What's the scope of a Javascript variable declared in a for() loop?
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by Dylan Beattie
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Published on 2010-04-28T21:57:14Z
Indexed on
2010/04/28
22:07 UTC
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Check out the following snippet of HTML/Javascript code:
<html>
<head>
<script type="text/javascript">
var alerts = [];
for(var i = 0; i < 3; i++) {
alerts.push(function() { document.write(i + ', '); });
}
for (var j = 0; j < 3; j++) {
(alerts[j])();
}
for (var i = 0; i < 3; i++) {
(alerts[i])();
}
</script>
</head><body></body></html>
This outputs:
3, 3, 3, 0, 1, 2
which isn't what I was expecting - I was expecting the output 0, 1, 2, 0, 1, 2,
I (incorrectly) assumed that the anonymous function being pushed into the array would behave as a closure, capturing the value of i that's assigned when the function is created - but it actually appears that i is behaving as a global variable.
Can anyone explain what's happening to the scope of i in this code example, and why the anonymous function isn't capturing its value?
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