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Can't change opacity with javascript function (html radio buttons) [duplicate]

I'm looking for a generalized solution for this.
Consider 2 radio type inputs with the same name. When submitted, the one that is checked determines the value that gets sent with the form:
<input type="radio" name="myRadios" onchange="handleChange1();" value="1" />
<input type="radio" name="myRadios" onchange="handleChange2();" value="2" />
The change event does not fire when a radio button is de-selected. So if the radio with value="1" is already selected and the user selects the second, handleChange1() does not run. This presents a problem (for me anyway) in that there is no event where I can catch this de-selection.
What I would like is a workaround for the onChange event for the checkbox group value or alternatively, an onCheck event that detects not only when a radio button is checked but also when it is unchecked.
I'm sure some of you have run into this problem before. What are some workarounds (or ideally what is the right way to handle this)? I just want to catch the change event, access the previously checked radio as well as the newly checked radio.
P.S.
onClick seems like a better (cross-browser) event to indicate when a radio button is checked but it still does not solve the unchecked problem.
I suppose it makes sense why onChange for a checkbox type does work in a case like this since it changes the value that it submits when you check or un-check it. I wish the radio buttons behaved more like a SELECT element's onChange but what can you do...

var rad = document.myForm.myRadios;
var prev = null;
for (var i = 0; i < rad.length; i++) {
rad[i].addEventListener('change', function() {
(prev) ? console.log(prev.value): null;
if (this !== prev) {
prev = this;
}
console.log(this.value)
});
}
<form name="myForm">
<input type="radio" name="myRadios" value="1" />
<input type="radio" name="myRadios" value="2" />
</form>
Here's a JSFiddle demo: https://jsfiddle.net/crp6em1z/

I would make two changes:
<input type="radio" name="myRadios" onclick="handleClick(this);" value="1" />
<input type="radio" name="myRadios" onclick="handleClick(this);" value="2" />
Use the onclick handler instead of onchange - you're changing the "checked state" of the radio input, not the value, so there's not a change event happening.
Use a single function, and pass this as a parameter, that will make it easy to check which value is currently selected.
ETA: Along with your handleClick() function, you can track the original / old value of the radio in a page-scoped variable. That is:
var currentValue = 0;
function handleClick(myRadio) {
alert('Old value: ' + currentValue);
alert('New value: ' + myRadio.value);
currentValue = myRadio.value;
}
var currentValue = 0;
function handleClick(myRadio) {
alert('Old value: ' + currentValue);
alert('New value: ' + myRadio.value);
currentValue = myRadio.value;
}
<input type="radio" name="myRadios" onclick="handleClick(this);" value="1" />
<input type="radio" name="myRadios" onclick="handleClick(this);" value="2" />

As you can see from this example: http://jsfiddle.net/UTwGS/
HTML:
<label><input type="radio" value="1" name="my-radio">Radio One</label>
<label><input type="radio" value="2" name="my-radio">Radio One</label>
jQuery:
$('input[type="radio"]').on('click change', function(e) {
console.log(e.type);
});
both the click and change events are fired when selecting a radio button option (at least in some browsers).
I should also point out that in my example the click event is still fired when you use tab and the keyboard to select an option.
So, my point is that even though the change event is fired is some browsers, the click event should supply the coverage you need.

You can add the following JS script
<script>
function myfunction(event) {
alert('Checked radio with ID = ' + event.target.id);
}
document.querySelectorAll("input[name='myRadios']").forEach((input) => {
input.addEventListener('change', myfunction);
});
</script>

What about using the change event of Jquery?
$(function() {
$('input:radio[name="myRadios"]').change(function() {
if ($(this).val() == '1') {
alert("You selected the first option and deselected the second one");
} else {
alert("You selected the second option and deselected the first one");
}
});
});
jsfiddle: http://jsfiddle.net/f8233x20/

