Mwahaha, I feel like a ninja. I mean a ninja by Doug Crockford's definition which means someone who finds a mistake in the language's design, decides it's cool and abuses it.

So there you go. Regular expression objects are created only once if you're using a literal. Such an object can be used to store the single instance of a Singleton() constructor.

Implementation

function Single() {
    "strict me not!";

    var re = / /;
    if (re.instance) {
        return re.instance;
    }
    re.instance = this;
    this.name = "Foo";

}

Single.prototype.getName = function () {
    return this.name;
};

Test

var s1 = new Single(),
    s2 = new Single();

console.log(s1 === s2); // true
console.log(s1.getName()); // "Foo"
s1.name = "dude";
console.log(s2.getName()); // "dude"

Pros

• The prototype chain works fine
• No closures
• No public properties or globals to store the single instance

Cons

• It's a hack
• ES5 defines that reg exp literals should no longer work like this

Verdict

Don't use. Please

Workaround

To work around that limitation, people often use ALLCAPPS to denote "hey, don't touch this var, it's meant to be a constant". It's constant-by-convention but the values can be changed by careless programmer on a bad day. If you really want to protect the value, you need to make it private.

PHP-inspired API

In PHP there are the functions define(name, value) to gefine a constant, defined(name) to check if a constant is defined and constant(name) to get the value of a constants when it's name is assembled at runtime and you don't know it a priori. So I thought - well, we can do the same in JavaScript. Only, let's not use global functions, but a constant global var and make the functions method of that global. constant.constant(name) is a little mouthful, so let's make that one constant.get(name)

Implementation

Here's the simple implementation:

"use strict";

var constant = (function () {
    var constants = {},
        ownProp = Object.prototype.hasOwnProperty,
        allowed = {
            string: 1,
            number: 1,
            boolean: 1
        };
    return {
        define: function (name, value) {
            if (this.defined(name)) {
                return false;
            }
            if (!ownProp.call(allowed, typeof value)) {
                return false;
            }
            constants[name] = value;
            return true;
        },
        defined: function (name) {
            return ownProp.call(constants, name);
        },
        get: function (name) {
            if (this.defined(name)) {
                return constants[name];
            }
            return null;
        }
    };
}());

This is it. Basically protect an object in a closure and don't provide means to change it, but only to add properties to it.

UPDATE: thanks to the comments (see below), there's extra care to check for own properties of the constants private object. This allows to define constants with weird names such as toString and hasOwnProperty. Also only primitive values are allowed to be constants.

Usage

// check if defined
constant.defined("bazinga"); // false

// define
constant.define("bazinga", "Bazinga!"); // true

// check again
constant.defined("bazinga"); // true

// attempt to redefine
constant.define("bazinga", "Bazinga2"); // false

// was it constant or it changed?
// get da, get da, get da value
constant.get("bazinga"); // "Bazinga!"

function aha(a, b) {
  console.log(arguments[0] === a); // true
  console.log(arguments[1] === b); // true
}

aha(1, 2);

However, it's not a good idea to use arguments for the reasons of :

• performance
• security

The arguments object is not automatically created every time the function is called, the JavaScript engine will only create it on-demand, if it's used. And that creation is not free in terms of performance. The difference between using arguments vs. not using it could be anywhere between 1.5 times to 4 times slower, depending on the browser (more info and bench)

As for the security, there is the POLA (Principle of Least Authority) which is violated when one function A passes arguments to another B. Then a number of bad things can happen including:

• B calls A through arguments.callee - something A never intended when calling B in the first place
• B overwrites some arguments and causes A to misbehave

While in these scenarios B looks a little malicious, it can actually cause trouble unvoluntarilly. Consider this example that illustrates the second case (B changing values behind A's unsuspecting back)

function A(obj, ar) {

  console.log(obj); // {p: 1}
  console.log(ar);  // [1, 2, 3]

  B(arguments);

  // oops!
  console.log(obj); // {p: 2}
  console.log(ar);  // [1, 2]
}

function B(args) {

  // convenient innocent-looking local vars
  var o=args[0],
      a=args[1];

  // do something with the local variables
  o.p = 2;
  a.pop();

  // now the original arguments is 
  // messed up because objects/arrays
  // are passed by reference
}

A({p: 1}, [1, 2, 3]);

ECMAScript 5

In ECMAScript's "strict mode", using arguments.callee will throw a syntax error.

