Cache Fetched AJAX Requests Locally: Wrapping the Fetch API

This article is by guest authorPeter Bengtsson. SitePoint guest posts aim to bring you engaging content from prominent writers and speakers of the JavaScript community

More from this author

• Smart Front-ends & Dumb Back-ends: Persisting State in AngularJS
• Face Proximity Detection with JavaScript

This article demonstrates how you implement a local cache of fetched requestsso that if done repeatedly it reads from session storage instead. The advantage of this is that you don’t need to have custom code for each resource you want cached.

Follow along if you want to look really cool at your next JavaScript dinner party,where you can show off various skills of juggling promises,state-of-the-art APIs and local storage.

The Fetch API

At this point you’re hopefully familiar withfetch. It’s a new native API in browsers to replace the oldXMLHttpRequestAPI.

Where it hasn’t been perfectly implemented in all browsers,you can useGitHub’s fetch polyfill(And if you have nothing to do all day,here’s theFetch Standard spec).

The Na?ve Alternative

Suppose you know exactly which one resource you need to download and only want to download it once. You could use a global variable as your cache,something like this:

let origin = null
fetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(information => {
    origin = information.origin  // your client's IP
  })

// need to delay to make sure the fetch has finished
setTimeout(() => {
  console.log('Your origin is ' + origin)
},3000)

On CodePen

That just relies on a global variable to hold the cached data. The immediate problem is that the cached data goes away if you reload the page or navigate to some new page.

Let’s upgrade our first naive solution before we dissect its shortcomings.

fetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(info => {
    sessionStorage.setItem('information',JSON.stringify(info))
  })

// need to delay to make sure the fetch has finished
setTimeout(() => {
  let info = JSON.parse(sessionStorage.getItem('information'))
  console.log('Your origin is ' + info.origin)
},3000)

On CodePen

The first an immediate problem is thatfetchis promise-based,meaning we can’t know for sure when it has finished,so to be certain we should not rely on its execution until its promise resolves.

The second problem is that this solution is very specific to a particular URL and a particular piece of cached data (keyinformationin this example). What we want is a generic solution that is based on the URL instead.

First Implementation – Keeping It Simple

Let’s put a wrapper aroundfetchthat also returns a promise. The code that calls it probably doesn’t care if the result came from the network or if it came from the local cache.

So imagine youusedto do this:

fetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(issues => {
    console.log('Your origin is ' + info.origin)
  })

On CodePen

And now you want to wrap that,so that repeated network calls can benefit from a local cache. Let’s simply call itcachedFetchinstead,so the code looks like this:

cachedFetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(info => {
    console.log('Your origin is ' + info.origin)
  })

The first time that’s run,it needs to resolve the request over the network and store the result in the cache. The second time it should draw directly from the local storage.

Let’s start with the code that simply wraps thefetchfunction:

const cachedFetch = (url,options) => {
  return fetch(url,options)
}

On CodePen

This works,but is useless,of course. Let’s implement thestoringof the fetched data to start with.

const cachedFetch = (url,options) => {
  // Use the URL as the cache key to sessionStorage
  let cacheKey = url
  return fetch(url,options).then(response => {
    // let's only store in cache if the content-type is
    // JSON or something non-binary
    let ct = response.headers.get('Content-Type')
    if (ct && (ct.match(/application/json/i) || ct.match(/text//i))) {
      // There is a .json() instead of .text() but
      // we're going to store it in sessionStorage as
      // string anyway.
      // If we don't clone the response,it will be
      // consumed by the time it's returned. This
      // way we're being un-intrusive.
      response.clone().text().then(content => {
        sessionStorage.setItem(cacheKey,content)
      })
    }
    return response
  })
}

On CodePen

There’s quite a lot going on here.

The first promise returned byfetchactually goes ahead and makes the GET request. If there are problems with CORS (Cross-Origin Resource Sharing) the.text(),.json()or.blob()methods won’t work.

The most interesting feature is that we have toclonetheResponseobject returned by the first promise. If we don’t do that,we’re injecting ourselves too much and when the final user of the promise tries to call.json()(for example) they’ll get this error:

TypeError: Body has already been consumed.

