Functional Programming in Javascript
August 31st, 2009 by James CarrJust a random, late night scribble for the boys and girls out there starting fresh with javascript and not understanding the power that you can harness with it. I’ve given this demonstration before, but it’s always fun to post because someone might learn something new. 
Anyway, you might be familiar with your usual function definition, let’s say a function that sums 2 numbers:
function sum(a,b){
return a+b;
}
sum(2,5); // returns 7
That’s fine and dandy… but now let’s change this around a bit to better illustrate what a function is:
var pre = function(a,b){
return a+b;
}
sum(2,5); // returns 7
Yep, that’s right… a function is just an object in javascript and the name of it is simply a variable. Now let’s make this a bit more interesting by creating a function that returns a running sum:
function runningSum(start){
var sum = start;
return function(a){
return sum+=a;
}
}
var sum = runningSum(3);
sum(2); // returns 5
sum(10); // returns 15
As you see, you can have a function return another function in javascript, and that function inherits the scope of the function that created it… in fact we could remove the temporary sum variable and get the same results:
function runningSum(start){
return function(a){
return start+=a;
}
}
var sum = runningSum(3);
sum(2); // returns 5
sum(10); // returns 15
now, lets introduce the arguments variable, which gives you access to an array of agu ments passed into the function. A revised sum function:
function sum(){
var sum = 0;
for(var i = 0; i < arguments.length; i++){
sum += arguments[i];
}
return sum;
}
sum(1,2,2,1); // returns six
The arguments object is a pretty damn cool object, if you want to look further, take a gander at the Mozilla documentation. Using the callee attribute of arguments (which returns the function instance itself), we can do something wicked:
var sum = 0;
function add(a){
sum+=a;
return arguments.callee;
}
add(1)(2)(3);
alert(sum); // displays 6
I find this can come in pretty handy when you need an accumulator of some sort… try it, it works! 
Also, you can assign the scope of this while using functions, which can be useful if, for example, you want to call a function where this might resolve to the local scope (think ajax callbacks):
function greeting(){
return "Hello " + this.name;
}
var person = {'name':'James'};
greeting.call(person); // returns "Hello James"
I also prefer to avoid polluting the global scope if I want a script that does something procedural when it’s included, in that case I just create a function and execute it on the spot:
(function(){
var bleh = 3;
alert(bleh+5); // alerts 8
})();
alert(bleh); // Reference Error: bleh is not defined
Finally, there’s a handy map function on arrays. It’s available in Mozilla, but not IE (afaik), however it is also available in jquery, so you can give it a try there.
[1,2,3,4,5].map(function(a){return a*a;}); // returns [1,4,9,16,25]
Overall, pretty basic stuff, but a nice refresher if you are unfamiliar.
