Detect Chrome incognito mode

It is a best practice to NOT check browser versions to do feature support, instead, use feature detection. When detecting Chrome incognito mode, using feature detection might be a solution in Javascript. Use storage API is not a solution due to cross browser implementation difference regarding private window browsing.

Here is an example of using File API.

https://www.html5rocks.com/en/tutorials/file/filesystem/

function main() {
  var fs = window.RequestFileSystem || window.webkitRequestFileSystem;
  if (!fs) {
    result.textContent = "check failed?";
    return;
  }
  fs(window.TEMPORARY, 100, function(fs) {
    result.textContent = "it does not seem like you are in incognito mode";
  }, function(err) {
    result.textContent = "it seems like you are in incognito mode";
  });
}