client/lib/analytics/ad-tracking.js
function setupFacebookGlobal() {
	if ( window.fbq ) {
		return;
	}

	const facebookEvents = ( window.fbq = function() {
		if ( facebookEvents.callMethod ) {
			facebookEvents.callMethod.apply( facebookEvents, arguments );
		} else {
			facebookEvents.queue.push( arguments );
		}
	} );

	if ( ! window._fbq ) {
		window._fbq = facebookEvents;
	}

	/*
	 * Disable automatic PageView pushState tracking. It causes problems when we're using multiple FB pixel IDs.
	 * The objective here is to avoid firing a PageView against multiple FB pixel IDs. By disabling pushState tracking,
	 * we can do PageView tracking for FB on our own. See: `retarget()` in this file.
	 *
	 * There's more about the `disablePushState` flag here:
	 * <https://developers.facebook.com/ads/blog/post/2017/05/29/tagging-a-single-page-application-facebook-pixel/>
	 */
	window._fbq.disablePushState = true;

	facebookEvents.push = facebookEvents;
	facebookEvents.loaded = true;
	facebookEvents.version = '2.0';
	facebookEvents.queue = [];
}
function setupFacebookGlobal() {
	if ( window.fbq ) {
		return;
	}

	const facebookEvents = ( window.fbq = function() {
		if ( facebookEvents.callMethod ) {
			facebookEvents.callMethod.apply( facebookEvents, arguments );
		} else {
			facebookEvents.queue.push( arguments );
		}
	} );

	if ( ! window._fbq ) {
		window._fbq = facebookEvents;
	}

	/*
	 * Disable automatic PageView pushState tracking. It causes problems when we're using multiple FB pixel IDs.
	 * The objective here is to avoid firing a PageView against multiple FB pixel IDs. By disabling pushState tracking,
	 * we can do PageView tracking for FB on our own. See: `retarget()` in this file.
	 *
	 * There's more about the `disablePushState` flag here:
	 * <https://developers.facebook.com/ads/blog/post/2017/05/29/tagging-a-single-page-application-facebook-pixel/>
	 */
	window._fbq.disablePushState = true;

	facebookEvents.push = facebookEvents;
	facebookEvents.loaded = true;
	facebookEvents.version = '2.0';
	facebookEvents.queue = [];
}
This sets up the globals that the Facebook event library expects.
More info here: https://www.facebook.com/business/help/952192354843755