index.js

var buildModel = function () {
  var model = tf.sequential();
  model.add(tf.layers.dense({ units: 24, inputShape: [6], activation: 'relu' })); // input shape === state shape
  model.add(tf.layers.dense({ units: 24, activation: 'relu' }));
  model.add(tf.layers.dense({ units: 9, activation: 'linear' }));  // units === action_size
  model.compile({ loss: tf.losses.huberLoss, optimizer: tf.train.adam(learningRate) });
  return model
}