Develop Reference

preventDefault property in JS

The book that I am learning JS from has code like below :
function check(e) {
if (!e){
e = window.event; // for IE
}
var target = e.target || e.srcTarget;
if (**e.preventDefault**){
e.preventDefault() ;
}
target.returnValue = false;
}
var el = document.getElementById("list");
el.addEventListener("click", check , false);
I understand that preventDefault is a method and not a property . I did not understand how they are doing a e.preventDefault in the if condition .I checked in chrome and did not find any property called preventDefault for e . There is a function under proto called preventDefault. Am I correct in assuming that all methods can be changed to a property removing the () and you can use it in your code to test if that method is available or not ?

the double asterisks should not be there.
if(e.preventDefault) will work because if checks if the passed argument isn't null. So when preventDefault function will be defined, the condition will be true.

This is just for checking cross browser support. IE 8 or under won't support preventDefault, they use returnValue.
//check preventDefault function is exists in event handler 'e'
if (e.preventDefault) {
// if browser support preventDefault, call preventDefault();
e.preventDefault();
}

Managing JavaScript hotkeys in Chrome Browser

I have the following jQuery code on a site I built:
$(document).ready(function() {
// Other Bindings/Initializations Removed
// Hotkey Event Handler to 'doSomething'
//-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
$(document).keypress("a",function(e) {
if(e.altKey) { // Doesn't work
doSomething();
}
});
// Hotkey Event Handler to 'doSomething'
//-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
$(document).keypress("a",function(e) {
if(e.shiftKey) { //Works as Expected?
doSomething();
}
});
});
The code catches key-press combination events, in this case "Alt-A", and then proceeds to call a function which preforms the appropriate action. I tested this feature in FireFox and the function was called as expected. When I tested the feature in Chrome the function was not called and an obnoxious error tone was emitted instead. I thought that perhaps "Alt-A" collided with an important browser hotkey combination so changed "A" to "N", "G", and then "K"; each time the function was not called and the error tone was emitted. However when I created a Shift-A/N/G/K hotkey combination, Chrome called the function as expected.
Why does Chrome handle the "Alt" key differently?
How to I define a hotkey for my site so that it will work in Chrome using the "Alt" key?

This works in Chrome and Firefox, however in IE Alt+a opens the favorites menu. I'm not sure how you would override that.
Fiddle
HTML:
<a accesskey="a">​
Javascript:
$(document).ready(function() {
// Other Bindings/Initializations Removed
// Hotkey Event Handler to 'doSomething'
//-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
$(document).keypress("a", function(e) {
if (e.shiftKey) { //Works as Expected?
alert("shift a");
}
if (e.altKey) {
alt_a_function();
}
});
$(document).on("click", "[accesskey=a]", function() {
alt_a_function();
});
});
function alt_a_function() {
alert("alt a");
}​

The jQuery docs say that the first argument to .keypress() is "A map of data that will be passed to the event handler." Perhaps jQuery is confused when that object is a string, which causes an error in Chrome.
To check for a particular key in the event handler, use e.which to get the character code instead.

How to register document.onkeypress event

I want to register keypress events for a document using javascript.
I have used:
document.attachEvent("onkeydown", my_onkeydown_handler);
It works fine with IE,
but not with Firefox and Chrome.
I also tried:
document.addEventListener("onkeydown", my_onkeydown_handler, true);
// (with false value also)
But it still doesn't work with Firefox and Chrome.
Is there a solution, am I missing something?

You are looking for:
EDIT:
Javascript:
document.addEventListener("keydown", keyDownTextField, false);
function keyDownTextField(e) {
var keyCode = e.keyCode;
if(keyCode==13) {
alert("You hit the enter key.");
} else {
alert("Oh no you didn't.");
}
}
DEMO: JSFIDDLE

You are probably looking for:
document.body.addEventListener('keydown', function (e) {
alert('hello world');
});​​​​​​​
But it is almost certainly going to be worth your time to use an existing library to abstract over the problems of the many browsers out there.

