reduce_wtf.js

[1,2,3].reduce(function(memo, current) {
  console.log(memo);
}, {});

// > {}
// > undefined
// > undefined

memo is based on the return value which is empty, try

[1,2,3].reduce(function(memo, current) {
  console.log(memo);
  return memo;
}, {});

When using reduce follow the 2 rules.

1. Always pass an initialValue
2. Always return the accumulator

var data = [1,2,3];
var initialValue = 0;
var reducer = function(accumulator, item) {
  return accumulator + item;
};
var total = data.reduce(reducer, initialValue);
console.log('Sum of the array', data, 'is: ', total);