How Can I Import LangChain (or JS npm Packages) into an HTML Page?

If you’ve ever tried to use a JavaScript package like LangChain in an HTML file and found yourself hitting a wall when running it through a web server such as Flask, you’re not alone. It’s a common issue that developers run into. In this post, I’ll walk you through why this happens and how to solve the problem.

Let’s say you have a simple HTML file that includes a JS script. Here’s our basic setup:

index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Test LangChain Integration</title>
</head>
<body>
    <h1>LangChain Test</h1>
    <script src="script.js"></script>
</body>
</html>

script.js

For the sake of simplicity, let’s assume you are trying to import from LangChain or any npm package.

import { SomeFunction } from 'langchain';

SomeFunction();

If you try to serve this HTML file using Flask and access it via a web browser, you’ll probably encounter an error in the browser’s console saying something along the lines of:

Uncaught SyntaxError: Cannot use import statement outside a module

The Problem

The issue here is that the ES module system (import/export) is not directly supported in HTML files when served traditionally. Browsers do not inherently understand module imports unless told specifically to treat the script as a module.

Solution

To use modern JavaScript modules in the browser, we need to make a few adjustments:

Use the type="module" attribute in the script tag.

Bundling the code using tools like Webpack, Rollup, or Vite.

Use a module CDN like jsdelivr or unpkg for module imports.

Using type=”module”

Start by updating the index.html file:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Test LangChain Integration</title>
</head>
<body>
    <h1>LangChain Test</h1>
    <script type="module" src="script.js"></script>
</body>
</html>

This tells the browser to treat script.js as a JavaScript module.

Using a CDN for Modules

Next, we can use a CDN like jsdelivr or unpkg for easy module access without setting up a local build process. Update script.js as follows:

import { SomeFunction } from 'https://cdn.jsdelivr.net/npm/langchain@latest/lib/index.js';

SomeFunction();

By referencing the URL of the package directly, the browser can dynamically load and parse the module.

Serving with Flask

Now, let’s get Flask to serve our static files properly. Set up a basic Flask server to serve the HTML file:

app.py

from flask import Flask, send_from_directory

app = Flask(__name__, static_url_path='', static_folder='.')

@app.route('/')
def serve_index():
    return send_from_directory('.', 'index.html')

if __name__ == "__main__":
    app.run(debug=True)

In this Flask setup, we’re serving the current directory, allowing index.html and script.js to be accessible.

Running the Solution

Start your Flask server:

python app.py

Open your web browser and navigate to http://127.0.0.1:5000.

Now the script.js should successfully import SomeFunction from the LangChain package, and you can utilize it within your HTML file.

Final Thoughts

Using JavaScript modules in an HTML file can be initially confusing, especially when served from a web server. However, with the modern type="module" attribute and the convenience of CDNs like jsdelivr, integrating npm packages like LangChain becomes straightforward. This setup not only helps you include npm packages but also keeps your code modular and organized.

I hope this helps you troubleshoot and integrate JavaScript modules more effectively into your web projects!