If a loader uses external resources, such as when reading from a filesystem, the loader must indicate this. This information is used to invalidate "cacheable" loaders and recompile them in watch mode. What follows is a brief example of how to accomplish this, using the addDependency method inside of loader.js:

import path from 'path';
export default function(source) {
  var callback = this.async();
  var headerPath = path.resolve('header.js');
  this.addDependency(headerPath);
  fs.readFile(headerPath, 'utf-8', function(err, header) {
    if(err) return callback(err);
    callback(null, header + '
' + source);
  });
 }

There are some differences between the loader and modular dependencies. Let's now discuss the latter.