{"type":"doc","content":[{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"1) Базовый паттерн: ключ объекта"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"function getProp(obj: T, key: K): T[K] {\n return obj[key];\n}\n\nconst user = { id: 1, name: \"Tim\" };\ngetProp(user, \"id\"); // ✅\ngetProp(user, \"age\"); // ❌ \"age\" ∉ keyof typeof user\n"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"keyof T"},{"type":"text","text":" — объединение строковых/числовых/символьных ключей типа "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":"."}]}]}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"2) Разрешить только подмножество ключей"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Иногда нужны не "},{"type":"text","marks":[{"type":"bold"}],"text":"все"},{"type":"text","text":" ключи "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":", а лишь часть:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"function getIdOrName<\n T,\n K extends keyof T & (\"id\" | \"name\")\n>(obj: T, key: K): T[K] {\n return obj[key];\n}\n"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Здесь "},{"type":"text","marks":[{"type":"code"}],"text":"K"},{"type":"text","text":" — пересечение «ключи T» c конкретным множеством "},{"type":"text","marks":[{"type":"code"}],"text":"\"id\" | \"name\""},{"type":"text","text":"."}]}]}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"3) Ключи, чьи "},{"type":"text","marks":[{"type":"bold"}],"text":"значения"},{"type":"text","text":" соответствуют типу (фильтрация по значению)"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type KeysOfType = {\n [K in keyof T]-?: T[K] extends V ? K : never\n}[keyof T];\n\nfunction getNumberField<\n T,\n K extends KeysOfType\n>(obj: T, key: K): number {\n return obj[key] as number;\n}\n\ntype U = { a: number; b: string; c: number };\n/* K = \"a\" | \"c\" */\n"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Похожий приём часто называют "},{"type":"text","marks":[{"type":"italic"}],"text":"value-based key filtering"},{"type":"text","text":"."}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"4) Обязательные/необязательные/незаписываемые ключи"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Иногда нужно ограничить ключи по «квалификатору» (optional/readonly):"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type RequiredKeys = {\n [K in keyof T]-?: {} extends Pick ? never : K\n}[keyof T];\n\ntype WritableKeys = {\n [K in keyof T]-?: { [P in K]: T[P] } extends { -readonly [P in K]: T[P] } ? K : never\n}[keyof T];\n\nfunction takeRequired>(obj: T, key: K) {\n return obj[key];\n}\n"}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"5) Ограничение ключами "},{"type":"text","marks":[{"type":"bold"}],"text":"индексной сигнатуры"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Если у типа словарная форма:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type Dict = Record; // индексная сигнатура\n\nfunction getDict<\n T extends Record,\n K extends keyof T\n>(obj: T, key: K) {\n return obj[key];\n}\n"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"keyof T"},{"type":"text","text":" при индексной сигнатуре ≈ "},{"type":"text","marks":[{"type":"code"}],"text":"string | number | symbol"},{"type":"text","text":" (зависит от ключей)."}]}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"6) Ключи массивов и кортежей"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"function first(arr: T) {\n const i: Extract = 0; // только числовые ключи\n return arr[i]; // T[number]\n}\n"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"У массива "},{"type":"text","marks":[{"type":"code"}],"text":"keyof T"},{"type":"text","text":" включает и методы ("},{"type":"text","marks":[{"type":"code"}],"text":"\"length\""},{"type":"text","text":", "},{"type":"text","marks":[{"type":"code"}],"text":"\"push\""},{"type":"text","text":"). Чтобы ограничиться индексами, пересекайте с "},{"type":"text","marks":[{"type":"code"}],"text":"number"},{"type":"text","text":"."}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"7) Ключи у литеральных объектов: "},{"type":"text","marks":[{"type":"code"}],"text":"as const"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Чтобы получить "},{"type":"text","marks":[{"type":"bold"}],"text":"литеральные"},{"type":"text","text":" ключи, а не «расширенные»:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"const cfg = {\n api: \"/api\",\n cdn: \"/cdn\",\n} as const;\n\ntype Cfg = typeof cfg; // { readonly api: \"/api\"; readonly cdn: \"/cdn\" }\ntype CfgKey = keyof Cfg; // \"api\" | \"cdn\"\n\nfunction fromCfg(k: K): Cfg[K] {\n return cfg[k];\n}\n"}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"8) Ключи у "},{"type":"text","marks":[{"type":"code"}],"text":"enum"},{"type":"text","text":" / объектов-справочников"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"enum Role { Admin = \"admin\", User = \"user\" }\n\nconst ROLES = { Admin: \"admin\", User: \"user\" } as const;\n\ntype RoleKey = keyof typeof ROLES; // \"Admin\" | \"User\"\ntype RoleVal = typeof ROLES[RoleKey]; // \"admin\" | \"user\"\n\nfunction allow(k: K) { /* ... */ }\n"}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"9) Утилиты для сборки API: "},{"type":"text","marks":[{"type":"code"}],"text":"Pick"},{"type":"text","text":"/"},{"type":"text","marks":[{"type":"code"}],"text":"Record"},{"type":"text","text":"/"},{"type":"text","marks":[{"type":"code"}],"text":"Omit"},{"type":"text","text":" с ограниченными ключами"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"function pick(obj: T, keys: K[]): Pick {\n const out = {} as Pick;\n keys.forEach(k => (out[k] = obj[k]));\n return out;\n}\n\nfunction mapValues(\n obj: T, keys: K[], fn: (v: T[K], k: K) => T[K]\n): Pick {\n const res = {} as Pick;\n for (const k of keys) res[k] = fn(obj[k], k);\n return res;\n}\n"}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"10) Частые грабли и подсказки"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"keyof never"},{"type":"text","text":" → "},{"type":"text","marks":[{"type":"code"}],"text":"never"},{"type":"text","text":": если "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":" может быть "},{"type":"text","marks":[{"type":"code"}],"text":"never"},{"type":"text","text":", "},{"type":"text","marks":[{"type":"code"}],"text":"K"},{"type":"text","text":" уже невыводим; добавляйте защиту."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"У массивов не забывайте фильтровать ключи до "},{"type":"text","marks":[{"type":"code"}],"text":"number"},{"type":"text","text":", иначе появятся строковые методы."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Если ключи приходят из значения (массива строк), отдавайте предпочтение "},{"type":"text","marks":[{"type":"code"}],"text":"as const"},{"type":"text","text":":"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"const allowed = [\"id\", \"name\"] as const;\ntype Allowed = typeof allowed[number]; // \"id\" | \"name\"\nfunction f(obj: T, key: K) {}\n"}]}]}]},{"type":"horizontalRule"},{"type":"heading","attrs":{"textAlign":null,"level":2},"content":[{"type":"text","text":"Резюме"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"База: "},{"type":"text","marks":[{"type":"code"}],"text":"K extends keyof T"},{"type":"text","text":" — ключи "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":"."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Подмножество: "},{"type":"text","marks":[{"type":"code"}],"text":"K extends keyof T & (\"a\" | \"b\")"},{"type":"text","text":"."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"По типу значений: "},{"type":"text","marks":[{"type":"code"}],"text":"KeysOfType"},{"type":"text","text":"."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Для массивов: "},{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":"."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Для литералов/enums: "},{"type":"text","marks":[{"type":"code"}],"text":"keyof typeof X"},{"type":"text","text":" (+ "},{"type":"text","marks":[{"type":"code"}],"text":"as const"},{"type":"text","text":")."}]}]}]},{"type":"paragraph","attrs":{"textAlign":null}}]}

Как ограничить generic-тип ключами другого типа?

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