Reduce関数ライブ講座で使用したコード

study-reduce.ts

// 一番シンプルな例
const input = [1, 2, 3, 4];

const output = input.reduce((prev, current) => {
  return prev + current;
});

console.log(output); // 10

初期値がある例（ループ回数が一度多くなります）

const input = [1, 2, 3, 4];

const output = input.reduce((prev, current) => {
  return prev + current;
}, 0);

console.log(output); // 10

オブジェクト配列内のプロパティを足す例

const input = [
  { name: "foo1", age: 10 },
  { name: "foo2", age: 20 },
  { name: "foo3", age: 30 },
  { name: "foo4", age: 40 },
];

const output = input.reduce((prev, current) => {
  return prev + current.age;
}, 0);

console.log(output); // output: 100

オブジェクト配列内のプロパティでgroupByする例

const input = [
  { name: "foo1", lang: "ja" },
  { name: "foo2", lang: "en" },
  { name: "foo3", lang: "ja" },
  { name: "foo4", lang: "en" },
];

const output = input.reduce((prev, current) => {
  return {
    ...prev,
    [current.lang]: prev[current.lang] ? [...prev[current.lang], current.name] : [current.name],
  };
}, {});

console.log(output);
// {
//     ja: ["foo1", "foo3"],
//     en: ["foo2", "foo4"],
// }

オブジェクト配列内のプロパティでgroupByする例（TypeScript）

const input = [
  { name: "foo1", lang: "ja" },
  { name: "foo2", lang: "en" },
  { name: "foo3", lang: "ja" },
  { name: "foo4", lang: "en" },
] as const;

type Lang = typeof input[number]["lang"];

const output = input.reduce((prev, current) => {
  return {
    ...prev,
    [current.lang]: prev[current.lang] ? [...prev[current.lang], current.name] : [current.name],
  };
}, {} as { [key in Lang]: string[] });

console.log(output);
// {
//     ja: ["foo1", "foo3"],
//     en: ["foo2", "foo4"],
// }

オブジェクト配列内のプロパティでgroupByする例（TypeScript + 関数の引数で分割代入）

const input = [
  { name: "foo1", lang: "ja" },
  { name: "foo2", lang: "en" },
  { name: "foo3", lang: "ja" },
  { name: "foo4", lang: "en" },
] as const;

type Lang = typeof input[number]["lang"];

const output = input.reduce((prev, { lang, name }) => {
  const prevArray = prev[lang];
  return {
    ...prev,
    [lang]: prevArray ? [...prevArray, name] : [name],
  };
}, {} as { [key in Lang]: string[] });

console.log(output);
// {
//     ja: ["foo1", "foo3"],
//     en: ["foo2", "foo4"],
// }