Edit: This answer is hopelessly out-of-date. For a more modern approach, look at the methods available on an array. Methods of interest might be:

• forEach
• map
• filter
• zip
• reduce
• every
• some

The standard way to iterate an array in JavaScript is a vanilla for-loop:

var length = arr.length,
    element = null;
for (var i = 0; i < length; i++) {
  element = arr[i];
  // Do something with element i.
}

Note, however, that this approach is only good if you have a dense array, and each index is occupied by an element. If the array is sparse, then you can run into performance problems with this approach, since you will iterate over a lot of indices that do not really exist in the array. In this case, a for .. in-loop might be a better idea. However, you must use the appropriate safeguards to ensure that only the desired properties of the array (that is, the array elements) are acted upon, since the for..in-loop will also be enumerated in legacy browsers, or if the additional properties are defined as enumerable.

In ECMAScript 5 there will be a forEach method on the array prototype, but it is not supported in legacy browsers. So to be able to use it consistently you must either have an environment that supports it (for example, Node.js for server side JavaScript), or use a "Polyfill". The Polyfill for this functionality is, however, trivial and since it makes the code easier to read, it is a good polyfill to include.

If you’re using the jQuery library, you can use jQuery.each:

$.each(yourArray, function(index, value) {
  // do your stuff here
});

EDIT : As per question, user want code in javascript instead of jquery so the edit is

var length = yourArray.length;   
for (var i = 0; i < length; i++) {
  // Do something with yourArray[i].
}

Some C-style languages use foreach to loop through enumerations. In JavaScript this is done with the for..in loop structure:

var index,
    value;
for (index in obj) {
    value = obj[index];
}

There is a catch. for..in will loop through each of the object's enumerable members, and the members on its prototype. To avoid reading values that are inherited through the object's prototype, simply check if the property belongs to the object:

for (i in obj) {
    if (obj.hasOwnProperty(i)) {
        //do stuff
    }
}

Additionally, ECMAScript 5 has added a forEach method to Array.prototype which can be used to enumerate over an array using a calback (the polyfill is in the docs so you can still use it for older browsers):

arr.forEach(function (val, index, theArray) {
    //do stuff
});

It's important to note that Array.prototype.forEach doesn't break when the callback returns false. jQuery and Underscore.js provide their own variations on each to provide loops that can be short-circuited.

Loop backwards

I think the reverse for loop deserves a mention here:

for (var i=array.length; i--; ) {
     // process array[i]
}

Advantages:

• You do not need to declare a temporary len variable, or compare against array.length on each iteration, either of which might be a minute optimisation.
• Removing siblings from the DOM in reverse order is usually more efficient. (The browser needs to do less shifting of elements in its internal arrays.)
• If you modify the array while looping, at or after index i (for example you remove or insert an item at array[i]), then a forward loop would skip the item that shifted left into position i, or re-process the ith item that was shifted right. In a traditional for loop, you could update i to point to the next item that needs processing - 1, but simply reversing the direction of iteration is often a simpler and more elegant solution.
• Similarly, when modifying or removing nested DOM elements, processing in reverse can circumvent errors. For example, consider modifying the innerHTML of a parent node before handling its children. By the time the child node is reached it will be detached from the DOM, having been replaced by a newly created child when the parent's innerHTML was written.
• It is shorter to type, and read, than some of the other options available. Although it loses to forEach() and to ES6's for ... of.

Disadvantages:

• It processes the items in reverse order. If you were building a new array from the results, or printing things on screen, naturally the output will be reversed with respect to the original order.
• Repeatedly inserting siblings into the DOM as a first child in order to retain their order is less efficient. (The browser would keep having to shift things right.) To create DOM nodes efficiently and in order, just loop forwards and append as normal (and also use a "document fragment").
• The reverse loop is confusing to junior developers. (You may consider that an advantage, depending on your outlook.)

Should I always use it?

Some developers use the reverse for loop by default, unless there is a good reason to loop forwards.

Although the performance gains are usually insignificant, it sort of screams:

"Just do this to every item in the list, I don't care about the order!"

However in practice that is not actually a reliable indication of intent, since it is indistinguishable from those occasions when you do care about the order, and really do need to loop in reverse. So in fact another construct would be needed to accurately express the "don't care" intent, something currently unavailable in most languages, including ECMAScript, but which could be called, for example, forEachUnordered().

