{"id":26387,"date":"2025-12-04T06:55:40","date_gmt":"2025-12-04T06:55:40","guid":{"rendered":"https:\/\/pokecon.jp\/job\/?p=26387"},"modified":"2025-12-04T06:55:40","modified_gmt":"2025-12-04T06:55:40","slug":"never-%e3%81%8c%e5%a5%bd%e3%81%8d-object-createnull","status":"publish","type":"post","link":"https:\/\/pokecon.jp\/job\/26387\/","title":{"rendered":"never \u304c\u597d\u304d – Object.create(null)"},"content":{"rendered":"\n<\/p>\n
\n

\u3053\u306e\u8a18\u4e8b\u306f \u306f\u3066\u306a\u30a8\u30f3\u30b8\u30cb\u30a2 Advent Calendar 2025<\/a> \u306e 4 \u65e5\u76ee\u306e\u8a18\u4e8b\u3067\u3059.<\/p>\n

\"\"id:susisu<\/a> \u3067\u3059. TypeScript \u306e never<\/code> \u578b\u304c\u305f\u3044\u3078\u3093\u5965\u3086\u304b\u3057\u3044\u306e\u3067\u898b\u3066\u3044\u3063\u3066\u304f\u3060\u3055\u3044.<\/p>\n

\u57fa\u790e\u7de8<\/h2>\n

\u307e\u305a\u306f never<\/code> \u578b\u304c\u3069\u3046\u3044\u3063\u305f\u3082\u306e\u306a\u306e\u304b\u3092\u898b\u3066\u307f\u307e\u3057\u3087\u3046.<\/p>\n

\u5024\u306e\u306a\u3044\u578b\u3068\u3057\u3066<\/h3>\n

\u4f8b\u3048\u3070 number<\/code> \u578b\u306b\u5bfe\u3057\u3066\u306f 42<\/code>, 3.14<\/code>, NaN<\/code> \u306a\u3069 , string<\/code> \u578b\u306b\u5bfe\u3057\u3066\u306f \"\"<\/code>, \"Hello\"<\/code> \u306a\u3069\u306e\u3088\u3046\u306b, \u4e00\u822c\u7684\u306a\u578b\u306b\u306f\u305d\u306e\u578b\u304c\u4ed8\u3051\u3089\u308c\u308b\u5024\u304c\u5b58\u5728\u3057\u307e\u3059.<\/p>\n

\u4e00\u65b9\u3067 never<\/code> \u578b\u306b\u306f\u305d\u3046\u3044\u3063\u305f\u5024\u304c\u5b58\u5728\u3057\u307e\u305b\u3093 (\u3053\u3046\u3044\u3063\u305f\u578b\u306f\u4e00\u822c\u306b\u30dc\u30c8\u30e0\u578b\u306a\u3069\u3068\u547c\u3070\u308c\u305f\u308a\u3057\u307e\u3059).
\nnever<\/code> \u578b\u306e\u5909\u6570\u306b\u306f\u4f55\u3092\u4ee3\u5165\u3057\u3088\u3046\u3068\u3057\u3066\u3082\u30a8\u30e9\u30fc\u306b\u306a\u308a\u307e\u3059.<\/p>\n

