Coming from old classic SharePoint pages you might have existing script solutions you want to re-use on a modern page without having to repackage it as a new SharePoint Framework web part. This web part is similar to the classic Script Editor Web Part, and allows you do drop arbitrary script or html on a modern page.
For more information, visit https://github.com/SharePoint/sp-dev-fx-webparts/tree/master/samples/react-script-editor.
Once you check out, you’ll receive a link to download the web part package for free.