If order doesn't matter, and efficiency is a concern (in the innermost loop of a game or animation engine), then it may be acceptable to use the reverse for loop as your go-to pattern. Just remember that seeing a reverse for loop in existing code does not necessarily mean that the order irrelevant!

It is better to use forEach()

In general for higher level code where clarity and safety are greater concerns, I would recommend using Array::forEach as your default pattern:

• It is clear to read.
• It indicates that i is not going to be shifted within the block (which is always a possible surprise hiding in long for and while loops.)
• It gives you a free scope for closures.
• It reduces leakage of local variables and accidental collision with (and mutation of) outer variables.
• But note that unlike map, forEach does not exclude the possibility that the loop will break out part-way through. This is a feature that can be triggered by returning false from the callback.

Then when you do see the reverse for loop in your code, that is a hint that it is reversed for a good reason (perhaps one of the reasons described above). And seeing a traditional forward for loop may indicate that shifting can take place.

(If the discussion of intent makes no sense to you, then you and your code may benefit from watching Crockford's lecture on Programming Style & Your Brain.)

How does it work?

for (var i=0; i<array.length; i++) { ... }   // Forwards

for (var i=array.length; i--; )    { ... }   // Reverse

You will notice that i-- is the middle clause (where we usually see a comparison) and the last clause is empty (where we usually see i++). That means that i-- is also used as the condition for continuation. Crucially, it is executed and checked before each iteration.

• How can it start at array.length without exploding?

  Because i-- runs before each iteration, on the first iteration we will actually be accessing the item at array.length-1 which avoids any issues with Array-out-of-bounds undefined items.

• Why doesn't it stop iterating before index 0?

  The loop will stop iterating when the condition i-- evaluates to a falsey value (when it yields 0).

  The trick is that unlike --i, the trailing i-- operator decrements i but yields the value before the decrement. Your console can demonstrate this:

  > var i = 5; [i, i--, i];

  [5, 5, 4]

  So on the final iteration, i was previously 1 and the i-- expression changes it to 0 but actually yields 1 (truthy), and so the condition passes. On the next iteration i-- changes i to -1 but yields 0 (falsey), causing execution to immediately drop out of the bottom of the loop.

  In the traditional forwards for loop, i++ and ++i are interchangeable (as Douglas Crockford points out). However in the reverse for loop, because our decrement is also our condition expression, we must stick with i-- if we want to process the item at index 0.

Trivia

Some people like to draw a little arrow in the reverse for loop, and end with a wink:

for (var i=array.length; i-->0 ;) {

Credits go to WYL for showing me the benefits and horrors of the reverse for loop.

A forEach implementation (see in jsFiddle):

function forEach(list,callback) {
  var length = list.length;
  for (var n = 0; n < length; n++) {
    callback.call(list[n]);
  }
}

var myArray = ['hello','world'];

forEach(
  myArray,
  function(){
    alert(this); // do something
  }
);

If you don't mind emptying the array:

var x;

while(x = y.pop()){ 

    alert(x); //do something 

}

x will contain the last value of y and it will be removed from the array. You can also use shift() which will give and remove the first item from y.

If you want to loop over an array, use the standard three-part for loop.

for (var i = 0; i < myArray.length; i++) {
    var arrayItem = myArray[i];
}

You can get some performance optimisations by caching myArray.length or iterating over it backwards.

There are three implementations of foreach in jQuery as follows.

var a = [3,2];

$(a).each(function(){console.log(this.valueOf())}); //Method 1
$.each(a, function(){console.log(this.valueOf())}); //Method 2
$.each($(a), function(){console.log(this.valueOf())}); //Method 3

An easy solution now would be to use the underscore.js library. It's providing many useful tools, such as each and will automatically delegate the job to the native forEach if available.

A CodePen example of how it works is:

var arr = ["elemA", "elemB", "elemC"];
_.each(arr, function(elem, index, ar)
{
...
});

See also

• Documentation for native Array.prototype.forEach().
• In for_each...in (MDN) it is explained that for each (variable in object) is deprecated as the part of ECMA-357 (EAX) standard.
• for...of (MDN) describes the next way of iterating using for (variable of object) as the part of the Harmony (ECMAScript 6) proposal.

