Make Beautifully Resilient JavaScript Apps With Progressive Enhancement | by Austin Gil | Mar, 2022

Picture Credit score: Writer

I’ve talked at size about the advantages and virtues of progressive enhancement. I received’t spend an excessive amount of time explaining it as we speak, however it boils right down to this: present at the least a working expertise to essentially the most customers attainable, and jazz issues up for customers whose browsers and gadgets can assist these enhancements (jazz fingers).

To me, it’s simple sufficient to nod alongside, however in follow we fail on a regular basis. And it’s no shock. In any case, as creators it’s engaging to construct novel, partaking experiences utilizing the most recent expertise. And with out prior expertise it’s onerous to know what quirks we’d like to concentrate on.

To color a extra practical image, let’s take a look at a real-life instance that I’ve handled.

Some time again, I used to be on a web site that regarded actually attention-grabbing and I believed, “Yeah, why don’t I enroll?”. I clicked the “Register” button, and you understand what occurred? Nothing.

Naturally, I clicked it 5 extra instances, then popped open my dev instruments and noticed the large block of pink textual content within the JavaScript console.

The web site was utilizing Sentry’s error monitoring script to catch any JavaScript errors (good considering). The issue, I used to be utilizing a browser extension that blocks Third-party trackers. The JavaScript on the web site relied on Sentry’s code being current. When it was blocked, every little thing blew up and I couldn’t join the service (presumably, a very powerful factor).

Whereas the answer might have been to be extra cautious managing JavaScript dependencies, this story highlights a missed alternative for practising progressive enhancement.

That is of essentially the most prevalent situations the place I see purposes fail; counting on JavaScript to ship knowledge backwards and forwards between a browser and a server (typically as JSON).

For instance, a <button> on the web page that when clicked, triggers and HTTP request with the Fetch API. It’d seem like this:

doc.querySelector('button').addEventListener('click on', () => 
technique: 'POST',
physique: someBodyData

This makes for a sublime and efficient person expertise. They click on a button, and the information flies off to the mom ship.

However there’s an issue with relying completely on JavaScript to ship this knowledge. JavaScript might not be obtainable in your app within the person’s browser.

Every time I point out this the response is invariably:

Who turns off JavaScript!?

And that fully misses the purpose. Sure, some customers may very well disable JavaScript, however I’m not too apprehensive about them. They knew what they have been signing up for.

Nonetheless, JavaScript can run into points for different customers (see Everyone has JavaScript, right?). Right here’s an inventory of attainable methods it might probably fail:

  • Customers might have JavaScript disabled.
  • Browsers might not acknowledge the JavaScript syntax (perhaps they’re outdated (the browser, not the person)).
  • Browser extensions block scripts from operating (<- hey, that’s me).
  • Customers might have a sluggish connection that instances out (cell knowledge).
  • Customers might have intermittent connection (on a prepare).
  • The machine could also be behind a firewall.

This isn’t the total record of ways in which JavaScript might fail, but when any of this occurs, you may kiss that candy person expertise goodbye. The person may see a button, however it wouldn’t do something.

In different phrases, in case your software solely works with JavaScript, there’s loads of situations the place it received’t work. Not solely are you doing a disservice to your customers, it’s possible you’ll be negatively impacting your objectives.

So what if we simply don’t use JavaScript?

For many years now, HTML has been in a position to ship HTTP requests utilizing and , however I’ll be specializing in simply <type>. This is a really minimal instance:

<label for="input-id">Label<label>
<enter id="input-id" identify="key" />
<button sort="submit">Submit</button>

In case you have been to open this HTML in a browser, you’d see a well-known enter factor with the label “Label”, adopted by a “Submit” button. Clicking that button will reload the browser to the identical URL the shape lives on, however appending the identify and worth from the enter to the URL as a question string (technically, this can be a navigation, not a reload).

We might additionally ship the information to a special URL by offering the attribute, and ship the information throughout the request physique by setting the attribute to ‘POST’.

It’s dependable, however the person expertise is meh. The browser navigates to the goal URL, inflicting a complete web page refresh. It really works, however it’s not very attractive.

We’ve all develop into accustomed to interactions taking place with out the browser refreshing. So asking people to return to solely making HTTP requests with <type> isn’t going to occur.

The excellent news is that we don’t have to decide on between HTML and JavaScript. We are able to use each!

