reduce.js

/**
 * 
 * @param {array} target 
 * @param {function} cb: (oldAcc, item, index) -> newAcc 
 * @param {*} seed
 */

function reduce(target, cb, seed){
  var runningAggregate = seed;
  target.forEach(function(item, index){
    runningAggregate = cb(runningAggregate, item, index);
  });
  return runningAggregate;
}

// console.log(reduce([3, 4, 5], function(acc, val){ 
//   return acc + val;
// }, 0), 3);

function sum(arr){
  var runningTotal = 0;
  arr.forEach(function(item){
    runningTotal += item;
  });
  return runningTotal;
}

function sumFromReduce(arr){
  return arr.reduce(function(runningTotal, item){
    console.log('Iteration', runningTotal, item);
    return runningTotal + item;
  }, 0);
}

function maxFromReduce(arr){
  return reduce(arr, function(runningMax, item){
    console.log('Iteration', runningMax, item);
    if(runningMax > item) return runningMax;
    else return item;
  }, -Infinity);
}

console.log('Total Sum:', sumFromReduce([3, 4, 5]));
console.log('Total Max:', maxFromReduce([4, 3, 5]));