反復処理のパターン

反復処理の概要

Map：entries/keys/values

// Map iterates as [key, value] pairs
const prices = new Map();
prices.set("apple", 3);
prices.set("banana", 2);

for (const [item, cost] of prices) {
  console.log(item, cost);
}

// You can also use keys() or values()
for (const k of prices.keys()) {
  console.log("key:", k);
}
for (const v of prices.values()) {
  console.log("value:", v);
}

Set：シンプルなループ

// Set iterates values in insertion order
const colors = new Set(["red", "green", "red", "blue"]);

for (const c of colors) {
  console.log("color:", c);
}

console.log("size:", colors.size);

Array.fromによる変換

// Convert Map to array of pairs, and back
const map1 = new Map([["a", 1], ["b", 2]]);
const pairs = Array.from(map1); // [["a",1],["b",2]]
console.log("pairs:", pairs);

const map2 = new Map(pairs);
console.log("map2 has a?", map2.has("a"));

// Set to array of values
const set1 = new Set([1, 1, 2, 3]);
const arr = Array.from(set1); // [1,2,3]
console.log("arr:", arr);

頻度カウンター

// Count items in an array using Map
function countItems(list) {
  const counts = new Map();
  for (const x of list) {
    const n = counts.get(x) || 0;
    counts.set(x, n + 1);
  }
  return counts;
}

const fruits = ["apple", "banana", "apple", "apple", "banana"];
const counts = countItems(fruits);

// Print counts as "item: n"
for (const [name, n] of counts) {
  console.log(name + ":", n);
}

一意性とメンバーシップ

// Unique values with Set, quick membership test
const numbers = [2, 3, 2, 4, 3, 5];
const unique = Array.from(new Set(numbers));
console.log("unique:", unique);

// Quick membership with Set.has
const allowed = new Set([2, 5]);
console.log("allowed has 2?", allowed.has(2));
console.log("allowed has 4?", allowed.has(4));

Mapの反復処理クイズ

まとめ