Easiest and power full way
read only radio inputs using getAttribute
document.addEventListener('input',(e)=>{
if(e.target.getAttribute('name')=="myRadios")
console.log(e.target.value)
})
<input type="radio" name="myRadios" value="1" /> 1
<input type="radio" name="myRadios" value="2" /> 2

Store the previous checked radio in a variable:
http://jsfiddle.net/dsbonev/C5S4B/
HTML
<input type="radio" name="myRadios" value="1" /> 1
<input type="radio" name="myRadios" value="2" /> 2
<input type="radio" name="myRadios" value="3" /> 3
<input type="radio" name="myRadios" value="4" /> 4
<input type="radio" name="myRadios" value="5" /> 5
JS
var changeHandler = (function initChangeHandler() {
var previousCheckedRadio = null;
var result = function (event) {
var currentCheckedRadio = event.target;
var name = currentCheckedRadio.name;
if (name !== 'myRadios') return;
//using radio elements previousCheckedRadio and currentCheckedRadio
//storing radio element for using in future 'change' event handler
previousCheckedRadio = currentCheckedRadio;
};
return result;
})();
document.addEventListener('change', changeHandler, false);
JS EXAMPLE CODE
var changeHandler = (function initChangeHandler() {
var previousCheckedRadio = null;
function logInfo(info) {
if (!console || !console.log) return;
console.log(info);
}
function logPrevious(element) {
if (!element) return;
var message = element.value + ' was unchecked';
logInfo(message);
}
function logCurrent(element) {
if (!element) return;
var message = element.value + ' is checked';
logInfo(message);
}
var result = function (event) {
var currentCheckedRadio = event.target;
var name = currentCheckedRadio.name;
if (name !== 'myRadios') return;
logPrevious(previousCheckedRadio);
logCurrent(currentCheckedRadio);
previousCheckedRadio = currentCheckedRadio;
};
return result;
})();
document.addEventListener('change', changeHandler, false);

I don't think there is any way other then storing the previous state.
Here is the solution with jQuery
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script type="text/javascript">
var lastSelected;
$(function () {
//if you have any radio selected by default
lastSelected = $('[name="myRadios"]:checked').val();
});
$(document).on('click', '[name="myRadios"]', function () {
if (lastSelected != $(this).val() && typeof lastSelected != "undefined") {
alert("radio box with value " + $('[name="myRadios"][value="' + lastSelected + '"]').val() + " was deselected");
}
lastSelected = $(this).val();
});
</script>
<input type="radio" name="myRadios" value="1" />
<input type="radio" name="myRadios" value="2" />
<input type="radio" name="myRadios" value="3" />
<input type="radio" name="myRadios" value="4" />
<input type="radio" name="myRadios" value="5" />
After thinking about it a bit more, I decided to get rid of the variable and add/remove class. Here is what I got: http://jsfiddle.net/BeQh3/2/

I realize this is an old issue, but this snippet of code works for me. Perhaps someone in the future will find it useful:
<h2>Testing radio functionality</h2>
<script type="text/javascript">var radioArray=[null];</script>
<input name="juju" value="button1" type="radio" onclick="radioChange('juju','button1',radioArray);" />Button 1
<input name="juju" value="button2" type="radio" onclick="radioChange('juju','button2',radioArray);" />Button 2
<input name="juju" value="button3" type="radio" onclick="radioChange('juju','button3',radioArray);" />Button 3
<br />
<script type="text/javascript">
function radioChange(radioSet,radioButton,radioArray)
{
//if(radioArray instanceof Array) {alert('Array Passed');}
var oldButton=radioArray[0];
if(radioArray[0] == null)
{
alert('Old button was not defined');
radioArray[0]=radioButton;
}
else
{
alert('Old button was set to ' + oldButton);
radioArray[0]=radioButton;
}
alert('New button is set to ' + radioArray[0]);
}
</script>

As you can see here:
http://www.w3schools.com/jsref/event_onchange.asp
The onchange attribute is not supported for radio buttons.
The first SO question linked by you gives you the answer: Use the onclick event instead and check the radio button state inside of the function it triggers.

