TensorFlow.js

TensorFlow.js Layers API for Keras Users

The Layers API of TensorFlow.js is modeled after Keras. As you might have noticed from the tutorials and examples, we strive to make the Layers API as similar to Keras as reasonable given the differences between JavaScript and Python. This makes it easier for users with experience developing Keras models in Python to migrate to TensorFlow.js Layers in JavaScript. For example, the following Keras code translates into JavaScript:

# Python:
import keras
import numpy as np

# Build and compile model.
model = keras.Sequential()
model.add(keras.layers.Dense(units=1, input_shape=[1]))
model.compile(optimizer='sgd', loss='mean_squared_error')

# Generate some synthetic data for training.
xs = np.array([[1], [2], [3], [4]])
ys = np.array([[1], [3], [5], [7]])

# Train model with fit().
model.fit(xs, ys, epochs=1000)

# Run inference with predict().
print(model.predict(np.array([[5]])))
// JavaScript:
import * as tf from '@tensorlowjs/tfjs';

// Build and compile model.
const model = tf.sequential();
model.add(tf.layers.dense({units: 1, inputShape: [1]}));
model.compile({optimizer: 'sgd', loss: 'meanSquaredError'});

// Generate some synthetic data for training.
const xs = tf.tensor2d([[1], [2], [3], [4]], [4, 1]);
const ys = tf.tensor2d([[1], [3], [5], [7]], [4, 1]);

// Train model with fit().
await model.fit(xs, ys, {epochs: 1000});

// Run inference with predict().
model.predict(tf.tensor2d([[5]], [1, 1])).print();

However, there are some differences we’d like to call out and explain in this document. Once you understand these differences and the rationale behind them, your Python-to-JavaScript migration (or migration in the reverse direction) should be a relatively smooth experience.

Constructors take JavaScript Objects as configurations

Compare the following Python and JavaScript lines from the example above: they both create a Dense layer.

# Python:
keras.layers.Dense(units=1, inputShape=[1])
// JavaScript:
tf.layers.dense({units: 1, inputShape: [1]});

JavaScript functions don’t have an equivalent of the keyword arguments in Python functions. We want to avoid implementing constructor options as positional arguments in JavaScript, which would be especially cumbersome to remember and use for constructors with a large number of keyword arguments (e.g., LSTM). This is why we use JavaScript configuration objects. Such objects provide the same level of positional invariance and flexibility as Python keyword arguments.

Some methods of the Model class, e.g., Model.compile(), also take a JavaScript configuration object as the input. However, keep in mind that Model.fit(), Model.evaluate() and Model.predict() are slightly different. Since these method take obligatory x (features) and y (labels or targets) data as inputs; x and y are positional arguments separate from the ensuing configuration object that plays the role of the keyword arguments. For example:

// JavaScript:
await model.fit(xs, ys, {epochs: 1000});

Model.fit() is async

Model.fit() is the primary method with which users perform model training in TensorFlow.js. This method can often be long-running, lasting for seconds or minutes. Therefore, we utilize the async feature of the JavaScript language, so that this function can be used in a way that does not block the the main UI thread when running in the browser. This is similar to other potentially long-running functions in JavaScript, such as the async fetch. Note that async is a construct that does not exist in Python. While the fit() method in Keras returns a History object, the counterpart of the fit() method in JavaScript returns a Promise of History, which can be awaited (as in the example above) or used with the then() method.

No NumPy for TensorFlow.js

Python Keras users often use NumPy to perform basic numeric and array operations, such as generating 2D tensors in the example above.

# Python:
xs = np.array([[1], [2], [3], [4]])

In TensorFlow.js, this kind of basic numeric operations are done with the package itself. For example:

// JavaScript:
const xs = tf.tensor2d([[1], [2], [3], [4]], [4, 1]);

The tf.* namespace also provides a number of other functions for array and linear algebra operations such as matrix multiplication. See the TensorFlow.js Core documentation for more information.

Use factory methods, not constructors

This line in Python (from the example above) is a constructor call:

# Python:
model = keras.Sequential()

If translated strictly into JavaScript, the equivalent constructor call would look like the following:

// JavaScript:
const model = new tf.Sequential();  // !!! DON'T DO THIS !!!

