🛠️ Solving “Deprecation Warning [legacy-js-api]” in Dart Sass While Building a React Project

While building a React application using react-scripts, you might encounter a warning like this during the npm run build process:

Deprecation Warning [legacy-js-api]: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0.
More info: https://sass-lang.com/d/legacy-js-api

This is not an error that stops your build, but it warns you that some part of your toolchain is using an outdated Sass API that will be removed in the future.

Let’s go over what this warning means, why it happens, and how to solve it step by step.

✅ What Is the Legacy JS API Warning?

Dart Sass (the primary implementation of Sass going forward) is phasing out its legacy JavaScript API. This means older packages or tools using this outdated API will eventually stop working with newer versions of Dart Sass.

🔍 What Causes This Warning?

You are likely using one or more of the following:

A package that depends on the outdated node-sass (which is now deprecated).
An older Sass plugin or loader (e.g., sass-loader in webpack).
A dependency that hasn’t been updated to use the newer Dart Sass API.

🧩 Solution: Update to Dart Sass and Clean Up Your Dependencies

Here’s how to resolve the warning:

1. Uninstall `node-sass` (if present)

npm uninstall node-sass

2. Install the latest version of `sass` (Dart Sass)

npm install sass@latest

This switches your project to the officially supported Dart Sass implementation.

3. Clean npm cache and reinstall dependencies (optional but helpful)

rm -rf node_modules package-lock.json
npm cache clean --force
npm install

4. Rebuild the project

npm run build

If all goes well, the warning should disappear or be silenced as dependencies now use the correct API.

🔄 Additional Tip: Check for Deprecated Dependencies

Use this command to check outdated packages:

npm outdated

Also, use this to update all packages:

npm update

✅ Final Thoughts

While the warning doesn’t break your build for now, ignoring it will eventually lead to errors once Dart Sass 2.0 is released. It’s good practice to keep your dependencies up-to-date to maintain compatibility and reduce tech debt.