Yes there is no change event for currently selected radio button. But problem is when each radio button is taken as a separate element. Instead a radio group should be considered a single element like select. So change event is triggered for that group. If it is a select element we never worry about each option in it, but take only the selected option. We store the current value in a variable which will become the previous value, when a new option is selected. Similarly you have to use a separate variable for storing value of checked radio button.
If you want to identify the previous radio button, you have to loop on mousedown event.
var radios = document.getElementsByName("myRadios");
var val;
for(var i = 0; i < radios.length; i++){
if(radios[i].checked){
val = radios[i].value;
}
}
see this : http://jsfiddle.net/diode/tywx6/2/

This is just off the top of my head, but you could do an onClick event for each radio button, give them all different IDs, and then make a for loop in the event to go through each radio button in the group and find which is was checked by looking at the 'checked' attribute. The id of the checked one would be stored as a variable, but you might want to use a temp variable first to make sure that the value of that variable changed, since the click event would fire whether or not a new radio button was checked.

<input type="radio" name="brd" onclick="javascript:brd();" value="IN">
<input type="radio" name="brd" onclick="javascript:brd();" value="EX">`
<script type="text/javascript">
function brd() {alert($('[name="brd"]:checked').val());}
</script>

If you want to avoid inline script, you can simply listen for a click event on the radio. This can be achieved with plain Javascript by listening to a click event on
for (var radioCounter = 0 ; radioCounter < document.getElementsByName('myRadios').length; radioCounter++) {
document.getElementsByName('myRadios')[radioCounter].onclick = function() {
//VALUE OF THE CLICKED RADIO ELEMENT
console.log('this : ',this.value);
}
}

this works for me
<input ID="TIPO_INST-0" Name="TIPO_INST" Type="Radio" value="UNAM" onchange="convenio_unam();">UNAM
<script type="text/javascript">
function convenio_unam(){
if(this.document.getElementById('TIPO_INST-0').checked){
$("#convenio_unam").hide();
}else{
$("#convenio_unam").show();
}
}
</script>

This is the easiest and most efficient function to use just add as many buttons as you want to the checked = false and make the onclick event of each radio buttoncall this function. Designate a unique number to each radio
button
function AdjustRadios(which)
{
if(which==1)
document.getElementById("rdpPrivate").checked=false;
else if(which==2)
document.getElementById("rdbPublic").checked=false;
}

For some reason, the best answer does not works for me.
I improved best answer by use
var overlayType_radio = document.querySelectorAll('input[type=radio][name="radio_overlaytype"]');
Original best answer use:
var rad = document.myForm.myRadios;
The others keep the same, finally it works for me.
var overlayType_radio = document.querySelectorAll('input[type=radio][name="radio_overlaytype"]');
console.log('overlayType_radio', overlayType_radio)
var prev = null;
for (var i = 0; i < overlayType_radio.length; i++) {
overlayType_radio[i].addEventListener('change', function() {
(prev) ? console.log('radio prev value',prev.value): null;
if (this !== prev) {
prev = this;
}
console.log('radio now value ', this.value)
});
}
html is:
<div id='overlay-div'>
<fieldset>
<legend> Overlay Type </legend>
<p>
<label>
<input class='with-gap' id='overlayType_image' value='overlayType_image' name='radio_overlaytype' type='radio' checked/>
<span>Image</span>
</label>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_tiled_image' value='overlayType_tiled_image' name='radio_overlaytype' type='radio' disabled/>
<span> Tiled Image</span>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_coordinated_tile' value='overlayType_coordinated_tile' name='radio_overlaytype' type='radio' disabled/>
<span> Coordinated Tile</span>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_none' value='overlayType_none' name='radio_overlaytype' type='radio'/>
<span> None </span>
</p>
</fieldset>
</div>
var overlayType_radio = document.querySelectorAll('input[type=radio][name="radio_overlaytype"]');
console.log('overlayType_radio', overlayType_radio)
var prev = null;
for (var i = 0; i < overlayType_radio.length; i++) {
overlayType_radio[i].addEventListener('change', function() {
(prev) ? console.log('radio prev value',prev.value): null;
if (this !== prev) {
prev = this;
}
console.log('radio now value ', this.value)
});
}
<div id='overlay-div'>
<fieldset>
<legend> Overlay Type </legend>
<p>
<label>
<input class='with-gap' id='overlayType_image' value='overlayType_image' name='radio_overlaytype' type='radio' checked/>
<span>Image</span>
</label>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_tiled_image' value='overlayType_tiled_image' name='radio_overlaytype' type='radio' />
<span> Tiled Image</span>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_coordinated_tile' value='overlayType_coordinated_tile' name='radio_overlaytype' type='radio' />
<span> Coordinated Tile</span>
</p>
<p>
<label>
<input class='with-gap' id='overlayType_none' value='overlayType_none' name='radio_overlaytype' type='radio'/>
<span> None </span>
</p>
</fieldset>
</div>
jsfiddle click here
https://jsfiddle.net/hoogw/jetmkn02/1/

