{"type":"doc","content":[{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"Что делает "},{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":"?"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Выбирает из "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":" только те подтипы, которые "},{"type":"text","marks":[{"type":"bold"}],"text":"совместимы"},{"type":"text","text":" (assignable) с "},{"type":"text","marks":[{"type":"code"}],"text":"U"},{"type":"text","text":". Реализация в lib.d.ts эквивалентна:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type Extract = T extends U ? T : never;\n"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Примеры:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type A = Extract; // number | boolean\ntype B = Extract<\"a\" | \"b\" | 1, string>; // \"a\" | \"b\"\ntype C = Extract<{a:1} | {b:2}, {a:1} | {c:3}>; // {a:1}\ntype D = Extract<\"get\" | \"set\" | \"reset\", `get${string}`>; // \"get\"\ntype E = Extract; // null | undefined\n"}]},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"Чем "},{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" противоположен "},{"type":"text","marks":[{"type":"code"}],"text":"Exclude"},{"type":"text","text":"?"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Exclude"},{"type":"text","text":" делает обратное: "},{"type":"text","marks":[{"type":"bold"}],"text":"удаляет"},{"type":"text","text":" из "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":" всё, что совместимо с "},{"type":"text","marks":[{"type":"code"}],"text":"U"},{"type":"text","text":"."}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type Exclude = T extends U ? never : T;\n"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Соотношения:"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract | Exclude === T"},{"type":"text","text":" (декомпозиция множества)"}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract & Exclude === never"},{"type":"text","text":" (они непересекаются)"}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Тождество: "},{"type":"text","marks":[{"type":"code"}],"text":"Extract === Exclude>"}]}]}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Мини-пример:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type T = string | number | boolean;\ntype OnlyStrings = Extract; // string\ntype NotStrings = Exclude; // number | boolean\n"}]},{"type":"heading","attrs":{"textAlign":null,"level":1},"content":[{"type":"text","text":"Как "},{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" работает с union-типами?"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Дистрибутивно по "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":": каждый член объединения из "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":" проверяется на совместимость с "},{"type":"text","marks":[{"type":"code"}],"text":"U"},{"type":"text","text":"; проходящие — объединяются, не прошедшие — отбрасываются."}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"type R1 = Extract<\"a\" | 1 | true, string | number>; // \"a\" | 1\ntype R2 = Extract<{x:1} | {y:2}, {x:1}>; // {x:1}\ntype R3 = Extract; // never\n"}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Тонкости:"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"С "},{"type":"text","marks":[{"type":"code"}],"text":"any"},{"type":"text","text":":"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" → всегда "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":"."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" → "},{"type":"text","marks":[{"type":"code"}],"text":"any"},{"type":"text","text":" (из-за особых правил условных типов с "},{"type":"text","marks":[{"type":"code"}],"text":"any"},{"type":"text","text":")."}]}]}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"С "},{"type":"text","marks":[{"type":"code"}],"text":"unknown"},{"type":"text","text":":"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" → "},{"type":"text","marks":[{"type":"code"}],"text":"T"},{"type":"text","text":" (всё совместимо с "},{"type":"text","marks":[{"type":"code"}],"text":"unknown"},{"type":"text","text":")."}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","marks":[{"type":"code"}],"text":"Extract"},{"type":"text","text":" → "},{"type":"text","marks":[{"type":"code"}],"text":"never"},{"type":"text","text":" (кроме случаев, где "},{"type":"text","marks":[{"type":"code"}],"text":"unknown"},{"type":"text","text":" явно совместим с "},{"type":"text","marks":[{"type":"code"}],"text":"U"},{"type":"text","text":", что обычно не так)."}]}]}]}]},{"type":"listItem","content":[{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"С "},{"type":"text","marks":[{"type":"code"}],"text":"never"},{"type":"text","text":": всегда "},{"type":"text","marks":[{"type":"code"}],"text":"never"},{"type":"text","text":"."}]}]}]},{"type":"paragraph","attrs":{"textAlign":null},"content":[{"type":"text","text":"Практические кейсы:"}]},{"type":"codeBlock","attrs":{"language":"javascript"},"content":[{"type":"text","text":"// Достаём ключи-строки из смешанного union\ntype Keys = Extract; // string | number | symbol ∩ string = string\n// Фильтруем union по формату строки:\ntype HttpGet = Extract<\"GET\" | \"POST\" | \"PUT\", \"GET\" | \"HEAD\">; // \"GET\"\n// Выбираем только функции:\ntype Fn = Extract void) | { run(): void }, Function>; // () => void"}]},{"type":"paragraph","attrs":{"textAlign":null}}]}

Что делает Extract<T, U>?

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