Javascript to check if PWA or Mobile Web

If this is for analytical purposes you could set the start URL in the manifest file to include a query string parameter, ex:

"start_url": "./?mode=standalone"

Then in your JavaScript you are able to check for this query string parameter.

Update (2020-08-19)

Pete LePage wrote a blog on how to setup a custom dimension in Google Analytics using the code below which checks the display mode using window.matchMedia:

let displayMode="browser";
  const mqStandAlone="(display-mode: standalone)";
  if (navigator.standalone || window.matchMedia(mqStandAlone).matches) {
    displayMode="standalone";
  }
ga('set', 'dimension1', displayMode);

Read more: https://petelepage.com/blog/2020/08/measure-understand-how-installed-pwa-users-differ-from-browser-tab-users/

Update (2017-01-20):

Alternatively you could check in JavaScript using:

if (window.matchMedia('(display-mode: standalone)').matches) {
  console.log("This is running as standalone.");
}