Issue #172  (defaultChecked property & Learning Tools)11/03/16

Here's an interesting DOM feature when working with forms that maybe you haven't seen before. Let's say you have a set of radio buttons or checkboxes in a form, like this:
<form id="form">
  <input type="radio" value="one" name="setOne">One
  <input type="radio" value="two" name="setOne" checked>Two
  <input type="radio" value="three" name="setOne">Three

There are three radio buttons and the second one is selected by default. As you probably know, you can use the checked attribute to determine which of those is selected at any given time. So even if the user switches the selection to a different radio button, you can find out which one it is:
let myForm = document.getElementById('form');

for (i = 0; i < myForm.setOne.length; i++) {
  if (myForm.setOne[i].checked === true) {
    // logs "two"

Even after the user switches the selected button, you can run the same type of script and get the newly selected value. Simple enough.

But what if you want to determine which radio button was selected by default when the page first loaded? You can use the defaultChecked property for that:
for (i = 0; i < myForm.setOne.length; i++) {
  if (myForm.setOne[i].defaultChecked === true) {
    // always logs "two"

Here's a JS Bin with a bunch of tests logged.

So no matter what gets changed on the form as a result of user input, the defaultChecked property will always be the value of the one that was selected by default. The only strange behaviour with this feature is the fact that you can define it with JavaScript and you can define it for more than one radio button in a set.

This means if you do:
myForm.setOne[0].defaultChecked = true;

on the above form, you will then have two radio buttons that log true for defaultChecked. So you have to unset the other one if you want to have only one have this attribute. Which I find a little strange, to be honest. The JS Bin includes a test for that too, towards the bottom.

Now on to this week's tools!
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Before I Go...

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