The trimCache function in Going Offline

Paul Yabsley wrote to let me know about an error in Going Offline. It���s rather embarrassing because it���s code that I���m using in the service worker for adactio.com but for some reason I messed it up in the book.

It���s the trimCache function in Chapter 7: Tidying Up. That���s the reusable piece of code that recursively reduces the number of items in a specified cache (cacheName) to a specified amount (maxItems). On page 95 and 96 I describe the process of creating the function which, in the book, ends up like this:

function trimCache(cacheName, maxItems) {
cacheName.open( cache => {
cache.keys()
.then( items => {
if (items.length > maxItems) {
cache.delete(items[0])
.then(
trimCache(cacheName, maxItems)
); // end delete then
} // end if
}); // end keys then
}); // end open
} // end function

See the problem? It���s right there at the start when I try to open the cache like this:

cacheName.open( cache => {

That won���t work. The open method only works on the caches object���I should be passing the name of the cache into the caches.open method. So the code should look like this:

caches.open( cacheName )
.then( cache => {

Everything else remains the same. The corrected trimCache function is here:

function trimCache(cacheName, maxItems) {
caches.open(cacheName)
.then( cache => {
cache.keys()
.then(keys => {
if (keys.length > maxItems) {
cache.delete(keys[0])
.then(
trimCache(cacheName, maxItems)
); // end delete then
} // end if
}); // end keys then
}); // end open then
} // end function

Sorry about that! I must���ve had some kind of brainfart when I was writing (and describing) that one line of code.

You may want to deface your copy of Going Offline by taking a pen to that code example. Normally I consider the practice of writing in books to be barbarism, but in this case ���go for it.