Tl;dr
'focusout' is dispatched before the 'change' event - example:
const radioName = 'radio';
// Add radios
document.body.innerHTML = `
<style>
input + label {
margin-left: 1rem;
}
</style>
<form action="#" name="example-form">
<fieldset>
${Array(5).fill(null, 0, 5).map((_, i) => {
const offsetId = i + 1;
const id = `radio-${offsetId}`;
return `<label for="${id}">Radio ${offsetId}</label>
<input type="radio" name="${radioName}" id="${id}" value="${offsetId}" />`;
}).join('\n')}
</fieldset>
</form>
`;
const {log} = console,
form = document.forms['example-form'];
form.addEventListener('submit', e => e.preventDefault());
form.addEventListener('change', e => {
const {target} = e;
if (target.matches(`[type="radio"][name="${radioName}"]`)) {
log(`[${e.type}]: "${target.id}" selected; Value: ${target.value}`);
}
});
form.addEventListener('focusout', e => {
const {target} = e,
soonToBePrevValue = target && target.form ?
target.form.elements[radioName].value : null;
if (!target.matches(`[type="radio"][name="${radioName}"]`) || !soonToBePrevValue) {
return;
}
// value, for '[name="radio"]', contained in form, will change after 'focusout' event
// has completed it's bubbling stage.
log(`[${e.type}]: previously selected radio value: ` +
`${soonToBePrevValue}`);
// log("Soon to be \"previous\" radio: ", target);
});
jsfiddle

<script>
function radioClick(radio){
alert()
}
</script>
<label>Cash on delivery</label>
<input type="radio" onclick="radioClick('A')" name="payment_method" class="form-group">
<br>
<label>Debit/Credit card, GPay, Paytm etc..</label>
<input type="radio" onclick="radioClick('B')" name="payment_method" class="form-group">

How to use on addEventListener on radio button in Plain Javascript?

How can i attach event listener on a radio button in my html? Considering this form:
<label>
<input name="contract_duration" type="radio" value="6-Months" {{ $contract && $contract->contract_duration === '6-Months' ? 'checked' : '' }} />
<span>6-Months</span>
</label>
<label>
<input name="contract_duration" type="radio" value="1-Year" {{ $contract && $contract->contract_duration === '1-Year' ? 'checked' : '' }} />
<span>1-Year</span>
</label>
<label>
<input name="contract_duration" type="radio" value="2-Years" {{ $contract && $contract->contract_duration === '2-Years' ? 'checked' : '' }} />
<span>2-Years</span>
</label>
and this is my Javascript:
if(document.querySelector('input[name="contract_duration"]')){
document.querySelector('input[name="contract_duration"]').addEventListener("click", function(){
var item = document.querySelector('input[name="contract_duration"]').value;
console.log(item);
});
}
during execution, my codes only log the first one, which is the 6-Months others if click shows nothing.

