構造化クローンとURLSearchParams

なぜこれらのツールなのか

型を保持するディープコピー

// Deep copy with structuredClone (Node 17+/modern browsers)
const original = { n: 1, list: [1, 2], when: new Date("2024-01-01") };
const copy = structuredClone(original);

copy.n = 2;
copy.list.push(3);

console.log("original.n:", original.n);
console.log("copy.n:", copy.n);
console.log("dates equal type?", copy.when instanceof Date);

JSONでは型が失われる

// JSON round-trip loses special types (e.g., Date becomes string)
const obj = { when: new Date("2024-01-01"), ok: true };
const round = JSON.parse(JSON.stringify(obj));

console.log("after JSON:", typeof round.when, round.when);
console.log("is Date?", round.when instanceof Date);

MapとSetを保持する

// structuredClone preserves Map and Set
const m1 = new Map([["a", 1]]);
const s1 = new Set([1, 2]);

const cloned = structuredClone({ m1, s1 });

console.log("map has a?", cloned.m1.has("a"));
console.log("set has 2?", cloned.s1.has(2));

クエリ文字列を解析する

// Parse a query string with URLSearchParams
const qs = "q=shoes&color=red&color=blue&page=2";
const params = new URLSearchParams(qs);

console.log("q:", params.get("q"));
console.log("first color:", params.get("color"));
console.log("all colors:", params.getAll("color"));
console.log("page:", params.get("page"));

クエリ文字列を構築する

// Build a query string from key/value pairs
const build = new URLSearchParams();
build.set("q", "shoes");
build.append("color", "red");
build.append("color", "blue");
build.set("page", String(2));

console.log("query:", build.toString());
// Example full URL
const url = "https://example.com/search?" + build.toString();
console.log("url:", url);

structuredCloneとJSONのクイズ

まとめ