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Reduce関数ライブ講座で使用したコード
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| // 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 |
Author
Author
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
Author
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"],
// }
Author
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"],
// }
Author
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"],
// }
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初期値がある例（ループ回数が一度多くなります）