test.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
  </head>
  <body>
    <br><br>
    Text area:<br>
    <textarea rows="15" cols="50"></textarea>
    <br><br><br>
    Div with contenteditable
    <div contenteditable="true" style="width:350px;height:200px;border:1px solid black;"/>

    <script type="text/javascript" charset="utf-8">

      try {
        const electronSpellchecker = require('electron-spellchecker');
        const SpellCheckHandler = electronSpellchecker.SpellCheckHandler;
        const ContextMenuListener = electronSpellchecker.ContextMenuListener;
        const ContextMenuBuilder = electronSpellchecker.ContextMenuBuilder;

        window.spellCheckHandler = new SpellCheckHandler();
        window.spellCheckHandler.attachToInput();

        window.spellCheckHandler.switchLanguage('en-GB');
        //window.spellCheckHandler._automaticallyIdentifyLanguages = false;

        let contextMenuBuilder = new ContextMenuBuilder(window.spellCheckHandler);
        let contextMenuListener = new ContextMenuListener(async (info) => {
          await contextMenuBuilder.showPopupMenu(info);
        });


      } catch (err) {
        console.log('error in spell checker: ' + err.message);
      }

    </script>
  </body>
</html>