However, we decided not to use the “new” constructors because 1) the “new” keyword would make the code more bloated and 2) the “new” constructor is regarded as a “bad part” of JavaScript: a potential pitfall, as is argued in JavaScript: the Good Parts. To create models and layers in TensorFlow.js, you call factory methods, which have lowerCamelCase names, for example:

// JavaScript:
const model = tf.sequential();

const layer = tf.layers.batchNormalization({axis: 1});

Option string values are lowerCamelCase, not snake_case

In JavaScript, it is more common to use camel case for symbol names (e.g., see Google JavaScript Style Guide), as compared with Python, where snake case is common (e.g., in Keras). As such, we decided to use lowerCamelCase for string values for options including the following:

For example, as in the example above:

// JavaScript:
model.compile({optimizer: 'sgd', loss: 'meanSquaredError'});

With regard to model serialization and deserialization, rest assured. TensorFlow.js’s internal mechanism ensure that snake cases in JSON objects are handled correctly, e.g., when loading pretrained models from Python Keras.

Run Layer objects with apply(), not by calling them as functions

In Keras, a Layer object has the __call__ method defined. Therefore the user can invoke the layer’s logic by calling the object as a function, e.g.,

# Python:
my_input = keras.Input(shape=[2, 4])
flatten = keras.layers.Flatten()

print(flatten(my_input).shape)

This Python syntax sugar is implemented as the apply() method in TensorFlow.js:

// JavaScript:
const myInput = tf.input{shape: [2, 4]});
const flatten = tf.layers.flatten();

console.log(flatten.apply(myInput).shape);

Layer.apply() supports imperative (eager) evaluation on concrete tensors

Currently, in Keras, the call method can only operate on (Python) TensorFlow’s Tensor objects (assuming TensorFlow backend), which are symbolic and do not hold actual numeric values. This is what’s shown in the example in the previous section. However, in TensorFlow.js, the apply() method of layers can operate in both symbolic and imperative modes. If apply() is invoked with a SymbolicTensor (a close analogy of tf.Tensor), the return value will be a SymbolicTensor. This happens typically during model building. But if apply() is invoked with an actual concrete Tensor value, it will return a concrete Tensor. For example:

// JavaScript:
const flatten = tf.layers.flatten();

flatten.apply(tf.ones([2, 3, 4])).print();

This feature is reminiscent of (Python) TensorFlow’s Eager Execution. It affords greater interactivity and debuggability during model development, in addition to opening doors to composing dynamic neural networks.

Optimizers are under train., not optimizers.

In Keras, the constructors for Optimizer objects are under the keras.optimizers.* namespace. In TensorFlow.js Layers, the factory methods for Optimizers are under the tf.train.* namespace. For example:

# Python:
my_sgd = keras.optimizers.sgd(lr=0.2)
// JavaScript:
const mySGD = tf.train.sgd({lr: 0.2});

loadModel() loads from a URL, not an HDF5 file

In Keras, models are usually saved as a HDF5 (.h5) file, which can be later loaded using the keras.models.load_model() method. The method takes a path to the .h5 file. The counterpart of load_model() in TensorFlow.js is tf.loadModel(). Since HDF5 is not a browser-friendly file format, tf.loadModel() takes a TensorFlow.js-specific format. tf.loadModel() takes a model.json file as its input argument. The model.json can be converted from a Keras HDF5 file using the tensorflowjs pip package.

// JavaScript:
const model = await tf.loadModel('https://foo.bar/model.json');

Also note that tf.loadModel() returns a Promise of tf.Model.

In general, saving and loading tf.Models in TensorFlow.js is done using the tf.Model.save and tf.loadModel methods, respectively. We designed these APIs to be similar to the save and load_model API of Keras. But the browser environment is quite different from the backend environment on which staple deep learning frameworks like Keras run, particularly in the array of routes for persisting and transimitting data. Hence there are some interesting differences between the save/load APIs in TensorFlow.js and in Keras. See our tutorial on Saving and Loading tf.Model for more details.