JavaScript for...in vs for

The choice should be based on the which idiom is best understood.

An array is iterated using:

for (var i = 0; i < a.length; i++)
   //do stuff with a[i]

An object being used as an associative array is iterated using:

for (var key in o)
  //do stuff with o[key]

Unless you have earth shattering reasons, stick to the established pattern of usage.

Douglas Crockford recommends in JavaScript: The Good Parts (page 24) to avoid using the for in statement.

If you use for in to loop over property names in an object, the results are not ordered. Worse: You might get unexpected results; it includes members inherited from the prototype chain and the name of methods.

Everything but the properties can be filtered out with .hasOwnProperty. This code sample does what you probably wanted originally:

for (var name in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, name)) {
        // DO STUFF
    }
}

FYI - jQuery Users

jQuery's each(callback) method uses for( ; ; ) loop by default, and will use for( in ) only if the length is undefined.

Therefore, I would say it is safe to assume the correct order when using this function.

Example:

$(['a','b','c']).each(function() {
    alert(this);
});
//Outputs "a" then "b" then "c"

The downside of using this is that if you're doing some non UI logic, your functions will be less portable to other frameworks. The each() function is probably best reserved for use with jQuery selectors and for( ; ; ) might be advisable otherwise.

there are performance differences depending on what kind of loop you use and on what browser.

For instance:

for (var i = myArray.length-1; i >= 0; i--)

is almost twice as fast on some browsers than:

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

However unless your arrays are HUGE or you loop them constantly all are fast enough. I seriously doubt that array looping is a bottleneck in your project (or for any other project for that matter)

I second opinions that you should choose the iteration method according to your need. I would suggest you actually not to ever loop through native Array with for in structure. It is way slower and, as Chase Seibert pointed at the moment ago, not compatible with Prototype framework.

There is an excellent benchmark on different looping styles that you absolutely should take a look at if you work with JavaScript. Do not do early optimizations, but you should keep that stuff somewhere in the back of your head.

I would use for in to get all properties of an object, which is especially useful when debugging your scripts. For example, I like to have this line handy when I explore unfamiliar object:

l = ''; for (m in obj) { l += m + ' => ' + obj[m] + '\n' } console.log(l);

It dumps content of the whole object (together with method bodies) to my Firebug log. Very handy.

Using forEach to skip the prototype chain

Just a quick addendum to @nailer's answer above, using forEach with Object.keys means you can avoid iterating over the prototype chain without having to use hasOwnProperty.

var Base = function () {
    this.coming = "hey";
};

var Sub = function () {
    this.leaving = "bye";
};

Sub.prototype = new Base();
var tst = new Sub();

for (var i in tst) {
    console.log(tst.hasOwnProperty(i) + i + tst[i]);
}

Object.keys(tst).forEach(function (val) {
    console.log(val + tst[val]);
});

I'd use the different methods based on how I wanted to reference the items.

Use foreach if you just want the current item.

Use for if you need an indexer to do relative comparisons. (I.e. how does this compare to the previous/next item?)

I have never noticed a performance difference. I'd wait until having a performance issue before worrying about it.

here is something i did.

function foreach(o, f) {
 for(var i = 0; i < o.length; i++) { // simple for loop
  f(o[i], i); // execute a function and make the obj, objIndex available
 }
}

this is how you would use it
this will work on arrays and objects( such as a list of HTML elements )

foreach(o, function(obj, i) { // for each obj in o
  alert(obj); // obj
  alert(i); // obj index
  /*
    say if you were dealing with an html element may be you have a collection of divs
  */
  if(typeof obj == 'object') { 
   obj.style.marginLeft = '20px';
  }
});

I just made this so I'm open to suggestions :)

With for (var i in myArray) you can loop over objects too, i will contain the key name and you can access the property via myArray[i]. Additionaly, any methods you will have added to the object will be included in the loop, too, i.e., if you use any external framework like jQuery or prototype, or if you add methods to object prototypes directly, at one point i will point to those methods.

For in loops on Arrays is not compatible with Prototype. If you think you might need to use that library in the future, it would make sense to stick to for loops.

http://www.prototypejs.org/api/array

I have seen problems with the "for each" using objects and prototype and arrays

my understanding is that the for each is for properties of objects and NOT arrays