Conditionally use loader in each watch build individually

Question

I have a webpack loader in my configuration which I want to run only

• in Watch Mode
• Starting with the second run (or any other condition that might change with each individual watch build triggered by file changes)

What I already have:

module.exports = (env, argv) {

  const isWatchMode = argv.watch;

  module: {
    rules: [
      ...(isWatchMode
          ? [{
              test: /\.js$/,
              exclude: /node_modules/,
              loader: 'babel-loader',
              options: {
                …
              }
            }]
          : []),
      …
    ],
  },
  …
};

This would trigger the loader only in watch mode, checked before the first run.

How do I include it depending on a condition that could change every run?