const<\/span> x: never<\/span> = 42<\/span>; \n<\/pre>\n
undefined<\/code> \u3084 null<\/code> \u3082\u300c\u5024\u304c\u5b58\u5728\u3057\u306a\u3044\u300d\u3068\u8aac\u660e\u3055\u308c\u308b\u3053\u3068\u304c\u3042\u308a\u307e\u3059\u304c, \u3053\u308c\u3089\u306f\u305b\u3044\u305c\u3044\u76ee\u7684\u306b\u5bfe\u3057\u3066\u5229\u7528\u53ef\u80fd\u306a\u5024 (\u5177\u4f53\u7684\u306a\u6570\u5024\u3084\u30aa\u30d6\u30b8\u30a7\u30af\u30c8\u306a\u3069) \u304c\u306a\u3044\u3068\u3044\u3046\u3060\u3051\u3067, undefined<\/code> \u3084 null<\/code> \u81ea\u4f53\u306f\u6b74\u3068\u3057\u305f\u5024\u3067\u3059.
\n\u305d\u306e\u305f\u3081\u5f53\u7136 never<\/code> \u578b\u306e\u5909\u6570\u306b\u306f\u4ee3\u5165\u3067\u304d\u307e\u305b\u3093.<\/p>\n
const<\/span> y: never<\/span> = undefined<\/span>; \nconst<\/span> z: never<\/span> = null<\/span>; \n<\/pre>\n
\u4efb\u610f\u306e\u578b\u306e\u90e8\u5206\u578b\u3068\u3057\u3066<\/h3>\n
\u578b S<\/code> \u306e\u5024\u3092\u578b T<\/code> \u306e\u5024\u3068\u3057\u3066\u6271\u3063\u3066\u3082\u554f\u984c\u304c\u306a\u3044\u3068\u304d, \u578b\u30b7\u30b9\u30c6\u30e0\u304c\u300c\u578b S<\/code> \u306f\u578b T<\/code> \u306e\u90e8\u5206\u578b\u3067\u3042\u308b\u300d\u3068\u5224\u65ad\u3057\u3066\u3053\u306e\u3088\u3046\u306a\u6271\u3044\u3092\u8a31\u3059\u3053\u3068\u304c\u3042\u308a\u307e\u3059.
\n\u4f8b\u3048\u3070 { id: string; name: string }<\/code> \u578b\u306e\u5024\u306f { id: string }<\/code> \u578b\u306e\u5024\u3068\u3057\u3066\u6271\u3063\u3066\u3082\u666e\u901a\u306f\u554f\u984c\u306a\u3044\u306e\u3067, TypeScript \u306e\u30b3\u30f3\u30d1\u30a4\u30e9<\/a>\u306f\u524d\u8005\u3092\u5f8c\u8005\u306e\u90e8\u5206\u578b\u3068\u307f\u306a\u3057\u307e\u3059.<\/p>\n
\u3055\u3066 never<\/code> \u578b\u306e\u5024\u306f\u3068\u3044\u3046\u3068\u5b58\u5728\u3057\u306a\u3044\u306e\u3067, \u3069\u3046\u6271\u3063\u3066\u3082\u554f\u984c\u304c\u8d77\u304d\u307e\u305b\u3093 (\u305d\u3082\u305d\u3082\u6271\u3046\u3053\u3068\u304c\u3042\u308a\u307e\u305b\u3093).
\n\u305d\u306e\u305f\u3081 TypeScript \u306e\u30b3\u30f3\u30d1\u30a4\u30e9<\/a>\u306f\u4efb\u610f\u306e\u578b T<\/code> \u306b\u5bfe\u3057\u3066\u300cnever<\/code> \u306f\u578b T<\/code> \u306e\u90e8\u5206\u578b\u3067\u3042\u308b\u300d\u3068\u5224\u65ad\u3057\u307e\u3059.<\/p>\n
\u5b9f\u969b\u306b never<\/code> \u578b\u306e\u5024\u304c\u5b58\u5728\u3059\u308b\u3068\u4eee\u5b9a\u3057\u3066 declare<\/code> \u3067\u5ba3\u8a00\u3057\u3066\u3042\u3052\u308b\u3068, \u305d\u306e\u5024\u3092\u4efb\u610f\u306e\u578b\u306e\u5024\u3068\u3057\u3066\u6271\u3048\u308b\u3053\u3068\u304c\u78ba\u8a8d\u3067\u304d\u307e\u3059.<\/p>\n