Let’s construct a pizza ordering type. When it’s submitted, we’ll wish to ship the information within the request physique to the URL “”. Within the request physique, we’ll embody a reputation, electronic mail deal with, and most well-liked toppings.

That is going to be essentially the most straight-forward half. In any case, that is how issues have labored for many years, so there isn’t any form of hand-waving we have to do to make it work. It simply works. That’s the great thing about HTML.

We inform the shape the place to ship the information, and to make use of the POST technique. Then inside the shape, every enter will get its personal and a attribute.

Labels are essential for accessibility and are related to their respective inputs by way of the attribute and the enter’s attribute.

The identify attribute is important for useful causes. It tells the shape methods to reference that bit of knowledge. A number of the inputs additionally share the identical identify which is necessary to notice as a result of it permits the identical knowledge property to have a number of values.

Along with doing the factor we would like (sending knowledge to a URL) utilizing HTML varieties additionally offers us some benefits constructed into the browser. The browser can can relay necessary semantic/accessibility info to customers counting on assistive expertise, and we even get client-side form validation totally free.

It’s not the most effective validation software, however it doesn’t price the person something to obtain it. And we will additionally progressively improve the validation expertise, however that’s past the scope of this text.

Subsequent, we’ll use JavaScript to connect an occasion listener to the “submit” event. Within the occasion handler, we will forestall the conventional HTML submit occasion from operating, and substitute the identical performance with JavaScript.

The tough half is to make it possible for requests made with JavaScript work the identical as with HTML. So the browser determines what we have to rebuild with JavaScript to be able to keep characteristic parity. In different phrases, we don’t wish to make some issues higher at the price of making different issues worse.

Let’s break it down, step-by-step. To connect an occasion handler to a type, we’d like a type DOM node. We are able to use document.querySelector for that. As soon as we now have the DOM node, we will connect the occasion handler with node.addEventListener().

doc.querySelector('type').addEventListener('submit', (occasion) => 
// Occasion handler goes in right here

We wish to make the HTTP request utilizing the fetch API. To take action, we’ll must know the URL, the information, and optionally, the request technique. For GET requests, we will ship all the information within the URL. For POST requests, we’ll must move an Object with the technique and physique properties.

Conveniently, if the HTML is finished correctly we will derive all the knowledge we’d like.

const type = occasion.goal;
const url = new URL(type.motion || window.location.href);
const formData = new FormData(type);
const choices =
technique: type.technique,
  • <type> DOM node is out there because the occasion’s .
  • URL comes from the type.motion attribute. If it is not outlined, the default HTML habits is to make use of the present URL. So we will default to window.location.href. We’ll use a URL object to make modifications afterward a bit less complicated.
  • The API makes it simple to seize knowledge from any type (so long as the inputs have identify attributes).
  • The request technique is out there from the type.technique property. It defaults to 'get'. We retailer this in an object to make it simple to move to the fetch name.

Subsequent, we have to decide methods to really ship the information. If the request ought to use the POST technique, we wish to add a “physique” to the request in fetch ‘s choices object. In any other case, we’ll wish to ship the information within the URL as a question string.

That is trickier than it sounds as a result of on POST requests, we will’t simply assign a FormData object because the request physique. Doing so will really modify request’s Content material-Sort header to 'multipart/form-data' which might break your HTTP request (extra on that shortly).

Thankfully, the net platform has one other helpful software in URLSearchParams (this truthfully stands out as the star of the present). We are able to use a URLSearchParams object because the request physique with out modifying the headers. We are able to additionally use it to assemble the question string for a GET request. Useful!

Okay, extra code…

if (choices.technique === 'put up') 
choices.physique = type.enctype === 'multipart/form-data' ? formData : new URLSearchParams(formData);
else = new URLSearchParams(formData);
  • For POST requests, we’ll ship the information within the request physique.
  • If the shape explicitly units the to 'multipart/form-data', it is protected to make use of FormData within the physique.
  • In any other case, we will fall again to URLSearchParams.
  • For GET requests, we’ll ship the information within the request URL with the URLSearchParams object.

As soon as once more, we should be particularly cautious about not modifying the default browser habits, notably across the request physique. HTML varieties can modify the Content-Type request header by assigning the attribute. The default is 'software/x-www-form-urlencoded', however in case you ever must ship information in a type, it’s important to use the 'multipart/form-data'.