The other thing to notice is the carefulness around what the response type is: we only store the response if the status code is200andif the content type isapplication/jsonortext/*. This is becausesessionStoragecan only store text.

Here’s an example of using this:

cachedFetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(info => {
    console.log('Your origin is ' + info.origin)
  })

cachedFetch('https://httpbin.org/html')
  .then(r => r.text())
  .then(document => {
    console.log('Document has ' + document.match(/<p>/).length + ' paragraphs')
  })

cachedFetch('https://httpbin.org/image/png')
  .then(r => r.blob())
  .then(image => {
    console.log('Image is ' + image.size + ' bytes')
  })

What’s neat about this solution so far is that it works,without interfering,for both JSONandHTML requests. And when it’s an image,it does not attempt to store that insessionStorage.

Second Implementation – Actually Return Cache Hits

So our first implementation just takes care ofstoringthe responses of requests. But if you call thecachedFetcha second time it doesn’t yet bother to try toretrieveanything fromsessionStorage. What we need to do is return,first of all,a promise and the promise needs to resolve aResponse object.

Let’s start with a very basic implementation:

const cachedFetch = (url,options) => {
  // Use the URL as the cache key to sessionStorage
  let cacheKey = url

  // START new cache HIT code
  let cached = sessionStorage.getItem(cacheKey)
  if (cached !== null) {
    // it was in sessionStorage! Yay!
    let response = new Response(new Blob([cached]))
    return Promise.resolve(response)
  }
  // END new cache HIT code

  return fetch(url,options).then(response => {
    // let's only store in cache if the content-type is
    // JSON or something non-binary
    if (response.status === 200) {
      let ct = response.headers.get('Content-Type')
      if (ct && (ct.match(/application/json/i) || ct.match(/text//i))) {
        // There is a .json() instead of .text() but
        // we're going to store it in sessionStorage as
        // string anyway.
        // If we don't clone the response,it will be
        // consumed by the time it's returned. This
        // way we're being un-intrusive.
        response.clone().text().then(content => {
          sessionStorage.setItem(cacheKey,content)
        })
      }
    }
    return response
  })
}

On CodePen

And it just works!

To see it in action,openthe CodePen for this codeand once you’re there open your browser’s Network tab in the developer tools. Press the “Run” button (top-right-ish corner of CodePen) a couple of times and you should see that only the image is being repeatedly requested over the network.

One thing that is neat about this solution is the lack of “callback spaghetti”. Since thesessionStorage.getItemcall is synchronous (aka. blocking),we don’t have to deal with “Was it in the local storage?” inside a promise or callback. And only if there was something there,do we return the cached result. If not,the if statement just carries on to the regular code.

Third Implementation – What About Expiry Times?

So far we’ve been usingsessionStoragewhich is just likelocalStorageexcept that thesessionStoragegets wiped clean when youstart a new tab. That means we’re riding a “natural way” of not caching things too long. If we were to uselocalStorageinstead and cache something,it’d simply get stuck there “forever” even if the remote content has changed. And that’s bad.

A better solution is to give theusercontrol instead. (The user in this case is the web developer using ourcachedFetchfunction). Like with storage such as Memcached or Redis on the server side,you set a lifetime specifying how long it should be cached.

For example,in Python (with Flask)

>>> from werkzeug.contrib.cache import MemcachedCache
>>> cache = MemcachedCache(['127.0.0.1:11211'])
>>> cache.set('key','value',10)
True
>>> cache.get('key')
'value'
>>> # waiting 10 seconds
...
>>> cache.get('key')
>>>

Now,neithersessionStoragenorlocalStoragehas this functionality built-in,so we have to implement it manually. We’ll do that by always taking note of the timestamp at the time of storing and use that to compare on a possible cache hit.