declare<\/span> const<\/span> x: never<\/span>;\n\nconst<\/span> y: number<\/span> = x; \nconst<\/span> z: string<\/span> = x; \n<\/pre>\n
\u7a7a\u306e\u30e6\u30cb\u30aa\u30f3\u578b\u3068\u3057\u3066<\/h3>\n
TypeScript \u306b\u306f number | string<\/code> \u306e\u3088\u3046\u306a\u30e6\u30cb\u30aa\u30f3\u578b\u304c\u5b58\u5728\u3057\u3066, JavaScript<\/a> \u6642\u4ee3\u304b\u3089\u3042\u308a\u304c\u3061\u306a\u300cnumber<\/code> \u3082\u3057\u304f\u306f string<\/code>\u300d\u3068\u3044\u3063\u305f\u66d6\u6627\u306a\u5024\u3082\u3046\u307e\u304f\u6271\u3046\u3053\u3068\u304c\u3067\u304d\u307e\u3059.<\/p>\n
\u3068\u3053\u308d\u3067 never<\/code> \u578b\u306b\u306f\u5024\u304c\u5b58\u5728\u3057\u306a\u3044\u3053\u3068\u304b\u3089\u300c\u7a7a\u306e\u30e6\u30cb\u30aa\u30f3\u578b\u300d\u3068\u307f\u306a\u3059\u3053\u3068\u304c\u3067\u304d\u3066, \u5b9f\u969b\u306b TypeScript \u306e\u30b3\u30f3\u30d1\u30a4\u30e9<\/a>\u306f\u305d\u306e\u3088\u3046\u306a\u6271\u3044\u3092\u3057\u3066\u304f\u308c\u307e\u3059.
\n\u307e\u305f\u3053\u308c\u306f\u578b\u306e\u6f14\u7b97 |<\/code> \u306e\u5358\u4f4d\u5143<\/a>\u3068\u3044\u3046\u3053\u3068\u3067\u3082\u3042\u308a, \u4efb\u610f\u306e\u578b T<\/code> \u306b\u5bfe\u3057\u3066 T | never<\/code> = never | T<\/code> = T<\/code> \u3067\u3059.
\n\u300cnumber<\/code> \u3082\u3057\u304f\u306f never<\/code> \u306e\u5024\u300d\u306f\u8981\u3059\u308b\u306b\u300cnumber<\/code> \u306e\u5024\u300d\u3067\u3042\u308b\u3068\u3044\u3046\u3053\u3068\u3067\u3059\u306d.<\/p>\n
type <\/span>X <\/span>=<\/span> number<\/span> |<\/span> never<\/span>;\n\n<\/pre>\n
\u5fdc\u7528\u7de8<\/h2>\n
never<\/code> \u578b\u304c\u4f55\u3067\u3042\u308b\u304b\u306b\u3064\u3044\u3066\u8aac\u660e\u3057\u3066\u304d\u307e\u3057\u305f\u304c, TypeScript \u3084 never<\/code> \u578b\u306b\u99b4\u67d3\u307f\u306e\u306a\u3044\u65b9\u306b\u3068\u3063\u3066\u306f, \u304a\u305d\u3089\u304f\u3053\u308c\u3060\u3051\u3067\u306f\u4f7f\u3044\u65b9\u306e\u898b\u5f53\u304c\u3064\u304d\u3065\u3089\u3044\u304b\u3068\u601d\u3044\u307e\u3059.
\n\u3068\u3044\u3046\u3053\u3068\u3067\u5177\u4f53\u7684\u306a\u4f7f\u3044\u65b9\u3092\u3044\u304f\u3064\u304b\u7d39\u4ecb\u3057\u307e\u3059.<\/p>\n
\u7d76\u5bfe\u306b\u5024\u3092\u8fd4\u3055\u306a\u3044\u95a2\u6570<\/h3>\n
