Can someone explain this JavaScript mystery?
Question
I was writing a simple recursive function in JavaScript and encountered some really weird behavior. At first I thought it's bug in a browser, but I tried it in FireFox, Chrome and IE9 and they all behave exactly the same way.
The HTML file below runs a simple JS function on page load. The function is recursive (calling itself exactly once). Essentially the function creates a new Array object and returns it. The weird thing is that after the function calls itself recursively, x and y reference to the same object, which as far as I understand it should not happen. Also if you uncomment the last line before return x , the alert "x == y" alert is not shown.
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title>JavaScript weirdness...</title>
<script type="text/javascript" language="javascript">
function RecursiveF(n) {
x = [ n ];
if (n > 0) {
y = RecursiveF(n - 1);
if (x == y)
alert('x == y');
}
//if (n == 0) return [ n ];
return x;
}
</script>
</head><body onload="javascript:RecursiveF(1);"></body></html>
Any hints to why "x == y" alert appears in this page?
3 answers
solution1
7 2010-10-07 05:34:14
x and y are by default global variables shared by all recursive calls of the function. If you want them to be local, declare them with the var keyword.
solution2
1 2010-10-07 05:33:22
您需要一个局部变量。
var x = [ n ];
solution3
1 2010-10-07 08:50:55
As the guys before said the problem is, that x and y are not local variables but global. Why is this causing a conflict right now?
What JavaScript does when it executes your function is pulling out x and y to the function that calls you function and declares them there. So now x and y are on the same scope. So at the point where you write:
y = RecursiveF(n - 1)
what will happen in the end is, that
return x;
is executed and so the value of x will be assigned to y resulting in
x == y
There you go. By declaring x and y as local variables via the var statement you can overcome this. This way you get new representations of them with each function call rather then overwriting them.
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