プロトタイプチェーンと自所有プロパティの基礎

プロトタイプの概要

プロトタイプの検索

// A simple object
const user = { name: "Ayla" };

// The prototype is another object used for fallback lookups
const proto = Object.getPrototypeOf(user);

console.log("name:", user.name);
console.log("has toString?", typeof user.toString === "function");
console.log("proto same as Object.prototype?", proto === Object.prototype);

自身のキーと継承されたキー

// Own vs inherited checks
const point = { x: 1 };

// "in" sees own or inherited
console.log("x in point:", "x" in point);
console.log("toString in point:", "toString" in point);

// Only own keys
console.log("hasOwn x:", Object.hasOwn(point, "x"));
console.log("hasOwn toString:", Object.hasOwn(point, "toString"));

Object.createの基本

// Use Object.create to set a prototype
const animal = { kind: "animal" };

// pet inherits from animal
const pet = Object.create(animal);
pet.name = "Milo";

console.log("pet name:", pet.name);
console.log("pet kind (inherited):", pet.kind);
console.log("hasOwn kind?", Object.hasOwn(pet, "kind"));

ディスクリプター（概要）

// Define simple descriptors
const prefs = {};
Object.defineProperty(prefs, "theme", {
  value: "dark",
  writable: false,
  enumerable: true
});

// Try to change (silently fails in non-strict)
prefs.theme = "light";

console.log("theme:", prefs.theme);

// List keys: theme is enumerable
console.log("keys:", Object.keys(prefs));

ヒントとチェックリスト

自身のキーと継承されたキーのクイズ

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