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Javascript Interview Questions and Answers



How to Handle Event Handlers?
You can add an event handler in the HTML definition of the element like this,
<script type="text/javascript"><!--
function hitme() {
alert("I've been hit!");
}
// -->
</script>
<input type="button" id="hitme" name="hitme" value="hit me" onclick="hitme()"

Or, interestingly enough you can just assign the event's name on the object directly with a reference to the method you want to assign.

<input type="button" id="hitme2" name="hitme2" value="hit me2"/>
<script type="text/javascript"><!--
function hitme2() {
alert("I've been hit too!");
}
document.getElementById("hitme2").onclick = hitme2;
// -->
</script>

You can also use an anonymous method like this:

document.getElementById("hitme3").onclick = function () { alert("howdy!"); }

You can also use the W3C addEvventListener() method, but it does not work in IE yet:

<input type="button" id="hitme4" name="hitme4" value="hit me4"/>
<script type="text/javascript"><!--
function hitme4() {
alert("I've been hit four!");
}
if(document.getElementById("hitme4").addEventListener) {
document.getElementById("hitme4").addEventListener("click", hitme4, false);
}
// -->
</script>

How to remove the event listener: ?
<script type="text/javascript"><!--
document.getElementById("hitme4").removeEventListener("click", hitme4, false);
// -->
</script>

Key Events

"onkeydown", "onkeypress", "onkeyup" events are supported both in ie and standards-based browsers.

<script type="text/javascript">
function setStatus(name,evt) {
evt = (evt) ? evt : ((event) ? event : null); /* ie or standard? */
var charCode = evt.charCode;
var status = document.getElementById("keyteststatus");
var text = name +": "+evt.keyCode;
status.innerHTML = text;
status.textContent = text;
}
</script>
<form action="">
<input type="text" name="keytest" size="1" value=""
onkeyup="setStatus('keyup',event)"
onkeydown="setStatus('keydown',event)"
/>
<p id="keyteststatus">status</p>
</form>

How to make elements invisible ?
Change the "visibility" attribute of the style object associated with your element. Remember that a hidden element still takes up space, use "display" to make the space disappear as well.

if ( x == y) {
myElement.style.visibility = 'visible';
} else {
myElement.style.visibility = 'hidden';
}

How to set the cursor to wait ?
In theory, we should cache the current state of the cursor and then put it back to its original state.
document.body.style.cursor = 'wait';
//do something interesting and time consuming
document.body.style.cursor = 'auto';

How to reload the current page ?
window.location.reload(true);

how to force a page to go to another page using JavaScript ?
<script language="JavaScript" type="text/javascript" ><!-- location.href="http://newhost/newpath/newfile.html"; //--></script>

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