Recursive anonymous function

Probably the biggest argument for keeping arguments and arguments.callee is so that recursive anonymous functions can be created, because by using the callee property a function can call itself without knowing its own name. Now, this is not such a common scenario, but even if so, you can wrap a named function inside of an anonymous function and voila! - call that named function recursively without leaking a variable to the global scope.

Why? For caching results of computations for example. Here's a memoization done this way.

function myFunc(param){
    if (!myFunc.cache) {
        myFunc.cache = {};
    }
    if (!myFunc.cache[param]) {
        var result = {}; // ...
        myFunc.cache[param] = result;
    }
    return myFunc.cache[param];
}

Here myFunc() gets a cache property which is an object. The function does the complicated computations based on a parameter and ends up with a result object. Using the parameter as a key in the cache object, it stores the result.

Consecutive calls with the same parameter will get the result from the cache, no need to compute the same result again.

It does some work initially and then caches the result. Then on consecutive calls it only returns the pre-computed result, it doesn’t really do anything.

In order to do so, the function redefines itself with a new implementation, one that simply points to the computed result.

function lazy() {
    var result = 2 + 2;
    lazy = function() {
        return result;
    };
    return lazy();
}

lazy(); // 4
lazy(); // 4

You call a function that does some complicated setup and as a results returns a new function. You assign the result to a new function and use it afterwards.

function setup() {
    alert(1);
    return function() {
        alert(2);
    };
}
var my = setup(); // alerts 1
my(); // alerts 2

Your setup function can store some private data in a closure and use that data somehow.

Here setup() creates a counter function, which gives a next ID for example. But the count variable is not exposed.

function setup() {
    var count = 0;
    return function() {
        return ++count;
    };
}
var next = setup();
next(); // 1
next(); // 2

function test(a, b, fn) {
    fn(a, b);
}

In this simple example we have a function test() that takes a function fn as a third parameter and calls that function passing the first two parameters.

When you cal the test() function, you can pass a pointer to an existing function myFunc() or you can also pass an anonymous function.

test(1, 2, myFunc);

test(1, 2, function(one, two){
    console.log(arguments);
});

Examples

For examples of this pattern in the wild – well, every time you attach an event listener, you pass a callback function.

document.addEventListener(
    'click',
    animateAndWowUser,
    false
);

Same with timeouts and intervals. setTimeout() and setInterval() expect a function and call it back after the specified number of milliseconds.

var thePlotThickens = function(){
  console.log('500ms later...');
};
setTimeout(thePlotThickens, 500);

And here's a common anti-pattern – passing a string where a function is expected. The JavaScript engine has no choice but to evaluate the string (like with eval()) and execute it, which is unnecessary work.

// anti-pattern
setTimeout("thePlotThickens()", 500);

Self-executable functions are simple and powerful.

All you do is add a set of parentheses after the function and this causes it to be executed right there. If it’s an anonymous function you also need to wrap it in parentheses.

(function(){
   var a = 1;
   var b = 2;
   alert(a + b);
})();

You can also pass parameters to the self-executing function if you want.

(function(a, b){
  var c = a + b;
  alert(c);
})(1, 2);

So how is this pattern useful?

It’s useful when you have some work to do, some initialization maybe. You need to do it only once and you don’t want to leave any globals lying around after the work is finished. All the temporary variables you need like c in the case above remain local and don't pollute the namespace.

Useful also when your code is kind of a guest to the page. Like a bookmarklet for example, or a widget of some sorts. You don’t want to add your variables and functions to the host page’s naming space and possibly causing a naming conflict with the page's core code.

Functions are objects so they can have properties. And they they have methods.

They can be copied, deleted, passed as arguments to other functions, returned by other functions.

They also provide scope.

And they have special feature that sets them apart from the other objects – they are invokable, they can be called.