That is necessary as a result of many backend frameworks don’t assist 'multipart/form-data' by default. So except you might be sending information, it is most likely finest to stay to the default.

On to the house stretch.

We have now all the information and configuration we’d like. The final half is to execute the fetch request.

fetch(url, choices)
  • With our URL and choices outlined above, we will move them to the fetch API.
  • Execute the event.preventDefault technique to forestall the HTML <type> from additionally submitting and reloading the web page.

You’ll have seen different tutorials and puzzled why we’re ready till the final minute to name the preventDefault technique. Even that could be a cautious consideration.

Take into account the chance that there could possibly be a JavaScript error hidden in our occasion handler. If we referred to as preventDefault on the very first line, and the error occurred earlier than our fetch name, the script would break and the HTTP request would by no means be despatched. By ready till all of the earlier JavaScript has executed, we will make it possible for there are not any errors earlier than stopping the “submit” occasion. Or, within the occasion of an error, the browser will nonetheless fall again to the default habits of submitting the shape the quaint manner.

The entire script may seem like this:

It’s a bit underwhelming contemplating how a lot thought and energy went into it. However I assume that’s a great factor as a result of it signifies that with only a little bit of JavaScript we will add a nicer person expertise. And since the HTML declaratively gives all the knowledge the JavaScript wants, this identical enchancment could be utilized to all varieties with none modification.

We give customers a greater expertise when JavaScript is enabled and a minimal working expertise when JavaScript is disabled or one thing goes incorrect.

Progressive enhancement FTW!!!

(we will do the identical with client-side validation as nicely, however it’s a bit concerned to get it proper)

However we’re nonetheless not accomplished. Up to now, we’ve solely lined the information sending facet of issues, however what in regards to the knowledge receiving half?

If all we have been doing was sending knowledge to the server, we might name it a day, however usually we wish to present the person some replace. Both a bit of knowledge has modified on the web page, or we wish to notify the person their request was profitable.

In an HTML-only world, the browser would navigate both to a brand new web page or to the identical URL. Both manner, that navigation occasion would rebuild the HTML on the server and ship it again to the person because the response. So exhibiting the up to date state of the appliance is simple sufficient.

With JavaScript, we don’t get the good thing about a full web page re-render. Technically, the browser might nonetheless reply with the HTML for the web page and we might use JavaScript to repaint the web page, or we might even set off a web page reload manually, however then what’s the purpose?

It’s way more widespread (and preferable) to reply with JSON. However that additionally introduces its personal dilemma. If we reply with JSON, the default HTML type submissions will reload the web page and present the person a bunch of nonsense.

What if we might reply to HTML requests with HTML and reply to JavaScript requests with JSON?

Properly, we will!

When an HTTP request is shipped from the consumer to a server, there’s some extra knowledge that tags alongside for the journey with out the developer or person needing to do something. A part of this knowledge is the HTTP Headers.

The cool factor is that in most fashionable browsers, there’s a header referred to as Sec-Fetch-Mode which tells the server the Request.mode. Apparently, for requests made with JavaScript, the worth is ready to cors, and for requests made with HTML, the worth is navigate.

The dangerous information is that it’s not supported in IE 11 or Safari. Boo!

The excellent news is we will nonetheless detect what the response sort must be by asking JavaScript builders to do just a bit bit extra leg work.

Once we create a fetch request, the second parameter is a configuration object. Inside that configuration, we will customise the request headers. Sadly, we will not customise the Sec-Fetch-Mode header from right here (browsers do not enable that), however we can set the Accept header.

This helpful little header lets us explicitly inform the server what sort of response we wish. The default worth is */* (like, no matter dude), however we will set it to software/json (JSON please!). We would want to manually add this to each request, which is type of annoying, however I feel it is value it.

So right here’s what our new fetch request might seem like:

technique: requestMethod,
physique: bodyData,
headers: new Headers( 'software/json' )

The primary parameter continues to be simply the URL. For POST requests, the second (init) parameter ought to already exists, so we solely want so as to add or modify the headers property. For GET requests, the second parameter might not already be outlined, so we may have to incorporate it with the headers property. And be aware that though I am utilizing the constructor right here, you might simply as nicely use an everyday Object.

In case you make loads of HTTP requests in your software, manually including this to each single one may get outdated. So I’d suggest utilizing a wrapper or curried function round fetch that automates this for you.

