Unexpected output when iterating through an array with a for( ... in ...) loop

Question

<html>
<head>
<script type="text/javascript">


Array.prototype.getUnique = function () {
    var o = new Object();
    var i, e;
    for (i = 0; e = this[i]; i++) {o[e] = 1};
    var a = new Array();
    for (e in o) {a.push (e)};
    return a;
}

function Perform(){
    var arr = new Array();

    arr[0] = "hello";
    arr[1] = "world";

    for(i in arr){
        console.log(arr[i]);
    }
}

</script>
</head>
<body onload="Perform()">
</body>
</html>

The result of above code in console is

hello
world
function()

From where does the last function() come from?

Stephen Chung · Accepted Answer · 2011-04-08 06:20:14Z

4

for ... in in JavaScript DOES NOT ITERATE AN ARRAY. Forget your other programming languages -- this is JavaScript, the language with similar syntax that means completely different things.

for ... in iterates through all the properties of the object, including all propertes in its prototype chain.

Therefore, always use hasOwnProperty to check whether the property is defined on the object itself:

for (var name in obj) {
  if (obj.hasOwnProperty(name)) {
    doSomething(obj[name]);
  }
}

You should not use for ... in to iterate an array. Use an index instead.

Search SO for hundreds of similar questions on this topic.

answered Apr 8, 2011 at 6:20

Stephen Chung

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David Tang · Accepted Answer · 2011-04-08 06:15:52Z

2

The function() is coming from the prototype - it is the getUnique function.

To avoid this, you shouldn't be iterating through arrays with a for .. in loop. Use the plain old:

for (var i = 0; i < arr.length; i++)

In cases where you must use for .. in (for example, iterating through keys of an object), you should always check hasOwnProperty():

for (var i in arr) {
    if (arr.hasOwnProperty(i)) {
        console.log(arr[i]);
    }
}

answered Apr 8, 2011 at 6:15

David Tang

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