The querySelector function only returns the first element of all the ones matched by the selector - hence your problem. You need to use querySelectorAll instead. This will return all the matched elements - and then you can loop through them to add an event listener to each.
Also, best practice in this scenario would be to listen to the "change" event rather than "click", then it doesn't fire unnecessarily when the user clicks an already-selected radio.
Demo:
if (document.querySelector('input[name="contract_duration"]')) {
document.querySelectorAll('input[name="contract_duration"]').forEach((elem) => {
elem.addEventListener("change", function(event) {
var item = event.target.value;
console.log(item);
});
});
}
<label>
<input name="contract_duration" type="radio" value="6-Months" checked />
<span>6-Months</span>
</label>
<label>
<input name="contract_duration" type="radio" value="1-Year" />
<span>1-Year</span>
</label>
<label>
<input name="contract_duration" type="radio" value="2-Years" />
<span>2-Years</span>
</label>

You need to use querySelectorAll instead, which returns a list of elements.
And the change event will run only for a chosen radio.
let contact = document.querySelectorAll('input[name="contract_duration"]');
// or '.your_radio_class_name'
for (let i = 0; i < contact.length; i++) {
contact[i].addEventListener("change", function() {
let val = this.value; // this == the clicked radio,
console.log(val);
});
}
label {
display: block;
}
<label>
<input name="contract_duration" type="radio" value="6-Months"/>
<span>6-Months</span>
</label>
<label>
<input name="contract_duration" type="radio" value="1-Year"/>
<span>1-Year</span>
</label>
<label>
<input name="contract_duration" type="radio" value="2-Years"/>
<span>2-Years</span>
</label>

The problem with your code is that querySelector only returns the first matched element. If you want the event listener on all of them, you should use querySelectorAll and loop through the elements.
Like this:
if(document.querySelector('input[name="contract_duration"]')){
document.querySelectorAll('input[name="contract_duration"]').forEach((elem) => {
elem.addEventListener("click", function(event){
var item = event.target.value;
console.log(item);
});
});
}

How can I get the value of the radio button after click

How will I get the value of the radio button after clicked on the button?
document.getElementById('btnKnop1').addEventListener('click', function(){
var kleuren = document.querySelectorAll('input[type=radio');
for (var i in kleuren) {
kleuren[i].onclick = function(){
document.getElementById('divResult'). innerHTML =
'Gekozen kleur: ' + this.value;
}
}
});
<input type="radio" name="radioGroup" value="rood" checked />Rood</br />
<input type="radio" name="radioGroup" value="blauw" />Blauw</br />
<input type="radio" name="radioGroup" value="geel" />Geel</br />
<input type="radio" name="radioGroup" value="groen" />Groen</br />
<button id="btnKnop1">Check de waarde!</button>
<div id="divResult"></div>
Now it depends on click on the radio button, but I'd to depend on click on the button

The issue seems to be be the inner click event on the radio button. If you change the loop to an if statement checking if it's checked then you can output the value on the button click:
document.getElementById('btnKnop1').addEventListener('click', function(){
var kleuren = document.querySelectorAll('input[type=radio');
for (var i in kleuren) {
if (kleuren[i].checked === true) {
document.getElementById('divResult'). innerHTML =
'Gekozen kleur: ' + kleuren[i].value;
}
}
});
<input type="radio" name="radioGroup" value="rood" checked />Rood</br>
<input type="radio" name="radioGroup" value="blauw" />Blauw</br>
<input type="radio" name="radioGroup" value="geel" />Geel</br>
<input type="radio" name="radioGroup" value="groen" />Groen</br>
<button id="btnKnop1">Check de waarde!</button>
<div id="divResult"></div>

First of all, please consider using english-named variables, it will improve readability by a lot.
Second of all, line
var kleuren = document.querySelectorAll('input[type=radio');
has a typo, it's missing a closing square bracket - ].
To check a checkbox/radio button value you can use checkbox.checked, where checkbox is your DOM object selected by querySelector.