At this level, the request is sending all the information that the server wants. Now, the server must deal with the request and decide methods to reply. For JSON responses, I’ll go away that as much as you. For HTML responses we will return the identical web page, or just reply with a redirect.

You may decide the place to redirect customers based mostly on the person request and your software logic. Or in case you simply wish to redirect customers again from whence they got here, we now have another HTTP header we will use: . The Referer header accommodates the URL from which the request was made, is mechanically set by the browser, can’t be modified by JavaScript, and is out there on all requests. It is excellent in each manner.

Instance time!

Right here I’m utilizing fastify, however the ideas ought to apply throughout any language or framework:

  • Create a server route that accepts GET or POST requests
  • I skipped the spicy enterprise logic, however that ought to go earlier than your response (clearly)
  • Seize the Settle for, Sec-Fetch-Mode, and Referer headers
  • Decide if the response must be JSON. In that case, reply with JSON. Word that the early return will forestall the remainder of the execution.
  • In any other case, both reply with HTML, redirect to a brand new URL, or redirect again to the place the request got here from. On this case, I did the latter.
  • Word that the request handler has to simply accept urlencoded knowledge (the HTML default encoding). It might optionally settle for JSON, however in case you solely settle for JSON, then the payload has a tough requirement of being created with JavaScript and subsequently makes supporting HTML type of pointless.

This works rather well in my testing, and in case you needed to, you might even create a plugin (or middleware) so as to add this logic to each route. That manner, you don’t should manually add it each time.

One draw back to all of this (perhaps you already caught it) is that if the aim is to assist HTML varieties, you may solely assist GET and POST request strategies.

It’s a bummer as a result of PUT, PATCH, and DELETE are very helpful strategies for dealing with CRUD operations. The excellent news is with a little bit of adjustments to the URL patterns, we will accomplish the identical factor, virtually as properly.

Right here’s what an API may seem like utilizing any technique we would like:

server.put up('/api/kitten', create);
server.get('/api/kitten', readCollection);
server.get('/api/kitten/:id', readSingle);
server.patch('/api/kitten/:id', updateSingle);
server.delete('/api/kitten/:id', deleteSingle);

Right here’s that very same API utilizing solely GET and POST strategies:

server.put up('/api/kitten', create);
server.get('/api/kitten/', readCollection);
server.get('/api/kitten/:id', readSingle);
server.put up('/api/kitten/:id/replace', updateSingle);
server.put up('/api/kitten/:id/delete', deleteSingle);
  • The GET and POST routes don’t change.
  • The PATCH and DELETE routes develop into POST.
  • We append the “strategies” /replace and /delete to their respective routes.

I’ll admit that I don’t love this tradeoff, however it’s a small annoyance and I feel I’ll survive. In any case, the advantages (as you’ll hopefully see) are so value it.

This has been only one instance of progressive enhancement because it applies to creating HTTP requests, however I believed it was an necessary one. Listed below are some issues I feel you need to take into accout:

  • Kinds can solely ship GET and POST requests. If you don’t specify attributes, they default sending to GET request to the present URL.
  • By default, knowledge is shipped to the server as URL encoded string (textual content=hiya&quantity=1) except you alter the enctype. For GET requests, it goes within the request URL. For POST requests, it goes within the request physique.
  • A number of inputs can have the identical identify and type knowledge can have a number of values for a similar knowledge property.
  • Once you submit a type, most text-style inputs will ship their knowledge as empty strings. There are some exceptions.
  • Default values for checkbox and radio is ‘on’, even when left unchecked. If it is not chosen, the information isn’t included.
  • The default worth for vary is ’50’.
  • The default worth for choose is regardless of the first chosen <choice>. If the choice doesn’t have a price, will use the contents of the tag. You may keep away from sending default knowledge by omitting disabling all choices.
  • The file enter sends the file identify by default. To ship the precise file as binary knowledge the request’s Content material-Sort have to be multipart/form-data which you’ll be able to set with the enctype attribute.
  • The default worth for shade inputs is '#000000'.
  • For JavaScript submissions, FormData and URLSearchParams are superior. Each can be utilized within the request physique, however utilizing FormData will change the default Content material-Sort to multipart/form-data.
  • To incorporate further knowledge in your request, you need to use and enter with the identify, worth, and the sort of “hidden”.

