VSC - How to detect a change of the langauge used by the active editor in a VSC Extension

vsc-extension-detect-active-editor-language-change.js

// Called when active editor language is changed:
// After the active editor language changes the content is "re-opened" so just listen for the open event
// And check the lang of the active editor
vscode.workspace.onDidOpenTextDocument(() => {
   const editor = vscode.window.activeTextEditor
   
   if (editor !== undefined) {
    const {
      document: { languageId },
    } = editor;

    if (languageId.indexOf('javascript') === 0) {
     // Do something...
    }
  }
});