functional-libraries.md

Item 26: Use Functional Constructs and Libraries to Help Types Flow

Things to Remember

• Use built-in functional constructs and those in utility libraries like Lodash instead of hand-rolled constructs to improve type flow, increase legibility, and reduce the need for explicit type annotations.

Code Samples

const csvData = "...";
const rawRows = csvData.split('\n');
const headers = rawRows[0].split(',');

const rows = rawRows.slice(1).map((rowStr) => {
  const row = {};
  rowStr.split(",").forEach((val, j) => {
    row[headers[j]] = val;
  });
  return row;
});

💻 playground

---

import _ from 'lodash';
const rows = rawRows.slice(1)
    .map(rowStr => _.zipObject(headers, rowStr.split(',')));

💻 playground

---

const rowsImperative = rawRows.slice(1).map(rowStr => {
  const row = {};
  rowStr.split(',').forEach((val, j) => {
    row[headers[j]] = val;
    // ~~~~~~~~~~~~ No index signature with a parameter of
    //              type 'string' was found on type '{}'
  });
  return row;
});
const rowsFunctional = rawRows.slice(1)
  .map((rowStr) =>
    rowStr
      .split(",")
      .reduce(
        (row, val, i) => ((row[headers[i]] = val), row),
        //                 ~~~~~~~~~~~~~~~ No index signature with a parameter of
        //                                 type 'string' was found on type '{}'
        {}
      )
  );

💻 playground

---

const rowsLodash =
  rawRows.slice(1).map(rowStr => _.zipObject(headers, rowStr.split(',')));
rowsLodash
// ^? const rowsLodash: _.Dictionary<string>[]

💻 playground

---

interface BasketballPlayer {
  name: string;
  team: string;
  salary: number;
}
declare const rosters: {[team: string]: BasketballPlayer[]};

💻 playground

---

let allPlayers = [];
//  ~~~~~~~~~~ Variable 'allPlayers' implicitly has type 'any[]'
//             in some locations where its type cannot be determined
for (const players of Object.values(rosters)) {
  allPlayers = allPlayers.concat(players);
  //           ~~~~~~~~~~ Variable 'allPlayers' implicitly has an 'any[]' type
}

💻 playground

---

let allPlayers: BasketballPlayer[] = [];
for (const players of Object.values(rosters)) {
  allPlayers = allPlayers.concat(players);  // OK
}

💻 playground

---

const allPlayers = Object.values(rosters).flat(); // OK
//    ^? const allPlayers: BasketballPlayer[]

💻 playground

---

const teamToPlayers: {[team: string]: BasketballPlayer[]} = {};
for (const player of allPlayers) {
  const {team} = player;
  teamToPlayers[team] = teamToPlayers[team] || [];
  teamToPlayers[team].push(player);
}

for (const players of Object.values(teamToPlayers)) {
  players.sort((a, b) => b.salary - a.salary);
}

const bestPaid = Object.values(teamToPlayers).map(players => players[0]);
bestPaid.sort((playerA, playerB) => playerB.salary - playerA.salary);
console.log(bestPaid);

💻 playground

---

const bestPaid = _(allPlayers)
  .groupBy(player => player.team)
  .mapValues(players => _.maxBy(players, p => p.salary)!)
  .values()
  .sortBy(p => -p.salary)
  .value();
console.log(bestPaid.slice(0, 10));
//          ^? const bestPaid: BasketballPlayer[]

💻 playground