test: add unit test for fix

.changeset/sixty-stingrays-glow.md

@@ -0,0 +1,5 @@
+---
+'lint-staged': patch
+---
+
+Previously it was possible for a function task to mutate the list of staged files passed to the function, and accidentally affect the generation of other tasks. This is now fixed by passing a copy of the original file list instead.

lib/makeCmdTasks.js

@@ -25,7 +25,7 @@ export const makeCmdTasks = async ({ commands, cwd, files, gitDir, shell, verbos
     // command function may return array of commands that already include `stagedFiles`
     const isFn = typeof cmd === 'function'
 
-    /** Pass copy of file list to prevent mutation by function from config file */
+    /** Pass copy of file list to prevent mutation by function from config file. */
     const resolved = isFn ? await cmd([...files]) : cmd
 
     const resolvedArray = Array.isArray(resolved) ? resolved : [resolved] // Wrap non-array command as array

test/unit/makeCmdTasks.spec.js

@@ -124,4 +124,25 @@ describe('makeCmdTasks', () => {
               Function task should return a string or an array of strings"
           `)
   })
+
+  it('should prevent function from mutating original file list', async () => {
+    const files = ['test.js']
+
+    const res = await makeCmdTasks({
+      commands: (files) => {
+        /** Array.splic() mutates the original array */
+        files.splice(0, 1)
+        expect(files).toEqual([])
+        return files.map((file) => `test ${file}`)
+      },
+      gitDir,
+      files,
+    })
+
+    /** Because function mutated file list, it was empty and no tasks were created... */
+    expect(res.length).toBe(0)
+
+    /** ...but the original file list was not mutated */
+    expect(files).toEqual(['test.js'])
+  })
 })