window.install

install.js

(function() {
  
  var deferredInstall;
  var promptTriggered = false;
  // The resolve function that will be called when we know we can prompt.
  var canPromptPromiseResolved;
  var canPromptPromise = new Promise(function(resolve, reject) {
    // The resolve will be called later when we know the prompt has been shown.
    // We might want to reject after a timeout of a couple of seconds. 
    canPromptPromiseResolved = resolve;
  });
  

  window.addEventListener('beforeinstallprompt',function(e) {

    promptTriggered = true;

    // Stop it doing what it needs to do;
    e.preventDefault();
    deferredInstall = e;
    
    // Resolve the promise, to say that we know we can prompt.
    canPromptPromiseResolved();
    
    return false;
  });
  
  var install = {};
  
  Object.defineProperty(install, 'isAvailable', { get: function() { return promptTriggered; } });
  
  install.canPrompt = function() {
    return canPromptPromise;
  };
  
  install.prompt = function () {
    return new Promise(function(resolve, reject){
      if(promptTriggered === false) {
        // There can be a whole host or reasons, we should determine them
        reject('User Agent decided not to prompt');
      };
      
      deferredInstall.prompt().then(function() {
        return deferredInstall.userChoice 
      }).then(function(choice) {
        resolve(choice.outcome);
      }).catch(function(reason) {
        reject(reason);
      });
    });
  };
  
  window.install = install;
})();