We've looked at some simple array functions which add filtering and pipes to arrays. Another useful tool is the accumulator. Accumulators aid in building up a single result by iterating over a collection. Many common operations such as summing up the elements of an array can be implemented using an accumulator instead of a loop.

Recursion is popular within functional programming languages and many of them actually offer an optimization called "tail call optimization". A language that supports this provides optimizations for functions using recursion in which the stack frame is reused. This is very efficient and can easily replace most loops. Details on whether tail call optimization is supported in any JavaScript interpreter are sketchy. For the most part it doesn't seem like it is but we can still make use of recursion.

The problem with for loops is that the control flow through the loop is mutable. Consider this rather easy-to-make mistake:

let result = "";
let multiArray = [[1,2,3], ["a", "b", "c"]];
for(vari=0; i<multiArray.length; i++)
  for(var j=0; i<multiArray[i].length; j++)
    result += multiArray[i][j];

Did you spot the error? It took me several attempts to get a working version of this code I could break. The problem is in the loop counter in the second loop, it should read as follows:

let result = "";
let multiArray = [[1,2,3], ["a", "b", "c"]];
for(let i=0; i<multiArray.length; i++)
  for(let j=0; j<multiArray[i].length; j++)
    result +=multiArray[i][j];

Obviously this could be somewhat mitigated through better variable naming but we would like to avoid the problem altogether.

Instead we can make use of an accumulator, a tool for combining multiple values from a collection into a single value. We've rather missed Westeros for a couple of patterns so let's get back to our mythical example land. Wars cost a great deal of money but fortunately there are a great number of peasants to pay taxes and finance the lords in their games for the throne.

let peasants = [
  {name: "Jory Cassel", taxesOwed: 11, bankBalance: 50},
  {name: "VardisEgen", taxesOwed: 15, bankBalance: 20}];

TaxCollector.prototype.collect = function (items, value, projection) {
  if (items.length> 1)
    return projection(items[0]) + this.collect(items.slice(1), value, projection);
  return projection(items[0]);
};

function (item) {
  return Math.min(item.moneyOwed, item.bankBalance);
}