I'm trying to build a website using Umbraco 8 CMS, but I want to write the front end code in React JS.
Is this possible?
Umbraco Heartcore would be the way to go. But I believe it's only available with the Cloud version.
https://umbraco.com/products/umbraco-heartcore/
If you're not using the cloud version, you could try using Umbraco API Controllers to expose your data as JSON.
https://our.umbraco.com/documentation/Reference/Routing/WebApi/
Then from your React application you can call the endpoints.