But before we do that,how is this going to look? How about something like this:

// Use a default expiry time,like 5 minutes
cachedFetch('https://httpbin.org/get')
  .then(r => r.json())
  .then(info => {
    console.log('Your origin is ' + info.origin)
  })

// Instead of passing options to `fetch` we pass an integer which is seconds
cachedFetch('https://httpbin.org/get',2 * 60)  // 2 min
  .then(r => r.json())
  .then(info => {
    console.log('Your origin is ' + info.origin)
  })

// Combined with fetch's options object but called with a custom name
let init = {
  mode: 'same-origin',seconds: 3 * 60 // 3 minutes
}
cachedFetch('https://httpbin.org/get',init)
  .then(r => r.json())
  .then(info => {
    console.log('Your origin is ' + info.origin)
  })

The crucial new thing we’re going to add is that every time we save the response data,wealsorecordwhenwe stored it. But note that now we can also switch to the braver storage oflocalStorageinstead ofsessionStorage. Our custom expiry code will make sure we don’t get horribly stale cache hits in the otherwise persistentlocalStorage.

So here’s our final working solution:

const cachedFetch = (url,options) => {
  let expiry = 5 * 60 // 5 min default
  if (typeof options === 'number') {
    expiry = options
    options = undefined
  } else if (typeof options === 'object') {
    // I hope you didn't set it to 0 seconds
    expiry = options.seconds || expiry
  }
  // Use the URL as the cache key to sessionStorage
  let cacheKey = url
  let cached = localStorage.getItem(cacheKey)
  let whenCached = localStorage.getItem(cacheKey + ':ts')
  if (cached !== null && whenCached !== null) {
    // it was in sessionStorage! Yay!
    // Even though 'whenCached' is a string,this operation
    // works because the minus sign converts the
    // string to an integer and it will work.
    let age = (Date.now() - whenCached) / 1000
    if (age < expiry) {
      let response = new Response(new Blob([cached]))
      return Promise.resolve(response)
    } else {
      // We need to clean up this old key
      localStorage.removeItem(cacheKey)
      localStorage.removeItem(cacheKey + ':ts')
    }
  }

  return fetch(url,it will be
        // consumed by the time it's returned. This
        // way we're being un-intrusive.
        response.clone().text().then(content => {
          localStorage.setItem(cacheKey,content)
          localStorage.setItem(cacheKey+':ts',Date.now())
        })
      }
    }
    return response
  })
}

On CodePen

Future Implementation – Better,Fancier,Cooler

Not only are we avoiding hitting those web APIs excessively,the best part is thatlocalStorageis a gazillion times faster than relying on network. See this blog post for a comparison oflocalStorageversus XHR:localForage vs. XHR. It measures other things but basically concludes thatlocalStorageis really fast and disk-cache warm-ups are rare.

So how could we further improve our solution?

Dealing with binary responses

Our implementation here doesn’t bother caching non-text things,like images,but there’s no reason it can’t. We would need a bit more code. In particular,we probably want to store more information about theBlob. Every response is a Blob basically. For text and JSON it’s just an array of strings. And thetypeandsizedoesn’t really matter because it’s something you can figure out from the string itself. For binary content the blob has to be converted to aArrayBuffer.

For the curious,to see an extension of our implementation that supports images,check outthis CodePen.

Using hashed cache keys

Another potential improvement is to trade space for speed by hashing every URL,which was what we used as a key,to something much smaller. In the examples above we’ve been using just a handful of really small and neat URLs (e.g.https://httpbin.org/get) but if you have really large URLs with lots of query string thingies and you have lots of them,it can really add up.

A solution to this is to usethis neat algorithmwhich is known to be safe and fast:

const hashstr = s => {
  let hash = 0;
  if (s.length == 0) return hash;
  for (let i = 0; i < s.length; i++) {
    let char = s.charCodeAt(i);
    hash = ((hash<<5)-hash)+char;
    hash = hash & hash; // Convert to 32bit integer
  }
  return hash;
}

If you like this,check outthis CodePen. If you inspect the storage in your web console you’ll see keys like557027443.

Conclusion

You now have a working solution you can stick into your web apps,where perhaps you’re consuming a web API and you know the responses can be pretty well cached for your users.

One last thing that might be a natural extension of this prototype is to take it beyond an article and into a real,concrete project,with tests and aREADME,and publish it on npm – but that’s for another time!

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!