matthew-gerstman

function chunk(list, size) {
  return list.reduce(
    (acc, item) => {
      let lastIndex = acc.length - 1;
      if (acc[lastIndex].length >= size) {
        acc.push([]);
        lastIndex++;
      }
      acc[lastIndex].push(item);
      return acc;

matthew-gerstman

{
  Gryffindor: [
    { name: "Harry Potter", house: "Gryffindor", points: 40 },
    { name: "Hermione Granger", house: "Gryffindor", points: 140 }
  ],
  Slytherin: [
    { name: "Draco Malfoy", house: "Slytherin", points: -20 },
    { name: "Lin Manuel Miranda", house: "Slytherin", points: 5000 },
    { name: "Taylor Swift", house: "Slytherin", points: 100 }
  ],

matthew-gerstman

function groupBy(list, iteratee) {
  return list.reduce((acc, item) => {
    const index = iteratee(item);
    if (!acc[index]) {
      acc[index] = [];
    }
    acc[index].push(item);
    return acc;
  }, {});
}

matthew-gerstman

{
  "Harry Potter": {name: "Harry Potter", house: "Gryffindor", points: 40},
  "Hermione Granger": {name: "Hermione Granger", house: "Gryffindor", points: 140},
  "Draco Malfoy": {name: "Draco Malfoy", house: "Slytherin", points: -20},
  "Lin Manuel Miranda": {name: "Lin Manuel Miranda", house: "Slytherin", points: 5000},
  "Taylor Swift": {name: "Taylor Swift", house: "Slytherin", points: 100},
  "Cedric Diggory": {name: "Cedric Diggory", house: "Hufflepuff", points: 12},
  "Sally Perks": {name: "Sally Perks", house: "Hufflepuff", points: 15},
  "Luna Lovegood": {name: "Luna Lovegood", house: "Ravenclaw", points: 100},
  "Cho Chang": {name: "Cho Chang", house: "Ravenclaw", points: 10 }

matthew-gerstman

function keyBy(list, key) {
  return list.reduce((acc, item) => {
    const index = item[key];
    acc[index] = item;
    return acc;
  }, {});
}

matthew-gerstman

const HarryPotter = {
  name: "Harry Potter",
  house: "Gryffindor",
  points: 40
};
const HermioneGranger = {
  name: "Hermione Granger",
  house: "Gryffindor",
  points: 140
};

matthew-gerstman

function reduce(list, callback, initialValue) {
  let accumulator = initialValue;
  for (let currentIndex = 0; currentIndex < list.length; currentIndex++) {
    currentValue = list[currentIndex];
    accumulator = callback(accumulator, currentValue, currentIndex, list);
  }
  return accumulator;
}

matthew-gerstman

function reduce(list, callback, initialValue) {
  if (list == null || !Array.isArray(list)) {
    throw new TypeError("list is not a valid array");
  }
  if (typeof callback !== "function") {
    throw new TypeError(callback + " is not a function");
  }
  if (initialValue === undefined && list.length === 0) {
    throw new TypeError("Reduce of empty array with no initial value");
  }

matthew-gerstman

list.reduce(callback, initialValue)

matthew-gerstman

const callback = (accumulator, currentValue, currentIndex, list) => accumulator;