Download A Google Drive File via Javascript

gdrive_download.js

function downloadGDriveFile (file) {
  if (file.downloadUrl) {
    var accessToken = gapi.auth.getToken().access_token;
    var xhr = new XMLHttpRequest();
    xhr.open('GET', file.downloadUrl);
    xhr.setRequestHeader('Authorization', 'Bearer ' + accessToken);
    xhr.onload = function() {
      var content = xhr.responseText;
      do_something_with_file_content(file.title, content);
    };
    
    xhr.onerror = function() {
      alert("Download failure.");
    };
    
    xhr.send();
  }
  
  else {
    alert("Unable to download file.");
  }
}

var request = gapi.client.drive.files.get({'fileId': fid});
request.execute(downloadGDriveFile);

Uncaught ReferenceError: gapi is not defined

What is the "file" in function downloadGDriveFile (file) ???