Watch all files saved in a folder and make changes to another file via gulp

Question

My basic requirement is to look for changes done to any file in js,css folder and update a JSON file.

---

Directory structure:

• html
  • css
    • x.css
    • y.css
  • js
    • a.js
    • b.js
    • c.js
  • server
    • versions.json

versions.json:

{
  "css": {
     "x.css": 1,
     "y.css": 1,
  },
  "js": {
     "a.js": 1,
     "b.js": 1,
     "c.js": 1,
  },
}

As I make any changes to any of the files in js and css folder, I want gulp to return the name of the file that was saved and look for the key in versions.json and increase its value by 1 i.e. if I save a.js then it should update only the value of "a.js" in versions.json to 2.

---

Here's what I've done:

gulp.task('upgrade-file-version', function() {
  gulp.src('server/versions.json')
    .pipe(modify({
        fileModifier: function(file, contents)
        {
           var fileContent = contents;
           var parsedObj = JSON.parse(fileContent);
           var dir = 'js';   // folder name
           var filename = 'a.js';  // file name
           var oldVersion = parsedObj[dir][filename];
           var newVersion = oldVersion + 1;
           parsedObj[dir][filename] = newVersion;
           var newFileContent = JSON.stringify(parsedObj, null, 4);

           return newFileContent;
        }
    }))
    .pipe(gulp.dest('server/'));
});

Plugin: gulp-modify

The variables dir and filename needs to have the value of the folder and name of the changed file.

Any improvement over this or even an another approach is fine