Probably the for(i = 0; i < array.length; i++) loop is not the best choice. Why? If you have this:

var array = new Array();
array[1] = "Hello";
array[7] = "World";
array[11] = "!";

The method will call from array[0] to array[2]. First, this will first reference variables you don't even have, second you would not have the variables in the array, and third this will make the code bolder. Look here, it's what I use:

for(var i in array){
    var el = array[i];
    //If you want 'i' to be INT just put parseInt(i)
    //Do something with el
}

And if you want it to be a function, you can do this:

function foreach(array, call){
    for(var i in array){
        call(array[i]);
    }
}

If you want to break, a little more logic:

function foreach(array, call){
    for(var i in array){
        if(call(array[i]) == false){
            break;
        }
    }
}

Example:

foreach(array, function(el){
    if(el != "!"){
        console.log(el);
    } else {
        console.log(el+"!!");
    }
});

It returns:

//Hello
//World
//!!!

I know this is an old post, and there are so many great answers already. For a little more completeness I figured I'd throw in another one using AngularJS. Of course, this only applies if you're using Angular, obviously, nonetheless I'd like to put it anyway.

angular.forEach takes 2 arguments and an optional third argument. The first argument is the object (array) to iterate over, the second argument is the iterator function, and the optional third argument is the object context (basically referred to inside the loop as 'this'.

There are different ways to use the forEach loop of angular. The simplest and probably most used is

var temp = [1, 2, 3];
angular.forEach(temp, function(item) {
    //item will be each element in the array
    //do something
});

Another way that is useful for copying items from one array to another is

var temp = [1, 2, 3];
var temp2 = [];
angular.forEach(temp, function(item) {
    this.push(item); //"this" refers to the array passed into the optional third parameter so, in this case, temp2.
}, temp2);

Though, you don't have to do that, you can simply do the following and it's equivalent to the previous example:

angular.forEach(temp, function(item) {
    temp2.push(item);
});

Now there are pros and cons of using the angular.forEach function as opposed to the built in vanilla-flavored for loop.

Pros

• Easy readability
• Easy writability
• If available, angular.forEach will use the ES5 forEach loop. Now, I will get to efficientcy in the cons section, as the forEach loops are much slower than the for loops. I mention this as a pro because it's nice to be consistent and standardized.

Consider the following 2 nested loops, which do exactly the same thing. Let's say that we have 2 arrays of objects and each object contains an array of results, each of which has a Value property that's a string (or whatever). And let's say we need to iterate over each of the results and if they're equal then perform some action:

angular.forEach(obj1.results, function(result1) {
    angular.forEach(obj2.results, function(result2) {
        if (result1.Value === result2.Value) {
            //do something
        }
    });
});

//exact same with a for loop
for (var i = 0; i < obj1.results.length; i++) {
    for (var j = 0; j < obj2.results.length; j++) {
        if (obj1.results[i].Value === obj2.results[j].Value) {
            //do something
        }
    }
}

Granted this is a very simple hypothetical example, but I've written triple embedded for loops using the second approach and it was very hard to read, and write for that matter.

Cons

• Efficiency. angular.forEach, and the native forEach, for that matter, are both so much slower than the normal for loop....about 90% slower. So for large data sets, best to stick to the native for loop.
• No break, continue, or return support. continue is actually supported by "accident", to continue in an angular.forEach you simple put a return; statement in the function like angular.forEach(array, function(item) { if (someConditionIsTrue) return; }); which will cause it to continue out of the function for that iteration. This is also due to the fact that the native forEach does not support break or continue either.

I'm sure there's various other pros and cons as well, and please feel free to add any that you see fit. I feel that, bottom line, if you need efficiency, stick with just the native for loop for your looping needs. But, if your datasets are smaller and a some efficiency is okay to give up in exchange for readability and writability, then by all means throw an angular.forEach in that bad boy.

var i, len;
for( i=0, len=array.length ; i<len ; i++){
    // here, array[i] is the current element
}

There isn't any for each loop in native JavaScript. You can either use libraries to get this functionality (I recommend Underscore.js), use a simple for in loop.

for (var instance in objects) {
   ...
}

However, note that there may be reasons to use an even simpler for loop (see Stack Overflow question Why is using “for…in” with array iteration such a bad idea?)

var instance;
for (var i=0; i < objects.length; i++) {
    var instance = objects[i];
    ...
}

