Building a Large Language Model (LLM) from Scratch with JavaScript: Comprehensive Guide.

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Building a Large Language Model (LLM) from Scratch with JavaScript: Comprehensive Guide.


6 min read


Creating a large language model like GPT-4 might seem daunting, especially considering the complexities involved and the computational resources required.

However, you can still build a smaller-scale language model that can perform tasks like text generation and sentiment analysis using JavaScript.

In this tutorial, we'll guide you through the process of creating a basic language model from scratch.

Step 1: Understanding the Basics

A language model is a type of artificial intelligence model that understands and generates human language. They can be used for tasks like speech recognition, translation, and text generation.

At its core, a language model learns to predict the next word in a sentence based on the words that came before it. For example, given the input "The cat sat on the...", the model should predict something like "mat" or "floor".

Step 2: Choose Your Toolset

To build a language model, we need a machine-learning library.

While JavaScript is not traditionally used for heavy machine learning tasks, there are still libraries available, such as TensorFlow, which is perfect for our needs.

To install TensorFlow.js in your project, run the following command in your terminal:

npm install @tensorflow/tfjs

Step 3: Prepare Your Dataset

For the model to learn from, we need a lot of text data, also known as a corpus. This could be anything from a collection of novels to tweets. For simplicity, you can start with a small dataset like a collection of sentences or paragraphs.

Step 4: Preprocessing the Data

Preprocessing involves cleaning the data and converting it into a format the model can understand. In the case of a language model, we'll convert words into numerical vectors in a process known as word embedding.

To achieve this, we'll first create a vocabulary of unique words from our corpus and then map each word to a unique integer. Here's a simple way to do it:

const corpus = "The cat sat on the mat. The dog sat on the log.";
const words = corpus.split(' ');
const vocab = Array.from(new Set(words));

const wordToId = {};
const idToWord = {};

vocab.forEach((word, i) => {
  wordToId[word] = i;
  idToWord[i] = word;

Step 5: Creating the Model

Next, we'll use TensorFlow.js to create a basic model. Here's an example of what that might look like:

const model = tf.sequential();

  tf.layers.embedding({inputDim: vocab.length, outputDim: 100, inputLength: 10})

  tf.layers.lstm({units: 100, returnSequences: true})

  tf.layers.timeDistributed({layer: tf.layers.dense({units: vocab.length})})

model.compile({optimizer: 'rmsprop', loss: 'categoricalCrossentropy'});

Here's a breakdown of each part of the code:

  1. const model = tf.sequential();

    This line of code creates a new sequential model. Sequential models are a type of model architecture that is a linear stack of layers, where each layer has exactly one input tensor and one output tensor.

  2. model.add(tf.layers.embedding({inputDim: vocab.length, outputDim: 100, inputLength: 10}))

    This adds an Embedding layer to the model. An Embedding layer is often used at the start of a model to transform integer representations of words (word indices) into dense vectors of fixed size. Here, the input dimension (inputDim) is the size of the vocabulary, the output dimension (outputDim) is the size of the embedding vectors, and the input length (inputLength) is the length of input sequences.

  3. model.add(tf.layers.lstm({units: 100, returnSequences: true}))

    This line adds an LSTM (Long Short-Term Memory) layer to the model. LSTM layers are a type of recurrent neural network (RNN) layer that are good at learning from sequences of data (like text). The units parameter specifies the dimensionality of the output space, and returnSequences being true means the LSTM will output a sequence matching the input sequence length, which is required when connecting to another LSTM layer or a TimeDistributed layer.

  4. model.add(tf.layers.timeDistributed({layer: tf.layers.dense({units: vocab.length})}))

    This adds a TimeDistributed wrapper around a Dense layer. The Dense layer is a regular fully-connected neural network layer, and it is being set to have as many units as there are words in the vocabulary. The TimeDistributed wrapper allows the Dense layer to be applied to every temporal slice of an input, which is necessary here because our LSTM layer is returning sequences.

  5. model.compile({optimizer: 'rmsprop', loss: 'categoricalCrossentropy'});

    Finally, the compile function configures the learning process before training the model. It's specifying that the model should use the RMSprop optimization algorithm and the categorical cross entropy loss function. RMSprop is a popular choice of optimizer for recurrent neural networks, and categorical cross entropy is a common choice of loss function for multi-class classification problems, which is what we have here as we are predicting the next word from our vocabulary.

Step 6: Training the Model

The final step is to train the model on your preprocessed data. This involves feeding your data into the model and allowing it to adjust its internal parameters to better predict the next word in a sentence.

const xs = tf.tensor2d(input_data, [input_data.length, input_data[0].length]);
const ys = tf.oneHot(tf.tensor1d(output_data, 'int32'), vocab.length);

await, ys, {epochs: 10});

Let's break this down:

  1. const xs = tf.tensor2d(input_data, [input_data.length, input_data[0].length]);

    This line is creating a 2D tensor (tf.tensor2d) for the input data (xs). Tensors are the primary data structure used in TensorFlow.js and they're similar to arrays and matrices. The shape of the tensor is determined by [input_data.length, input_data[0].length], which specifies the number of rows and columns respectively.

  2. const ys = tf.oneHot(tf.tensor1d(output_data, 'int32'), vocab.length);

    This is creating one-hot encoded labels for the output data (ys). One-hot encoding is a process of converting integer labels into a binary vector where the index of the integer label is marked with a 1, while the rest of the vector is filled with 0s. Here, tf.oneHot takes a 1D tensor of integers (tf.tensor1d(output_data, 'int32')) and the depth of the one-hot dimension (vocab.length). The result is a 2D tensor where each row is a one-hot vector.

  3. await, ys, {epochs: 10});

    This line is training the model for a fixed number of iterations (epochs) on the dataset. The fit function takes the input data (xs), the target data (ys), and an optional configuration object where we specify the number of epochs. Each epoch is an iteration over the entire input and target data. The await keyword is used because fit returns a Promise representing the asynchronous operation of training the model for a given number of epochs. It allows the JavaScript runtime to handle other tasks in the meantime, and resumes the function execution once the awaited Promise is fulfilled.

    Note: Using the fit function might throw an error if the shapes of the input data and the model input layer, or the shapes of the output data and the model output layer, do not match. It's crucial to ensure that these shapes align properly for the model to train successfully.

    Please note that creating and training a large language model from scratch can be very resource-intensive and time-consuming, especially without specialized hardware like GPUs.


Building a language model from scratch is a complex process,

especially for novice programmers. This guide should give you a solid start, but remember that creating a robust, large-scale language model requires advanced knowledge in machine learning and substantial computational resources. Nonetheless, getting started on this journey can be a rewarding learning experience. Remember, every expert was once a beginner!

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