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Asked: 2026-05-18T22:27:17+00:00

anyone can explain how to use for…in statement in javascript. I had read the

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anyone can explain how to use for…in statement in javascript. I had read the w3school article but i think it is not so clear.Below is the code, please explain this:

<html>
<body>
<script type="text/javascript">
var x;
var mycars = new Array();
mycars[10] = "Saab";
mycars[20] = "Volvo";
mycars[30] = "BMW";

for (x in mycars)
{
document.write(mycars[x] + "<br />");
}
</script>
</body>
</html>
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1 Answer

  1. Editorial Team

    A for in loop will iterate through every property in an object.

    In your example, the x variable will cycle through every property in the mycars object.

    If you add mycars.expensive = "Porsche";, it will find that too.

    Note that, as stated by MDC, for in loops should not be used to loop through ordinary arrays:

    Although it may be tempting to use
    this as a way to iterate over an Array,
    this is a bad idea. The
    for...in statement
    iterates over user-defined properties
    in addition to the array elements, so
    if you modify the array’s non-integer
    or non-positive properties (e.g. by
    adding a "foo" property
    to it or even by adding a method or
    property to
    Array.prototype), the
    for...in statement will
    return the name of your user-defined
    properties in addition to the numeric
    indexes. Also, because order of
    iteration is arbitrary, iterating over
    an array may not visit elements in
    numeric order. Thus it is better to
    use a traditional for loop with a numeric index when
    iterating over arrays. Similar
    arguments might be used against even
    using for…in at all (at least
    without propertyIsEnumerable()
    or hasOwnProperty()
    checks), since it will also iterate
    over Object.prototype (which, though
    usually discouraged, can, as in the
    case of Array.prototype, be usefully
    extended by the user where are no
    namespacing concerns caused by
    inclusion of other libraries which
    might not perform the above checks on
    such iterations and where they are
    aware of the effect such extension
    will have on their own use of
    iterators such as for…in).

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