mapとfilter — 基本

mapとfilterの導入

mapの基本

// map transforms each value and returns a new array
const nums = [1, 2, 3];

const doubled = nums.map(function (n) {
  return n * 2;
});

console.log("nums:", nums);
console.log("doubled:", doubled);

filterの基本

// filter keeps values that pass the test
const values = [1, 2, 3, 4, 5];

const evens = values.filter(function (n) {
  return n % 2 === 0;
});

console.log("values:", values);
console.log("evens:", evens);

チェーンの例

// Chain methods for clarity
const prices = [4, 7, 10];

// First add tax, then keep items over 8
const withTax = prices.map(function (p) {
  return p * 1.1;
});

const expensive = withTax.filter(function (p) {
  return p > 8;
});

console.log("withTax:", withTax);
console.log("expensive:", expensive);

ミューテーションなしの例

// map and filter do not mutate the original array
const base = [2, 4, 6];

const plusOne = base.map(function (n) {
  return n + 1;
});

const small = base.filter(function (n) {
  return n < 5;
});

console.log("base:", base);
console.log("plusOne:", plusOne);
console.log("small:", small);

読みやすい配列のヒント

mapとfilterのクイズ

振り返り