fix: keep untracked files around by backing them up

Git diff completely ignores new, untracked files, but they can be read/written separately

lib/gitWorkflow.js

@@ -1,6 +1,7 @@
 'use strict'
 
 const debug = require('debug')('lint-staged:git')
+const normalize = require('normalize-path')
 const path = require('path')
 
 const execGit = require('./execGit')
@@ -68,6 +69,31 @@ class GitWorkflow {
       debug('Done backing up merge state!')
     }
 
+    // Git stash ignores new, untracked files, so back them up separately
+    // Use `git ls-files` to list untracked files:
+    // `--others includes` all untracked files, including ignored files
+    // `--exclude-standard` excludes ignored files, the .git/ dir and so on...
+    let untrackedFiles = await this.execGit(['ls-files', '--others', '--exclude-standard'])
+    if (untrackedFiles) {
+      debug('Detected untracked files:')
+      debug(untrackedFiles)
+      debug('Backing up untracked files...')
+      // Resolve untrackedFiles output into filenames
+      untrackedFiles = untrackedFiles
+        .split('\n')
+        .map(file => normalize(path.resolve(this.cwd, file)))
+      this.untrackedFiles = new Map()
+      const readPromises = []
+      for (const file of untrackedFiles) {
+        // Read all untracked files into buffers, and save them into internal Map
+        readPromises.push(
+          readBufferFromFile(file).then(buffer => this.untrackedFiles.set(file, buffer))
+        )
+      }
+      await Promise.all(readPromises)
+      debug('Done backing up untracked files!')
+    }
+
     // Get stash of entire original state, including unstaged changes
     // Keep index so that tasks only work on those files
     await this.execGit(['stash', 'save', '--quiet', '--include-untracked', '--keep-index', STASH])
@@ -107,6 +133,16 @@ class GitWorkflow {
       }
     }
     debug('Done restoring unstaged changes!')
+
+    if (this.untrackedFiles) {
+      debug('Restoring untracked files...')
+      const writePromises = []
+      this.untrackedFiles.forEach((buffer, file) => {
+        writePromises.push(writeBufferToFile(file, buffer))
+      })
+      await Promise.all(writePromises)
+      debug('Done restoring untracked files!')
+    }
   }
 
   /**

test/runAll.unmocked.spec.js

@@ -450,4 +450,23 @@ describe('runAll', () => {
                     `)
     expect(await readFile('test.js')).toEqual(fileInBranchBFixed)
   })
+
+  it('should keep untracked files', async () => {
+    // Stage pretty file
+    await appendFile('test.js', testJsFilePretty)
+    await execGit(['add', 'test.js'])
+
+    // Add another file, but keep it untracked
+    await appendFile('test-untracked.js', testJsFilePretty)
+
+    // Run lint-staged with `prettier --list-different` and commit pretty file
+    await gitCommit({ config: { '*.js': 'prettier --list-different' } })
+
+    // Nothing is wrong, so a new commit is created
+    expect(await execGit(['rev-list', '--count', 'HEAD'])).toEqual('2')
+    expect(await execGit(['log', '-1', '--pretty=%B'])).toMatch('test')
+    expect(await readFile('test.js')).toEqual(testJsFilePretty)
+    expect(await readFile('test-untracked.js')).toEqual(testJsFilePretty)
+    console = makeConsoleMock()
+  })
 })