This is an iterator for NON-sparse list where the index starts at 0, which is the typical scenario when dealing with document.getElementsByTagName or document.querySelectorAll)

function each( fn, data ) {

    if(typeof fn == 'string')
        eval('fn = function(data, i){' + fn + '}');

    for(var i=0, L=this.length; i < L; i++) 
        fn.call( this[i], data, i );   

    return this;
}

Array.prototype.each = each;  

Examples of usage:

Example #1

var arr=[];
[1,2,3].each( function(a){ a.push( this * this},arr);

arr is now = [1, 4, 9]

Example #2

each.call(document.getElementsByTagName('p'), "this.className = data;",'blue');

Each p tag gets class="blue"

Example #3

each.call(document.getElementsByTagName('p'), 
    "if( i % 2 == 0) this.className = data;",
    'red'
);

Every other p tag gets class="red">

Example #4

each.call(document.querySelectorAll('p.blue'), 
    function(newClass, i) {
        if( i < 20 )
            this.className = newClass;
    },'green'
);

And finally, the first 20 blue p tags are changed to green

Caution when using string as function: the function is created out-of-context and ought to be used only where you are certain of variable scoping. Otherwise, better to pass functions where scoping is more intuitive.

Internet Explorer 8 and below don't support forEach. Here's a polyfill for non supporting browsers;

if ( !Array.prototype.forEach ) {
  Array.prototype.forEach = function(fn, scope) {
    for(var i = 0, len = this.length; i < len; ++i) {
      if (i in this) {
        fn.call(scope, this[i], i, this);
      }
    }
  };
}

It is taken from Mozilla's documentation.

I think that the best and standard way is to use the for loop, and I like the short way to write it:

var list = ["a", "b", "c"];
for (var i = 0, letter; letter=list[i]; i++) {
     alert(letter) ;
}

ECMAScript 6 will maybe contain the "for...of" construct. See for...of (MDN) for more information. You can already try it out with recent Firefox versions.

jQuery way using $.map:

var data = [1, 2, 3, 4, 5, 6, 7];

var newData = $.map(data, function(element) {
    if (element % 2 == 0) {
        return element;
    }
});

// newData = [2, 4, 6];

Use:

$.each([ 52, 97 ], function( index, value ) {
    alert( index + ": " + value );
});

where [ 52, 97 ] is your array.

This produces two messages:

0: 52
1: 97

$.each() is essentially a drop-in replacement of a traditional for or for-in loop.

Example

// **Traditional JavaScript code**

var sum = 0;
var arr = [ 1, 2, 3, 4, 5 ];

for ( var i = 0, l = arr.length; i < l; i++ ) {
    sum += arr[ i ];
}
console.log( sum ); // 15

It can be replaced with this using jQuery:

$.each( arr, function( index, value ){
    sum += value;
});
console.log( sum ); // 15

Using JavaScript

var myStringArray = ["Hello","World"];
var arrayLength = myStringArray.length;
for (var i = 0; i < arrayLength; i++) {
    alert(myStringArray[i]);
    //Do something
}

There's no built-in ability to break in forEach. To interrupt execution use of Array#some:

[1,2,3].some(function(number) {
    return number === 1;
});

This works because some returns true as soon as any of the callbacks, executed in array order, return true, short-circuiting the execution of the rest. Original Answer see Arrary prototype for some

One more thing about array.forEach. You can get not only index, but also the iterated object with forEach, if you have array of objects:

var fruits = [{'name': 'banana'}, {'name': 'apple'}, {'name': 'pear'}, {'name': 'watermelon'}];

fruits.forEach(function(fruit, i) {
  console.log('name of fruit no. ' + i + ' is: ' + fruit.name);
});

This code list array value using forEach loop.

<script>
(function(){
var names = new Array();
    names[0]='sunny';
    names[1]= 'Raj';
    names[2]='Reena'

    var showList = function(element, index, array){
        console.log('Name[' + index + '] = ' + element);
    }

    names.forEach(showList );
})();
</script>

I also would like to add this as a composition of reverse loop and an answer above for someone that would like this syntax too.

var foo = [object,object,object];
for(var i = foo.length , item ; item = foo[--i] ;){
   console.log(item);
}

Pros : The benefit for this , you have the reference already in the first like that wont need to declared later with another line. Handy when looping trough the object array.

Cons: This will break whenever reference is false - falsey (undefined etc). Can be used as an advantage though , however it would make it to read a little bit harder