Daniel Carrera

In my previous post I talked about JavaScript being a functional language. But those familiar with traditional functional languages like Haskell and ML will notice that JavaScript lacks traditional functional constructs like map, foldl and currying.

I do wish that JavaScript had these constructs by default, and it doesn’t. But at least we can add these constructs ourselves.

var map = function(fn , xs) {
    if (xs.length == 1) return [fn(xs[0])];
    return [fn(xs[0])].concat(map(fn, xs.slice(1)));
};
var foldl = function(fn , x0, xs) {
    if (xs.length == 1) return fn(x0, xs[0]);
    return foldl(fn, fn(x0, xs[0]), xs.slice(1) );
};
var curry = function(fn) {
    /* Yes, I hate this function, why do you ask? */
    function toArray(pseudoarray) {
        var array = [];
        for (var i=0; i<pseudoarray.length; i++)
            array.push(pseudoarray[i]);
        return array;
    }
    var xs = toArray(arguments).slice(1);
    return function() {
        return fn.apply(this, xs.concat(toArray(arguments)));
    };
};

Then you can do things like:

/* Ex 1 */
var double = function(x) {return 2*x};
map(double, [1,2,3,4,5]);

/* Ex 2 */
var mult = function(x,y) { return x*y };
foldl(mult, 1, [2,3,4,5]);

/* Ex 3 */
var product = function(xs) {
    return foldl(function(x,y) { return x*y }, 1, xs);
};
product([1,2,3,4,5]);

/* Ex 4 */
var add = function(a,b) { return a+b; }
var add1 = curry(add, 1);
add1(2);