mapreduce.js

const totalLine = line =>
  line.quantity * line.price;

const sum = (a, b) => a + b;

const map = fn => xs => xs.map(fn);

const reduce = (fn, ini) => xs => xs.reduce(fn, ini)

const sumTotal = lines =>
  reduce(sum, 0)(map(totalLine)(lines));

console.log(sumTotal(lineItems));