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Exposing Additional Form Fields via Checked Radio Buttons

There is a :checked pseudo class in CSS. I often think of it in connection with the “checkbox hack”, in which you use it on a hidden checkbox with the ~ adjacent sibling combinator to do simulate toggling behavior without any JavaScript. It’s a hack because now you have these stray form elements on the page that really aren’t for a form. Not a huge deal, as I’m sure you can work around it accessibility wise, but there is a way to use this same concept in a totally non-hacky way, on an actual form!

I used this technique on the CodePen job posting form to only reveal additional form fields as needed.

Let’s get to the point with a demo:

See the Pen LEGXOK by Chris Coyier (@chriscoyier) on CodePen.


Let’s say we’re using radio buttons here. Either a checkbox or a radio button can be “checked” (:checked). The div we’re going to reveal has to be after it, and a sibling element (within the same parent).

<input type="radio" name="choice-animals" id="choice-animals-dogs">
<label for="choice-animals-dogs">I like dogs more</label> <div class="reveal-if-active"> Anything you want in here.


By default, that “reveal-if-active” div will be hidden. We’re going to do that in a number of different ways. They aren’t all required just to hide it, but they will play a factor in how it reveals itself.

.reveal-if-active { opacity: 0; max-height: 0; overflow: hidden;

Now when a radio button or checkbox at the same level as it becomes checked, it reveals:

input[type="radio"]:checked ~ .reveal-if-active,
input[type="checkbox"]:checked ~ .reveal-if-active { opacity: 1; max-height: 100px; /* little bit of a magic number šŸ™ */ overflow: visible;

That works, but let’s transition everything fancy style, and Sass it up:

.reveal-if-active { opacity: 0; max-height: 0; overflow: hidden; transform: scale(0.8); transition: 0.5s; input[type="radio"]:checked ~ &, input[type="checkbox"]:checked ~ & { opacity: 1; max-height: 100px; overflow: visible; padding: 10px 20px; transform: scale(1); }

That’ll do the trick.

What about required fields?

This gets a little tricker. Let’s say in the “reveal-if-active” divs you’re putting more form fields (that’s the whole point really). You might want to require a field if it’s revealed, but not if it’s not. A required field that is hidden is still required, and so you can’t just put the required attribute on those hidden fields and forget about, lest get yourself in a nasty UX situation.

I handle this by putting a “require-if-active” class on the input, and a data-* attribute which references the ID it’s paired with.

... <div class="reveal-if-active"> <label for="which-dog">Good call. What's the name of your favorite dog?</label> <input class="require-if-active" type="text" id="which-dog" name="which-dog" data-require-pair="#choice-animals-dogs">

Then, when the page loads, and whenever a checkbox or radio button changes value, you run some JavaScript to determine whether that input should be required or not.

var FormStuff = { init: function() { // kick it off once, in case the radio is already checked when the page loads this.applyConditionalRequired(); this.bindUIActions(); }, bindUIActions: function() { // when a radio or checkbox changes value, click or otherwise $("input[type='radio'], input[type='checkbox']").on("change", this.applyConditionalRequired); }, applyConditionalRequired: function() { // find each input that may be hidden or not $(".require-if-active").each(function() { var el = $(this); // find the pairing radio or checkbox if ($(el.data("require-pair")).is(":checked")) { // if its checked, the field should be required el.prop("required", true); } else { // otherwise it should not el.prop("required", false); } }); } }; FormStuff.init();

Yep, little JavaScript required. We could do the hide/showing through JavaScript as well, but CSS is quite well suited for it. You might even say it’s a design concern, so it belongs in CSS. #debate

Here’s that demo again.

Exposing Additional Form Fields via Checked Radio Buttons is a post from CSS-Tricks

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