JavaScript - onclick event getting called automati

Question

I wrote a javascript class called "MyClass" in which I've defined a method "closeThis"

MyClass = function(){

 this.closeThis = function(){
      document.getElementById("hidePane").style.display = 'none';
 }

}

Now, in my html, i'm trying to call that as follows...

<script type="text/javascript">
     function callThis(){
        var myclassObj = new MyClass();
        document.getElementById("closeButton").onclick = myclassObj.closeThis();
     }
</script>

The above callThis will be called when I clicked on a button. The problem here is, "onclick" event on top of "clsoeButtion" is getting called automatically when page loads. What could be wrong in this?

Answer 1

You need to remove () from it otherwise it gets called immediately because that's how you call a function by suffixing (), so simply remove these braces:

document.getElementById("closeButton").onclick = myclassObj.closeThis;

Answer 2

You're calling the function right away.

When you leave the parentheses on the function reference, what you're basically saying is:

Evaluate the closeThis function and assign the result to onclick

when what you really want to do is assign the function reference to the click handler:

document.getElementById("closeButton").onclick = myclassObj.closeThis;

Leave out the parentheses instead, and you'll bind the closeThis function to the onclick. What this instead says is:

Assign the function closeThis to the click handler.

You are essentially assigning the function to the variable as a first-class object, or a reference to a function.

As an aside, my personal preference is to always use an anonymous function wrapper. Sometimes you need to be able to pass parameters into your function, and this makes sure that you can more easily do so:

document.getElementById("closeButton").onclick = 
    function() {
        myclassObj.closeThis();
    };

Answer 3

it should be document.getElementById("closeButton").onclick = myclassObj.closeThis; not myclassObj.closeThis();

myclassObj.closeThis() will call the function then assign value to onclick