Please go through following links for detailed description.
https://developer.mozilla.org/en-US/docs/DOM/element.addEventListener?redirectlocale=en-US&redirectslug=DOM%3Aelement.addEventListener
http://www.reloco.com.ar/mozilla/compat.html
In short, write handler as
function myFunction(e)
{
///For IE
if(!e)
e=window.event;
// use e as event in rest of code.
}

Javascript: IE event.preventDefault is null or not an object

My code works fine in FF/Chrome/Everything except IE(failed in both 7/8, didn't bother going furthur down). Due to restrictions, I cannot use jQuery and am hard-coding the Javascript. The issue is with IE, I am getting "preventDefault is null or not an object". Hoping one of you has the answer, and here's relevant code:
AddEvent Method:
function addEvent( obj, type, fn ) {
if ( obj.attachEvent ) {
obj['e'+type+fn] = fn;
obj[type+fn] = function(){obj['e'+type+fn]( window.event );}
obj.attachEvent( 'on'+type, obj[type+fn] );
} else
obj.addEventListener( type, fn, false );
};
Event handler throwing error:
function mousedown(e){
if(e.preventDefault){
e.preventDefault();
} else {
e.returnValue = false;
e.cancelBubble=true;
}
//Processing
};
Also DOCTYPE and charset are both set on the calling HTML page. Any ideas would be appreciated. The error is thrown in the if statement for the mousedown method.
EDIT:
Due to the fact that grabbing window.event did "fix" the main issue, I discovered the problem was a different section of code. Basically I am adding a element ontop of a pre-placed element, and then registering mousedown/mousemove events to that div. When firing the event on the <div>, THAT'S where the error is thrown.
Could this be something due to the fact that I have events registered to 2
rect = document.createElement('div');
rect.className='square';
rect.id='chooser_rectangle';
rect.style.left=initx+'px';
rect.style.top=inity+'px';
addEvent(rect,"mousemove",mousemove);
addEvent(rect,"mousedown",mousedown);

In IE the event object is not passed as the first argument to an event handler. Instead it is a global variable:
function mousedown(e){
var evt = e || window.event; // IE compatibility
if(evt.preventDefault){
evt.preventDefault();
}else{
evt.returnValue = false;
evt.cancelBubble=true;
}
//Processing
};

IE has a different event model to other browsers (even IE8). In IE you would call this to do the same thing:
event.returnValue = false;
You need to determine what the browser supports and call the correct method. So first check if event.returnValue is set, if so then call it, otherwise call preventDefault().

event.preventDefault() function not working in IE

Following is my JavaScript (mootools) code:
$('orderNowForm').addEvent('submit', function (event) {
event.preventDefault();
allFilled = false;
$$(".required").each(function (inp) {
if (inp.getValue() != '') {
allFilled = true;
}
});
if (!allFilled) {
$$(".errormsg").setStyle('display', '');
return;
} else {
$$('.defaultText').each(function (input) {
if (input.getValue() == input.getAttribute('title')) {
input.setAttribute('value', '');
}
});
}
this.send({
onSuccess: function () {
$('page_1_table').setStyle('display', 'none');
$('page_2_table').setStyle('display', 'none');
$('page_3_table').setStyle('display', '');
}
});
});
In all browsers except IE, this works fine. But in IE, this causes an error. I have IE8 so while using its JavaScript debugger, I found out that the event object does not have a preventDefault method which is causing the error and so the form is getting submitted. The method is supported in case of Firefox (which I found out using Firebug).
Any Help?

in IE, you can use
event.returnValue = false;
to achieve the same result.
And in order not to get an error, you can test for the existence of preventDefault:
if(event.preventDefault) event.preventDefault();
You can combine the two with:
event.preventDefault ? event.preventDefault() : (event.returnValue = false);

If you bind the event through mootools' addEvent function your event handler will get a fixed (augmented) event passed as the parameter. It will always contain the preventDefault() method.
Try out this fiddle to see the difference in event binding.
http://jsfiddle.net/pFqrY/8/
// preventDefault always works
$("mootoolsbutton").addEvent('click', function(event) {
alert(typeof(event.preventDefault));
});
// preventDefault missing in IE
<button
id="htmlbutton"
onclick="alert(typeof(event.preventDefault));">
button</button>
For all jQuery users out there you can fix an event when needed. Say that you used HTML onclick=".." and get a IE specific event that lacks preventDefault(), just use this code to get it.
e = $.event.fix(e);
After that e.preventDefault(); works fine.

