MediaWiki:Gadget-InstallApp.js

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const InstallApp = {

	init( $content ) {
		window.onbeforeinstallprompt = event => InstallApp.bind( event, $content );
	},

	bind( event, $content ) {
		event.preventDefault();
		const $buttons = $content.find( '.template-button.install-app' );
		$buttons.css( 'display', 'inline' );
		$buttons.on( 'click', () => {
			event.prompt();
			$buttons.hide();
		} );
	}
};

mw.hook( 'wikipage.content' ).add( InstallApp.init );