Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
        modified:   src/index.js

This isn’t an error — Git is telling you that you have changes that haven’t been staged (added) for the next commit.

Fix: Stage your changes

# Stage specific files
git add src/index.js

# Stage all changes
git add -A

# Then commit
git commit -m "Update index.js"

Understanding the Git workflow

Working Directory → Staging Area → Repository
    (edit files)     (git add)     (git commit)

1. You edit files — they show as “not staged”
2. git add moves them to staging — they show as “to be committed”
3. git commit saves them permanently

Common situations

You want to commit everything:

git add -A && git commit -m "my changes"

You want to commit only some files:

git add src/index.js src/utils.js
git commit -m "update specific files"

You want to undo changes (discard edits):

git checkout -- src/index.js    # Discard changes to one file
git checkout -- .               # Discard all changes

You want to stash changes for later:

git stash                       # Save changes, clean working directory
git stash pop                   # Restore them later

See also: Git cheat sheet | Git stash cheat sheet