You're basically already doing it. When you click the button, in the click handler for the button, just grab the radio button element using a selector (either class or id), with a "querySelector" call (just like you're doing). Inspect that element for whatever property makes sense (probably "checked").
Something like:
<button onclick="onClick ()">Click Me</button>
...
onClick () {
const kleuren = document.querySelector ( [mySelectorByIDorClass, etc.] );
console.log ( kleuren.checked );
}
See here:
https://www.w3schools.com/jsref/prop_radio_checked.asp

The checked property will tell you whether the element is selected:
if (document.getElementById('id').checked) {
var variable = document.getElementById('id').value;
}

How to show jquery dialog with radio buttons when checkbox is checked and post value in textbox

I have 41 checkboxes like this
HTML
<input id="1" type="checkbox" onclick="updatebox()" />
<input id="2" type="checkbox" onclick="updatebox()" />
<input id="3" type="checkbox" onclick="updatebox()" />
<input id="4" type="checkbox" onclick="updatebox()" />
Javascript
<script type="text/javascript">
function updatebox()
{
var textbox = document.getElementById("list");
var values = [];
if(document.getElementById('1').checked) {values.push("1");}
if(document.getElementById('2').checked) {values.push("2");}
if(document.getElementById('3').checked) {values.push("3");}
if(document.getElementById('4').checked) {values.push("4");}
textbox.value = values.join(", ");
}
</script>
When checkbox is checked the value is posted in textbox,
now what i want is when the user clicks the checkbox the jquery dialog popups and the user will have two radio buttons with Male or Female options along with ok button so when the user will click on ok the value should be posted on textbox depending on selection M for male F for female along with number like 1M or 1F, 2M or 2F and so on.
P.S user can select multiple checkboxes.
Thanks You!

Here is something that does what you want. HTML:
<body>
<form id="form">
<input id="1" type="checkbox" /> 1
<input id="2" type="checkbox" /> 2
<input id="3" type="checkbox" /> 3
<input id="4" type="checkbox" /> 4
...
<input id="10" type="checkbox" /> 10
...
<input id="41" type="checkbox" /> 41
<input id="list" />
</form>
<div id="prompt" style="display:none;" title="Gender">
<form>
<input type="radio" name="gender" id="radio" value="male" />
<label for="radio">Male</label>
<input type="radio" name="gender" id="radio2" value="female" />
<label for="radio2">Female</label>
</form>
</div>
</body>
The JavaScript:
$(function() {
var form = document.getElementById("form");
var textbox = document.getElementById("list");
var $prompt = $("#prompt");
// We record what is currently checked, and the user's answers in this `pairs` object.
var pairs = [];
// Listen to `change` events.
$("input[type='checkbox']", form).on('change', function (ev) {
var check = ev.target;
if (check.checked) {
// Checked, so prompt and record.
$prompt.dialog({
modal: true,
buttons: {
"Ok": function() {
var gender = $prompt.find("input[name='gender']:checked")[0];
var letter = {"male":"M", "female":"F"}[gender.value];
pairs[check.id] = '' + check.id + letter;
$( this ).dialog( "close" );
refresh();
}
}
});
}
else {
// Unchecked, so forget it.
delete pairs[check.id];
refresh();
}
function refresh() {
// Generate what we must now display in the textbox and refresh it.
// We walk the list.
var keys = Object.keys(pairs);
var values = [];
for (var i = 0, key; (key = keys[i]); ++i) {
values.push(pairs[key]);
}
textbox.value = values.join(", ");
}
});
});
Here is a jsbin with the code above.
Salient points:
This code adds the event handlers using JavaScript rather than use onclick in the HTML. It is not recommended to associated handlers directly in the HTML.
It listens to the change event rather than click. Some clicks can sometimes not result in a change to an input element.
It uses $.dialog to prompt the user for M, F.
The refresh function is what recomputes the text field.
It keeps a record of what is currently checked rather than requery for all the check boxes when one of them changes.