I know this is quite an old post but I just spent some time trying to make this work in IE8.
It appears that there are some differences in IE8 versions because solutions posted here and in other threads didn't work for me.
Let's say that we have this code:
$('a').on('click', function(event) {
event.preventDefault ? event.preventDefault() : event.returnValue = false;
});
In my IE8 preventDefault() method exists because of jQuery, but is not working (probably because of the point below), so this will fail.
Even if I set returnValue property directly to false:
$('a').on('click', function(event) {
event.returnValue = false;
event.preventDefault();
});
This also won't work, because I just set some property of jQuery custom event object.
Only solution that works for me is to set property returnValue of global variable event like this:
$('a').on('click', function(event) {
if (window.event) {
window.event.returnValue = false;
}
event.preventDefault();
});
Just to make it easier for someone who will try to convince IE8 to work. I hope that IE8 will die horribly in painful death soon.
UPDATE:
As sv_in points out, you could use event.originalEvent to get original event object and set returnValue property in the original one. But I haven't tested it in my IE8 yet.

Mootools redefines preventDefault in Event objects. So your code should work fine on every browser. If it doesn't, then there's a problem with ie8 support in mootools.
Did you test your code on ie6 and/or ie7?
The doc says
Every event added with addEvent gets the mootools method automatically, without the need to manually instance it.
but in case it doesn't, you might want to try
new Event(event).preventDefault();

if (e.preventDefault) {
e.preventDefault();
} else {
e.returnValue = false;
}
Tested on IE 9 and Chrome.

To disable a keyboard key after IE9, use : e.preventDefault();
To disable a regular keyboard key under IE7/8, use : e.returnValue = false; or return false;
If you try to disable a keyboard shortcut (with Ctrl, like Ctrl+F) you need to add those lines :
try {
e.keyCode = 0;
}catch (e) {}
Here is a full example for IE7/8 only :
document.attachEvent("onkeydown", function () {
var e = window.event;
//Ctrl+F or F3
if (e.keyCode === 114 || (e.ctrlKey && e.keyCode === 70)) {
//Prevent for Ctrl+...
try {
e.keyCode = 0;
}catch (e) {}
//prevent default (could also use e.returnValue = false;)
return false;
}
});
Reference : How to disable keyboard shortcuts in IE7 / IE8

Here's a function I've been testing with jquery 1.3.2 and 09-18-2009's nightly build. Let me know your results with it. Everything executes fine on this end in Safari, FF, Opera on OSX. It is exclusively for fixing a problematic IE8 bug, and may have unintended results:
function ie8SafePreventEvent(e) {
if (e.preventDefault) {
e.preventDefault()
} else {
e.stop()
};
e.returnValue = false;
e.stopPropagation();
}
Usage:
$('a').click(function (e) {
// Execute code here
ie8SafePreventEvent(e);
return false;
})

preventDefault is a widespread standard; using an adhoc every time you want to be compliant with old IE versions is cumbersome, better to use a polyfill:
if (typeof Event.prototype.preventDefault === 'undefined') {
Event.prototype.preventDefault = function (e, callback) {
this.returnValue = false;
};
}
This will modify the prototype of the Event and add this function, a great feature of javascript/DOM in general. Now you can use e.preventDefault with no problem.

return false in your listener should work in all browsers.
$('orderNowForm').addEvent('submit', function () {
// your code
return false;
}

FWIW, in case anyone revisits this question later, you might also check what you are handing to your onKeyPress handler function.
I ran into this error when I mistakenly passed onKeyPress(this) instead of onKeyPress(event).
Just something else to check.

I was helped by a method with a function check. This method works in IE8
if(typeof e.preventDefault == 'function'){
e.preventDefault();
} else {
e.returnValue = false;
}