mass add event handlers to objects in array

**axelsword** · Jun 2, 2010 04:51

I am trying to add onclick event handler to many objects but I can't understand why it doesn't work.

I would appreciate nothing too advanced. To assign event handler I use traditional approach as described in http://www.quirksmode.org/js/events_tradmod.html

Heres the code (extract.js):

Code JavaScript:

//the class
function extract(){
	this.iForm=document.getElementById('iNew')
	this.navSel=new Array()
	this.x=0
	this.navSel[this.x]=new Array(3)
	this.navSel[this.x][0]=this.iForm.iVarrantyYears
	this.navSel[this.x][1]=document.getElementById('nSel')
	this.navSel[this.x][2]=document.getElementById('pSel')
	this.x++
	this.navSel[this.x]=new Array(3)
	this.navSel[this.x][0]=this.iForm.bFloors
	this.navSel[this.x][1]=document.getElementById('nSel2')
	this.navSel[this.x][2]=document.getElementById('pSel2')
 
	this.selectNext=function(select){
		if(select.selectedIndex<select.length){select.selectedIndex+=1}
	}
	this.selectPrevious=function(select){
		if(select.selectedIndex>=1){select.selectedIndex-=1}
	}
}
//function to execute onload
function prepare(){
	o=new extract()
	var c=o.navSel.length
	for(i=0;i<c;i++){
		//The following does NOT work
		o.navSel[i][1].onclick=function(){o.selectNext(o.navSel[i][0])}
		o.navSel[i][2].onclick=function(){o.selectPrevious(o.navSel[i][0])}
	}
	//The following approach works
	/*
	o.navSel[0][1].onclick=function(){o.selectNext(o.navSel[0][0])}
	o.navSel[0][2].onclick=function(){o.selectPrevious(o.navSel[0][0])}
	o.navSel[1][1].onclick=function(){o.selectNext(o.navSel[1][0])}
	o.navSel[1][2].onclick=function(){o.selectPrevious(o.navSel[1][0])}
	*/
	//But obviously I want to add event automatically
}

This JavaScript code is used in extract.html (excerpt):

Code HTML4Strict:

<html><head>
<script type="text/javascript" src="extract.js"></script>
</head><body onload="prepare()">
<form action="" method="post" id="iNew">
<p>Years of Varranty: <select name="iVarrantyYears" /></select>
<input type="button" id="nSel" value="+" />
<input type="button" id="pSel" value="-" /></p>
 
<p>Floor count in the building: <select name="bFloors" /></select>
<input type="button" id="nSel2" value="+" />
<input type="button" id="pSel2" value="-" /></p>
</form>
</body></html>

I know that both select tags don't have options, but I generate them with JS because they hold sequential numbers and this part has no impact on the problem at hand.
Both functions help select next or previous index in a given select tag for greater comfort

Thanks!

**paul_wilkins** · Jun 2, 2010 05:29

Inside the attached function, the variable i does not have the value that you think it has.
By the time the event occurs, the variable i will always be the same number, that being the length of the array.

Do not create the function from inside the loop. Instead, pass the variables o and i to a function that returns the appropriate function instead.

Code javascript:

function selectNextHandler(obj, index) {
    return function () {
        obj.selectNext(obj.navSel[index][0]);
    }
}
function selectPreviousHandler(obj, index) {
    return function () {
        obj.selectPrevious(obj.navSel[index][0]);
    }
}
...
for (...) {
    o.navSel[i][1].onclick = selectNextHandler(o, i);
    o.navSel[i][2].onclick = selectPreviousHandler(o, i);
}

Because the variables are now more removed from their context, I have also renamed o and i to be obj and index within the external functions.

**axelsword** · Jun 2, 2010 06:13

Yes, that works! Thank you.

A minor detail remains: how to make this code feel more OOP like. I know JS is not like that but defining two separate functions along the one necessary "prepare()" that is called after onLoad so that all the defined "getElementById" actually work - is there a neat approach? Because those functions have hard-coded variable names anyway - function name "selectNext" and array name "navSel" won't wokr for different objects.

I noticed the scope problem after some time but my quick fix didn't work:
o.navSel[i][1].onclick=function(e,i){o.selectNext(o.navSel[i][0])}
o.navSel[i][2].onclick=function(e,i){o.selectPrevious(o.navSel[i][0])}

Anyway great stuff. It's always hard to get hung up over syntax errors.

**paul_wilkins** · Jun 2, 2010 06:20

You could always do it this way, so that the inner function retains the scope of the outer one.

Code javascript:

o.navSel[i][1].onclick = function(){
    return function () {
        o.selectNext(o.navSel[i][0]);
    }
}(i);
o.navSel[i][2].onclick=function(){
    return function () {
        o.selectPrevious(o.navSel[i][0]);
    }
}(i);

**axelsword** · Jun 4, 2010 13:14

I couldn't get your code to work. Could you demonstrate using function as return value?

Similar example: selecting radio button fires an event
Javascript (extract.js)

Code JavaScript:

//the class
function extract(){
    this.catRadio=document.getElementById('radioCat')
 
	this.before_filter=function(){
		//here, this assignment didn't work for me
		this.catRadio.onclick=function(){
			return function(){
				o.yuba(o.catRadio,'drop')
			}
		}
	}
 
	this.yuba=function(radio,msg){
		alert('value is '+radio.value+'\nmessage reads '+msg)
	}
}
//function to execute onload
function prepare(){
    o=new extract()
	o.before_filter()
}

HTML

Code HTML4Strict:

<html><head>
<script type="text/javascript" src="extract.js"></script>
</head><body onload="prepare()">
<form action="" method="post" id="iNew">
<p><input type="radio" id="radioCat" name="group1" value="category"> <span id="checkCat">Category</span>
 <input type="radio" id="radioSubA" name="group1" value="subA" checked="checked"> <span id="checkSubA">subA</span></p>
</form>
</body></html>

**paul_wilkins** · Jun 4, 2010 15:22

The first of the two function needs to be instantiated, which in my example is the (i) part at the end of the function.

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