function updatebox()
{
var textbox = document.getElementById("list");
var values = [];
for (var i = 1; i <= 41; ++i) {
var id = '' + i;
if (document.getElementById(id).checked) {
var gender = prompt('Male (M) or female (F)?');
values.push(gender + id);
}
}
textbox.value = values.join(", ");
}
A few things to note:
I got rid of all that code repetition by simply using a for loop from 1 to 41.
I also fixed the strange indentation you had there.
You may want to use a method of getting user input other than prompt, but it'll work the same way.
(If you're going to keep using prompt, you might also want to add input validation as well to make sure the user didn't input something other than M or F.)

How to set radio button status with JavaScript

What method would be best to use to selectively set a single or multiple radio button(s) to a desired setting with JavaScript?

Very simple
radiobtn = document.getElementById("theid");
radiobtn.checked = true;

the form
<form name="teenageMutant">
<input value="aa" type="radio" name="ninjaTurtles"/>
<input value="bb" type="radio" name="ninjaTurtles"/>
<input value="cc" type="radio" name="ninjaTurtles" checked/>
</form>
value="cc" will be checked by default, if you remove the "checked" non of the boxes will be checked when the form is first loaded.
document.teenageMutant.ninjaTurtles[0].checked=true;
now value="aa" is checked. The other radio check boxes are unchecked.
see it in action: http://jsfiddle.net/yaArr/
You may do the same using the form id and the radio button id. Here is a form with id's.
<form id="lizardPeople" name="teenageMutant">
<input id="dinosaurs" value="aa" type="radio" name="ninjaTurtles"/>
<input id="elephant" value="bb" type="radio" name="ninjaTurtles"/>
<input id="dodoBird" value="cc" type="radio" name="ninjaTurtles" checked/>
</form>
value="cc" is checked by default.
document.forms["lizardPeople"]["dinosaurs"].checked=true;
now value="aa" with id="dinosaurs" is checked, just like before.
See it in action: http://jsfiddle.net/jPfXS/

Vanilla Javascript:
yourRadioButton.checked = true;
jQuery:
$('input[name=foo]').prop('checked', true);
or
$("input:checkbox").val() == "true"

You can also explicitly set value of radio button:
<form name="gendersForm">
<input type="radio" name="gender" value="M" /> Man
<input type="radio" name="gender" value="F" /> Woman
</form>
with the following script:
document.gendersForm.gender.value="F";
and corresponding radio button will be checked automatically.
Look at the example on JSFiddle.

/**
* setCheckedValueOfRadioButtonGroup
* #param {html input type=radio} vRadioObj
* #param {the radiobutton with this value will be checked} vValue
*/
function setCheckedValueOfRadioButtonGroup(vRadioObj, vValue) {
var radios = document.getElementsByName(vRadioObj.name);
for (var j = 0; j < radios.length; j++) {
if (radios[j].value == vValue) {
radios[j].checked = true;
break;
}
}
}

Try
myRadio.checked=true
<input type="radio" id="myRadio">My radio<br>

$("#id_of_radiobutton").prop("checked", true);

I am configuring a radio button within a document fragment and tried using radiobtn.checked = true;.
That didn't work so I instead went with this solution:
radiobtn.setAttribute("checked", "checked");

This sets checked using name to cycle through the elements and a value check to set the desired element to true. I kept it as simple as possible, its pretty easy to put it into a function or a loop, etc.
variable 'nameValue' is the radio elements name value
variable 'value' when matched sets the radio button
Array.from( document.querySelectorAll('[name="' + nameValue + '"]') ).forEach((element,index) =>
{
if (value === element.value) {
element.checked = true;
} else {
element.checked = false;
}
});