reduce、some/every、find — シンプルなパターン

reduceと仲間たちの概要

reduceによる合計

// reduce: sum numbers with an initial value
const prices = [4, 6, 3];

const total = prices.reduce(function (acc, n) {
  // Add current number to the accumulator
  return acc + n;
}, 0);

console.log("Total:", total);

reduceによる最小値

// reduce: smallest number
const nums = [7, 2, 9, 4];

const min = nums.reduce(function (acc, n) {
  // Keep the smaller of acc or n
  return n < acc ? n : acc;
}, Infinity);

console.log("Min:", min);

someの基本

// some: is there any even number?
const list = [1, 5, 8, 11];

const hasEven = list.some(function (n) {
  return n % 2 === 0;
});

console.log("Has even?", hasEven);

everyの基本

// every: are all ages 18+ ?
const ages = [22, 19, 31];

const allAdults = ages.every(function (a) {
  return a >= 18;
});

console.log("All adults?", allAdults);

findの基本

// find: get the first matching value (or undefined)
const users = [
  { name: "Ayla", active: true },
  { name: "Mina", active: false },
  { name: "Lena", active: true }
];

const firstInactive = users.find(function (u) {
  return u.active === false;
});

console.log("First inactive:", firstInactive);

reduceの概念クイズ

振り返り