The BeforeInstallPromptEvent is the interface of the beforeinstallprompt event fired at the Window object before a user is prompted to "install" a website to a home screen on mobile.

This interface inherits from the Event interface.

BeforeInstallPromptEvent() Non-standard Experimental

Creates a new BeforeInstallPromptEvent object.

Inherits properties from its parent, Event.

BeforeInstallPromptEvent.platforms Read only Non-standard Experimental

Returns an array of string items containing the platforms on which the event was dispatched. This is provided for user agents that want to present a choice of versions to the user such as, for example, "web" or "play" which would allow the user to choose between a web version or an Android version.

BeforeInstallPromptEvent.userChoice Read only Non-standard Experimental

Returns a Promise that resolves to an object describing the user's choice when they were prompted to install the app.

BeforeInstallPromptEvent.prompt() Non-standard Experimental

Show a prompt asking the user if they want to install the app. This method returns a Promise that resolves to an object describing the user's choice when they were prompted to install the app.

In the following example an app provides its own install button, which has an id of "install". Initially the button is hidden.

<button id="install" hidden>Install</button>

The beforeinstallprompt handler:

• Cancels the event, which prevents the browser displaying its own install UI on some platforms
• Assigns the BeforeInstallPromptEvent object to a variable, so it can be used later
• Reveals the app's install button.

let installPrompt = null;
const installButton = document.querySelector("#install");

window.addEventListener("beforeinstallprompt", (event) => {
  event.preventDefault();
  installPrompt = event;
  installButton.removeAttribute("hidden");
});

When clicked, the app's install button:

• Calls the prompt() method of the stored event object, to trigger the installation prompt.
• Resets its state by clearing the installPrompt variable and hiding itself again.

installButton.addEventListener("click", async () => {
  if (!installPrompt) {
    return;
  }
  const result = await installPrompt.prompt();
  console.log(`Install prompt was: ${result.outcome}`);
  installPrompt = null;
  installButton.setAttribute("hidden", "");
});

• Making PWAs installable
• How to provide your own in-app install experience on web.dev (2021)