1

Why isit when I do

http://jsfiddle.net/xe4Ph/1/

var footnoteLinks = [1,2,3];
for (var i in footnoteLinks) {
   document.write(footnoteLinks[i] + ", ");
}

What I get is something like ...

1, 2, 3, function () { return lower; }, function Array() { [native code] }, function pop() { [native code] }, function push() { [native code] }, function reverse() { [native code] }, function shift() { [native code] }, function sort() { [native code] }, function splice() { [native code] }, function unshift()

Why is that? Whats with the functions etc... I think it worked ok b4, I dunno what caused this now, it seems the same for all browsers I tried. Firefox 3.6, Chrome 6 (i think?), IE9

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2
  • 2
    Take a look through the related questions. You're not safe to use for .. in without a hasOwnProperty check - and the old-style loops are very significantly faster than for .. in .. hasOwnProperty. Commented Dec 7, 2010 at 7:11
  • possible duplicate of JavaScript: Looping over array Commented Dec 7, 2010 at 7:35

4 Answers 4

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6

The for(var in obj) is for iterating over the properties of an object. You're getting the properties of the Array object you've created.

You want a more traditional looping/index construct:

for(var i=0,z=footnoteLinks.length; i<z; i++)

Some JavaScript runtimes also have map and reduce methods on Array objects, but this isn't guaranteed. Most JavaScript libraries have something like this (or perhaps an each method), though.

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Comments

1

In javascript, 

for (var k in {a:1, b:2}) {

doesn't just iterate over a and b; it also iterates over all of the members of the prototype of that object. Since functions are values in javascript, that includes all of the methods of Object. There are three ways to avoid this:

  1. Check that obj.hasOwnProperty(k) before you use k
  2. If you're iterating over an array, use the forEach method of array (or equivalent in your favorite javascript library)
  3. If you're iterating over an array, use the for(var i=0; i < obj.length; i++) construct (length counts only the elements of the array)
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0

An alternative to Weston C's answer would be using the hasOwnProperty function in the for in loop, as suggested by Chris Morgan:

for(var i in obj) if(obj.hasOwnProperty(i)) {
  document.write(obj[i]);
}

And yes, it is slower than iterating using the traditional for loop.

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0

The for/in loop cannot be used to iterate over an array, since it also enumerates user-defined properties (see the MDC documentation). There’s a forEach method on array objects and for each/in loop, but only in JavaScript 1.6+ (= not much usable right now). You might want to use some of the many JavaScript frameworks or code your own utility function to iterate over arrays:

function iterate(a, func) {
    for (var i=0; i<a.length; i++)
        func(a[i]);
}

iterate([1, 2], function(x) { console.log(x) });
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1 Comment

It can be used to iterate over an array, albeit usually unwisely as only all numeric non-negative keys are of interest. However, since the order is also not specified... but it can be done, even if it [normally] shouldn't be done.

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