How to prevent default event handling in an onclick method?

Try this (but please use buttons for such cases if you don't have a valid href value for graceful degradation)

<a href="#" onclick="callmymethod(24); return false;">Call</a>

In my opinion the answer is wrong! He asked for event.preventDefault(); when you simply return false; it calls event.preventDefault(); AND event.stopPropagation(); as well!

You can solve it by this:

<a href="#" onclick="callmymethod(event, 24)">Call</a>

function callmymethod(e, myVal){
    //doing custom things with myVal

    //here I want to prevent default
    e = e || window.event;
    e.preventDefault();
}

Let your callback return false and pass that on to the onclick handler:

<a href="#" onclick="return callmymethod(24)">Call</a>

function callmymethod(myVal){
    //doing custom things with myVal
    //here I want to prevent default
    return false;
}

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To create maintainable code, however, you should abstain from using "inline Javascript" (i.e.: code that's directly within an element's tag) and modify an element's behavior via an included Javascript source file (it's called unobtrusive Javascript).

The mark-up:

<a href="#" id="myAnchor">Call</a>

The code (separate file):

// Code example using Prototype JS API
$('myAnchor').observe('click', function(event) {
    Event.stop(event); // suppress default click behavior, cancel the event
    /* your onclick code goes here */
});

You can catch the event and then block it with preventDefault() -- works with pure Javascript

document.getElementById("xyz").addEventListener('click', function(event){
    event.preventDefault();
    console.log(this.getAttribute("href"));
    /* Do some other things*/
});