fetch.js

function getProductData (productId) {
	let productCount = 0
	indexedDB.open('store', 1).then ( (db) => {
		let transaction = db.transaction(['products'])
		let productData = transaction.ObjectStore('products').get(productId).then (item => productCount = item.count)							
	})
	return productCount
}

self.addEventListener('fetch', (event) => {
	event.waitUnitil(
		if (event.request.url.includes('/product') {
			let productId = event.data.productId
			let productCount = getProductData(productId)
			indexedDB.open('store', 1, (db) => {
				let productStore = db.createObjectStore('products', { keyPath: 'id' })
				productStore.put({ id: productId, count: ++productCount })
			})
		}
	)
})

Line 12 is invalid.