Javascript variable scope in addEventListener anonymous function
Question
When clicking on each div it should alert '1' if div 1 was clicked on or '5' if div 2 was clicked on. I have tried to make this code as easy to as possible because this is needed in a much larger application.
<html>
<head>
<style type="text/css">
#div1 { background-color: #00ff00; margin: 10px; padding: 10px; }
#div2 { background-color: #0000ff; margin: 10px; padding: 10px; }
</style>
<script type="text/javascript">
function init()
{
var total = 1;
var div1 = document.getElementById('div1'),
div2 = document.getElementById('div2');
var helper = function(event, id)
{
if (event.stopPropagation) event.stopPropagation();
if (event.preventDefault) event.preventDefault();
alert('id='+id);
}
div1.addEventListener('click', function(event) { helper(event, total); }, false);
total += 4;
div2.addEventListener('click', function(event) { helper(event, total); }, false);
}
</script>
</head>
<body onload="init();">
<div id="div1">1</div>
<div id="div2">2</div>
</body>
</html>
Thanks for your help! :-)
2 answers
solution1
8 ACCPTED 2009-11-22 20:26:24
The problem is that the event listeners and 'total' both exist in the same scope (init())
The event functions are always going to reference total within the init() scope, even if it is changed after the event functions are declared
To get around this, the event functions need to have a 'total' in their own scope which will not change. You can add another layer of scope using an anonymous function
For example:
(function (total) {
div1.addEventListener('click', function(event) { helper(event, total); }, false);
}(total));
total += 4;
(function (total) {
div2.addEventListener('click', function(event) { helper(event, total); }, false);
}(total));
The anonymous functions are passed init()'s current 'total' value as a parameter. This sets another 'total' to the anonymous function's scope, so it does not matter if init()'s total changes or not, because the event function will FIRST reference the anonymous function's scope.
Also, you need to place a semicolon after the closing brace of the helper function, otherwise the script will complain that 'event' is undefined.
var helper = function(event, id)
{
if (event.stopPropagation) event.stopPropagation();
if (event.preventDefault) event.preventDefault();
alert('id='+id);
};
solution2
2 2009-11-22 21:19:09
This is almost the same but i think it will be better:
div1.addEventListener('click', function(t){ return function(event) { helper(event, t); }}(total), false);
instead of:
(function (total) {
div2.addEventListener('click', function(event) { helper(event, total); }, false);
}(total));
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