npm i transform-loader --save

Documentation: Using loaders

Pass the module name as query parameter.

var x = require("!transform-loader?brfs!./file.js");
var x = require("!transform-loader/cacheable?brfs!./file.js"); // cacheable version

If you pass a number instead it will take the function from this.options.transforms[number].

module.exports = {
  module: {
    rules: [
      {
        loader: "transform-loader?brfs",
        enforce: "post"
      },
			{
        test: /\.coffee$/,
        loader: "transform-loader/cacheable?coffeeify"
      },
      {
        test: /\.weirdjs$/,
        loader: "transform-loader?0"
      }
    ]
  },
  plugins: [
    new webpack.LoaderOptionsPlugin({
      options: {
        transforms: [
          function(file) {
            return through(function(buf) {
              this.queue(buf.split("").map(function(s) {
                return String.fromCharCode(127-s.charCodeAt(0));
              }).join(""));
            }, function() { this.queue(null); });
          }
        ]
      }
    })
  ]
};

module.exports = {
	module: {
		postLoaders: [
			{
				loader: "transform-loader?brfs"
			}
		]
		loaders: [
			{
				test: /\.coffee$/,
				loader: "transform-loader/cacheable?coffeeify"
			},
			{
				test: /\.weirdjs$/,
				loader: "transform-loader?0"
			}
		]
	},
	transforms: [
		function(file) {
			return through(function(buf) {
				this.queue(buf.split("").map(function(s) {
					return String.fromCharCode(127-s.charCodeAt(0));
				}).join(""));
			}, function() { this.queue(null); });
		}
	]
};

Say you have the following Node source:

var test = require('fs').readFileSync('./test.txt', 'utf8');

After npm install transform-loader brfs --save, add the following loader to your config:

module.exports = {
    context: __dirname,
    entry: "./index.js",
    module: {
        loaders: [
            {
                test: /\.js$/,
                loader: "transform-loader?brfs"
            }
        ]
    }
}

The loader is applied to all JS files, which can incur a performance hit with watch tasks. So you may want to use transform-loader/cacheable?brfs instead.