client/lib/purchases/index.js
function handleRenewNowClick( purchase, siteSlug ) {
	const renewItem = cartItems.getRenewalItemFromProduct( purchase, {
		domain: purchase.meta,
	} );
	const renewItems = [ renewItem ];

	// Track the renew now submit.
	analytics.tracks.recordEvent( 'calypso_purchases_renew_now_click', {
		product_slug: purchase.productSlug,
	} );

	addItems( renewItems );

	page( '/checkout/' + siteSlug );
}
function handleRenewNowClick( purchase, siteSlug ) {
	const renewItem = cartItems.getRenewalItemFromProduct( purchase, {
		domain: purchase.meta,
	} );
	const renewItems = [ renewItem ];

	// Track the renew now submit.
	analytics.tracks.recordEvent( 'calypso_purchases_renew_now_click', {
		product_slug: purchase.productSlug,
	} );

	addItems( renewItems );

	page( '/checkout/' + siteSlug );
}
Adds a purchase renewal to the cart and redirects to checkout.

@param {Object} purchase - the purchase to be renewed
@param {string} siteSlug - the site slug to renew the purchase for