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how to change value of html element by classname using javascript

Here is the code i am using to change value of html element ***
<a class="classname" href="Vtech.com"> This text to be chnage</a>
<script type="text/javascript">
document.getElementsByClassName("classname")[0].innerHTML = "aaaaaaqwerty";
</script>
how can I change this text on page load instans

Seems you need to add DOMContentLoaded or put your script before </body>
Native JavaScript solution
document.addEventListener("DOMContentLoaded", function(event) {
document.getElementsByClassName("classname")[0].innerHTML = "qwerty";
});
Add your script before </body>
Version with jQuery
$(funtion(){
$(".classname:first").text("qwerty");
});

You can use css selector, but it can be not safe, because this method return first occurance:
document.querySelector(".classname");
By the way, almost all developers use some js framework: jQuery, prototype, extJs, etc

$(document).ready(funtion(){
$(".classname").text("aaaaaaqwerty");
});

Using jquery (I refered to jquery, since you have tagged it in your question), you could achieve this like below:
$(funtion(){
$("a .classname")[0].text("aaaaaaqwerty");
});

You could use this
document.addEventListener("DOMContentLoaded", function(event) {
document.getElementsByClassName("classname")[0].innerHTML = "some texts";
});
Using jQuery
$(document).ready(function(){
$(".classname").eq(0).val("aaaaaaqwerty");
});

Auto click a link by class name using javascript or jquery?

I am trying to auto click a link using a class name instead of the ID name.
however my approach doesn't do anything!
Here is what I have done:
<script type="text/javascript">
$(document).ready(function(){
document.getElementsByClassName("some-iclass").click();
});
</script>
Could someone point me in the right direction please?
EDIT:
I've used the following code and still doesn't work:
<script type="text/javascript">
$(document).ready(function(){
$(".myLink").click();
});
</script>
<a class="myLink" href="http://yahoo.com"> CLICK HERE NOW </a>
and I have this right at the top of my page header:
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/jquery-ui.min.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
EIDT:
i've tried this as well and still doesn't work:
<script type="text/javascript">
$(document).ready(function(){
$('.myLink').trigger('click');
});
</script>

here you go:
<script type="text/javascript">
$(function(){
$('.className').trigger('click');
});
</script>
hope that helps.
UPDATE:
try:
<script type="text/javascript">
$(function(){
window.location.href = $('.className').attr('href');
});
</script>
after your edit, i think this is what you need.

getElementsByClassName doesn't return an element but a NodeList which may contain more than one element.
You may do this :
document.getElementsByClassName("some-iclass")[0].click();
or if you want to click all elements :
var list = document.getElementsByClassName("some-iclass");
for (var i=0; i<list.length; i++) list[i].click();
But as you use jQuery, it would be simpler to do
$('.some-iclass').click();
but only when the click event handler was added with jQuery (in other cases, like for example in case of an href attribute, use the standard dom functions).

$(document).ready(function(){
$(".some-iclass").trigger('click');
});

Simple with jquery $(".some-iclass").click();
if you have a lot of elements with this class - point to the wanted element:
i.e. $($(".some-iclass")[0]).click();

for auto-clicking a button or a link
"<"body onload="document.getElementById('some-class')[0].click()" ">"
this works...:)

if you want to autoclick a link and you are using jQuery, you could use
$('.yourClass').click();
if you need this to be one link in a collection of multiple links, you could do this:
$($('.yourClass')[0]).click();
Where 0 is the index of the element in the jQuery object.
document.getElementsByClassName('yourClass'); does not work in older browsers so it's best to use jQuery here for cross-browser compatibility.

For me, I managed to make it work that way. I deployed the automatic click in 5000 milliseconds and then closed the loop after 1000 milliseconds. Then there was only 1 automatic click.
<script>
var myVar = setInterval(function ({document.getElementById("test").click();}, 500);
setInterval(function () {clearInterval(myVar)}, 1000);
</script>

how i can add a id in js on click event

Hey all i am new in java script so please help me out
i have a inputtag so what i want is when some body click on input tag so the #helptext will call automatically by js.
how i can achieve this below the demo code and desired result image....
DEMO
i want like this image :-
please check my updated DEMO bit of change in my markup i need like my current demo

try this :
$(document).ready(function() {
$("a").click(function(event) {
$(this).append(event.target.id);
});
});

$('input').focus(function() {
$('#helptext').show();
});

Here is the working JSFiddle
http://jsfiddle.net/tn98m/9/
$('#myInput').focus(function() {
$('#helptext').show();
});
$('#myInput').blur(function() {
$('#helptext').hide();
});

Can I toggle specific instances of a class that occurs multiple times?

I'm building a working mockup of a site design and I want the ad spaces to disappear to illustrate what the page would look like if they're empty. Can I accomplish this without having the uniquely identify each space? I was hoping this would work:
<script type="text/javascript">
$('.adSpace').click(function() {
$this.toggle('fast');
});
</script>

Use $(this) instead of $this. $this is not defined here.
Live Demo
<script type="text/javascript">
$('.adSpace').click(function() {
$(this).toggle('fast');
});
</script>

$this is not the object. You should use $(this).
this : java-script object
$(this) : converting this to jQuery object
You can use
$('.adSpace').click(function() {
$(this).toggle('fast');
});

JQuery missing : after property ID

So I am working with some jquery code to do a simple hide of a p and a show of a p. I am simple adding a class called showp and showing it while making sure the others are not shown by hiding them first. But I keep getting an error missing : after property ID. Any help would be greatly appreciated.
$(document).ready({
$("#phone").click(function(){
$(".hide2").addClass(".hide2").hide("slow");
$(".hide3").addClass(".hide3").hide("slow");
$(".hide1").addClass("showp").show("slow");
});
});

Change this:
$(document).ready({
To this
$(document).ready(function(){

Try this:
$(document).ready(function() {
$("#phone").click(function() {
$(".hide2, .hide3").hide();
$(".hide1").show();
});
});

Your document ready is invalid.
$(document).ready(function() {
// stuff here.
});
Or better yet, a shorthand:
$(function() {
// stuff here.
});