration} from './utils'
+import {
+  retrieveRepositoryPath,
+  resolveConfiguration,
+  writeOutput
+} from './utils'
 import * as github from '@actions/github'
 import {ReleaseNotesBuilder} from './releaseNotesBuilder'
 
@@ -43,6 +47,13 @@ async function run(): Promise<void> {
     ).build()
 
     core.setOutput('changelog', result)
+
+    // write the result in changelog to file if possible
+    const outputFile: string = core.getInput('outputFile')
+    if (outputFile !== '') {
+      core.debug(`Enabled writing the changelog to disk`)
+      writeOutput(repositoryPath, outputFile, result)
+    }
   } catch (error) {
     core.setFailed(error.message)
   }

src/utils.ts

@@ -111,3 +111,22 @@ export function directoryExistsSync(
 
   throw new Error(`Directory '${inputPath}' does not exist`)
 }
+
+/**
+ * Writes the changelog to the given the file
+ */
+export function writeOutput(
+  githubWorkspacePath: string,
+  outputFile: string,
+  changelog: string | null
+): void {
+  if (outputFile && changelog) {
+    const outputPath = path.resolve(githubWorkspacePath, outputFile)
+    core.debug(`outputPath = '${outputPath}'`)
+    try {
+      fs.writeFileSync(outputPath, changelog)
+    } catch (error) {
+      core.warning(`⚠️ Could not write the file to disk - ${error.message}`)
+    }
+  }
+}