var myArray = ["a", "c", "b"];
myArray.swap(1, 2);
console.log(myArray);
// ["a", "b", "c"];
 
Array.prototype.swap=function(a, b) {
    this[a]= this.splice(b, 1, this[a])[0];
    return this;
}

It could also be done with a temp var, but this solution seems to be the most elegant. The key to this is in the additional arguments the splice() function can accept.

splice(start, itemCount, additionalItems ...)

splice() removes elements denominated by the start and itemCount argument, and – if specified – inserts additional items at the same position.

Also important to note is that splice() returns an array. And to turn that array into an item again we need to select it directly with [0].

<div class="myClass">One Title</div>
<div class="myClass">Another Title</div>
<div class="myClass">Brrrr</div>
<div class="myClass">Zzzzzz</div>

And you want to get a particular element from this list.

var myArray = $(".myClass");
var thirdElement = myArray[2];
console.log(thirdElement);
// <div class="myClass">Brrrr</div>

But note, that myArray[2] is the HTML element, not an jQuery object. If you want to access it via jQuery, you’ll have to “arm” it…

var myArray = $(".myClass");
var thirdElement = $(myArray[2]);
console.log(thirdElement);
// [<div class="myClass">Brrrr</div>]

Interrating over a jQuery selected list can be done with the each() function:

myArray.each(function(i, element){
	var myElement = myArray[i];
	// this
	// element
}

myElement, element and this would all hold the same value at each iteration. i is obviously the index counter.

It’s also described quite clearly at the jQuery Docs, but I think it’s important enough to reiterate.

var oneArray = ["a", "b", "c"];
var anotherArray = oneArray;
anotherArray[0] = "z";
 
console.log(oneArray, anotherArray);
// ["z", "b", "c"], ["z", "b", "c"]

Ok, but let’s say you’ll need to make a copy of the array. How do you do that?

var oneArray = ["a", "b", "c"];
var anotherArray = oneArray.slice(0);
anotherArray[0] = "z";
 
console.log(oneArray, anotherArray);
// ["a", "b", "c"], ["z", "b", "c"]

If you’d really, really want you could also wrap it into a prototype function. Although the code-characters savings are minimal.

Array.prototype.copy = function() {
	return this.slice(0);
}
 
var anotherArray = oneArray.slice(0);
//vs
var anotherArray = oneArray.copy();

Minimal, but maybe contextually for self-explaining.

So far so good.

As in Javascript Arrays also inherit from Object, I could see not reason, why freezing an array should not work.

var obj = {'a':1, 'b:2'};
Object.freeze(obj);
Object.isFrozen(obj);       // returns true
obj.a = 10;                 // new assignment has no affect
obj.a;                      // returns 1
 
var arr = [1, 2];
Object.freeze(arr);
Object.isFrozen(arr);      // returns true
arr[0] = 10;
arr;                       // returns [10, 2] ... ouch!

It turns out, that it seems to be an implementation bug in Safari (5.1). It’s working in the latest Firefox and Chrome releases.

To check whether freezing arrays works in your brower, click the following button:
Check the Freezer!

ps. I asked the question first on Stack Overflow: ‘Freezing’ Arrays in Javascript?. Thanks for the constructive answers.

Object.keys(myObject).length;

More details at Stackoverflow. This functionality is part of EMCAScript 5 specs.

var array = [ 	{a:13, b:false },
		{a:7, b:true },
		{a:78, b:true } ]

The old, ECMAScript 3 way of checking for objects, in which b is true would be a simple for-loop:

var results = [];
for (var i=0; i<array.length; i++) {
	if (array[i].b) {
		results.push(array[i]);
	}
}

Ok, that’s working, but not very elegant nor concise. Luckily, ECMAScript 5 offers a nicer way:

var results = array.filter( function(item) {
	return item.b;		// if item.b is true
});

var myArray = ["a", "b", "c", "z"];
 
for (int i=0; i<myArray.length; i++) {
	console.log(myArray[i]);
}

But I recently came across a Javascript loop that puzzled me slightly.

var myArray = ["a", "b", "c", "z"];
 
for (var i=0,f ; f=myArray[ i++]; ) {
	console.log(f);
}

Let’s have a closer look what’s happening here. As a reminder, For-statements have the following structure:
for (initiation; condition; increment) { block }

In the initiation, variables are initiated; Condition tests this var, and if it returns true, continues another loop; and here’s something funny; the increment is left blank. Don’t panic, because we are already incrementing i at the condition test.

This is equivalent:

var myArray = ["a", "b", "c", "z"];
 
for (var i=0,f ; f=myArray[ i]; i++) {
	console.log(f);
}

This, however, is not the same:

var myArray = ["a", "b", "c", "z"];
 
for (var i=0,f ; f=myArray[ ++i]; ) {
	console.log(f);
}

i gets incremented before it is used to traverse myArray, therefor resulting in this output:

b
c
z

In nice and simple arrays like in this example, it should not make any difference to test the array length at every time coming around, but a larger ones it might be beneficial.

As usual, take care that the array does not contain any falsy objects. And don’t loop an object that way.

Thanks to the Re-introduction to Javascript https://developer.mozilla.org/en/a_re-introduction_to_javascript for clarifying that issue for me.

Turns out things are not so clear after all. Let’s consider the following case: I have an array of numbers, I’d like to get the sum of all numbers in the array.
One way to do it:

var myArray = [1,2,3,5,7];
for (var i=0, sum=0; i<this.length; i++) {
	sum += this[i];
}
console.log(sum);		// 18

So, shouldn’t this work too…?

var myArray = [1,2,3,5,7];
for (var i=0, sum=0; sum = myArray[i]; i++) {};
console.log(sum);		// undefined

Apparently not… I am trying to find out why not.

This, on the other hand works:

for (var i=0, sum=0; this[i]; sum+=this[i++]){};

array_filter($my_array)

array_filter WITHOUT a callback function removes ‘false’, ‘null’ and ” fields from an array.

var arr = [1233, 32, 442];
var largest = Math.max.apply(Math, arr);

http://ejohn.org/blog/fast-javascript-maxmin/

http://stackoverflow.com/questions/1379553/how-might-i-find-the-largest-number-contained-in-a-javascript-array

Objective-C
NSArray *listItems = [@"One, Two, Three" componentsSeparatedByString:@", "];