high-order-function.js

function head(l) { return l[0];}

function tail(l) { return l.slice(1, l.length);}

function map(f,l) { if(l.length ==0 ) {return l;} 
		else {return [f(head(l))].concat(map(f,tail(l)));}}

map(function (i) {return i+1}, [1,2,3]) //[2,3,4]

function filter(p,l) {
	if(l.length == 0) {return l}
	else { 
		if(p(head(l))) {return [head(l)].concat(filter(p,tail(l)));} 
		else {return filter(p,tail(l));}	
	}	
}
filter(function(x) {return x > 0},[0,-1,2,3,4]) //[2,3,4]

function foldr(f,v,l) {
	if(l.length == 0) {return v;}
	else{
		return foldr(f,f(v,head(l)),tail(l));
	}
}
foldr(function(x,y){return x+y;},0,[1,2,3]) //6

function reverse(l) {
	if(l.length == 0) return l; 
	else { return reverse(tail(l)).concat(head(l));} }

function reverse2(l) {
	return foldr(function(acc,e) {return [e].concat(acc);},[],l);
	}
reverse2([1,2,3]) //[3,2,1]

function map2(f,l) {
    return foldr(function(acc, e) {return acc.concat(f(e))}, [] , l);
}