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.

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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.");
}

Edit 11 Oct 2019: Added an extra switch to check if the app is launched via TWA - document.referrer.includes('android-app://')

This works for all - TWA, Chrome & Safari:

const isInStandaloneMode = () =>
      (window.matchMedia('(display-mode: standalone)').matches) || (window.navigator.standalone) || document.referrer.includes('android-app://');

 if (isInStandaloneMode()) {
    console.log("webapp is installed")
}