Making use of what we’ve discovered to the very first instance, a button that sends knowledge when clicked, we will accomplish the identical factor extra reliably with a little bit of a change to the HTML:

<type motion="" technique="POST">
<enter sort="hidden" identify="data-key" worth="data-value">
<button>Ship knowledge</button>

Sprinkle our JavaScript type submitter on high of that and we’ll have a contemporary person expertise that also works if the web page experiences some situation with JavaScript.

No answer is ideal and it will be dishonest of me to say that what I like to recommend above is with out flaws. In truth, there’s a actually massive one.

Counting on HTML varieties to assemble knowledge means you may’t use nested Objects.

I imply, perhaps you might create inputs with dot or bracket notations within the identify (<enter identify="person.first-name"> <enter identify="person.last-name">), then do some logic on the server to assemble a nested Object. However I have never accomplished it myself, and I am not satisfied it is value it, so I am not going to attempt to persuade you. Anyway, in case you begin with the flat knowledge mannequin in thoughts from the start, this shouldn’t be an issue.

Since nested Objects are out of the query, that additionally means you may’t make GraphQL queries.

Having used GraphQL on the consumer and the server, with and with out libraries, I’ll say it’s tremendous cool and modern however I don’t prefer it. It provides extra complexity than it’s value and the prices outweigh the advantages for many initiatives.

There are, nevertheless, few initiatives the place there are sufficient knowledge and HTTP requests flying round that GraphQL is sensible. For these instances, it’s value it.

(Hopefully, I didn’t make a very biased ass of myself)

Having made my case, I’ll pose this query to you. You probably have a checkout type that solely works with JavaScript, is it value a rewrite in order that it falls again to HTML if JavaScript fails? Properly, it relies upon, proper?

If your organization earns 1,000,000 greenback via that type, and the shape fails 1% of the time resulting from JavaScript points, that repair could possibly be value ten thousand {dollars}.

Why 1%? According to research by, JavaScript will fail for 1% or customers. That’s all customers (or web page views), not solely the sting instances the place somebody turns or JavaScript or is utilizing an outdated browser on an outdated cell phone. There’s a superb explainer referred to as Why Availability Matters that goes into extra particulars, however now I’ve digressed.

Getting again to the unique query, is $10,000 sufficient so that you can make some adjustments to that type? It could be for me, and that call is predicated on a single type dealing with $1,000,000. That’s really not that a lot income for a corporation, and most web sites which have varieties, normally have a couple of.

I’ve used the identical instance of HTTP requests in HTML vs. JavaScript to drive dwelling the idea, however it’s necessary to level out that not all progressive enhancement is a zero-sum game.

It’s not all the time about it working or being damaged. Generally it’s about working nice or working simply OK. Generally it pertains to safety, efficiency, accessibility, or design. Whatever the situation or expertise, progressive enhancement is about constructing with resilient fallbacks in thoughts.

There are loads of fantastic options obtainable to newer browsers, however it’s necessary, to start with one thing that can work for many customers, and supply an enhanced expertise for the parents that may assist it. With CSS, for instance, you might seek the advice of to verify for browser compatibility, write kinds for older browsers, and add fashionable enhancements inside an at-rule that detects that characteristic.

Lastly, I’ll go away you with this quote.

This code is each backwards and forwards appropriate. Browsers being evergreen and JavaScript being pressured to backwards compat by the legacy of the net is a HUGE characteristic… many dev groups blow many hours of cycles to maintain performance they have already got as a result of they’re chasing dependency updates. This system is each forwards and backwards appropriate with out chasing updates compounding your funding of time. Write as soon as, run eternally is a really distinctive characteristic to JS that many ‘fashionable’ frameworks go away behind.

Brian LeRoux

I loved this text and wish to take a look at extra issues I’ve created alongside the identical traces listed here are a number of:

Listed below are among the sources I discovered notably attention-grabbing:

[Update: I listed ways JavaScript could fail some of these include network conditions. Next, I go into an example of progressively enhanced form submissions. It’s worth calling out that the list of ways JS can fail is a good general thing to keep in mind, but if it errors due to network conditions, it’s possible that an HTML form will fail anyway.]

Thanks a lot for studying.

Need to Join?You can too sign up for my newsletter or follow me on Twitter if you wish to know when new articles are revealed.Initially revealed on

More Posts