\u4f8b\u3048\u3070\u5e38\u306b\u7121\u9650\u30eb\u30fc\u30d7\u3057\u305f\u308a\u30a8\u30e9\u30fc\u3092 throw \u3057\u305f\u308a\u3059\u308b\u95a2\u6570\u306f\u5024\u3092\u8fd4\u3057\u307e\u305b\u3093.
\n\u3053\u3046\u3044\u3063\u305f\u95a2\u6570\u306e\u623b\u308a\u5024\u3068\u3057\u3066 never<\/code> \u304c\u4f7f\u3048\u307e\u3059.
\n\u30b7\u30b0\u30cd\u30c1\u30e3<\/a>\u304b\u3089\u632f\u308b\u821e\u3044\u304c\u63a8\u6e2c\u3067\u304d\u3066\u4fbf\u5229\u3067\u3059\u306d.<\/p>\n
function<\/span> infiniteLoop<\/span>(): never<\/span> {<\/span>\n  while<\/span> (true<\/span>) {}<\/span>\n}<\/span>\n\nfunction<\/span> throw_<\/span>(error<\/span>: <\/span>Error<\/span>): never<\/span> {<\/span>\n  throw<\/span> error;\n}<\/span>\n<\/pre>\n
JavaScript<\/a> \u3084 TypeScript \u306e throw<\/code> \u306f\u5f0f\u3067\u306f\u3042\u308a\u307e\u305b\u3093\u304c, \u4e0a\u8a18\u306e\u3088\u3046\u306a throw_<\/code> \u95a2\u6570\u3092\u5b9a\u7fa9\u3057\u3066\u304a\u304f\u3068 throw<\/code> \u5f0f\u306e\u3088\u3046\u306a\u3082\u306e\u3068\u3057\u3066\u4fbf\u5229\u306b\u4f7f\u3048\u307e\u3059.
\n\u4efb\u610f\u306e\u578b T<\/code> \u306b\u5bfe\u3057\u3066 T | never<\/code> = T<\/code> \u306a\u3053\u3068\u3082\u3042\u308a, \u4e0b\u8a18\u306e\u4f8b\u306e foo<\/code> \u306b\u5bfe\u3057\u3066\u306f\u304d\u3061\u3093\u3068 options.foo<\/code> \u304c\u5b58\u5728\u3059\u308b\u6642\u306e\u578b\u304c\u63a8\u8ad6\u3055\u308c\u307e\u3059.<\/p>\n
const<\/span> foo = options?.foo ?? throw_(new<\/span> Error<\/span>(\"'foo' is required\"<\/span>));\n<\/pre>\n
declare<\/span> const<\/span> x: number<\/span> |<\/span> string<\/span>;\n\nif<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  infiniteLoop();\n  \n}<\/span>\n\nconst<\/span> y: string<\/span> = x; \n<\/pre>\n
\u6761\u4ef6\u5206\u5c90\u306e\u7db2\u7f85\u6027\u30c1\u30a7\u30c3\u30af<\/h3>\n
\u4e0a\u306e\u4f8b\u3067\u3082\u3057\u308c\u3063\u3068\u767b\u5834\u3057\u3066\u3044\u307e\u3059\u304c, TypeScript \u306b\u306f\u30d5\u30ed\u30fc\u89e3\u6790\u304c\u3042\u3063\u3066, if<\/code> \u3084 switch<\/code> \u306a\u3069\u3067\u5909\u6570\u306e\u578b\u3092\u7d5e\u308a\u8fbc\u3080\u3088\u3046\u306a\u6761\u4ef6\u3092\u66f8\u304f\u3068, \u5b9f\u969b\u306b\u305d\u306e\u5909\u6570\u304c\u7d5e\u308a\u8fbc\u307e\u308c\u305f\u5f8c\u306e\u578b\u3068\u3057\u3066\u4f7f\u3048\u308b\u3088\u3046\u306b\u306a\u308a\u307e\u3059.<\/p>\n
declare<\/span> const<\/span> x: number<\/span> |<\/span> string<\/span>;\n\nif<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  const<\/span> y: number<\/span> = x; \n}<\/span> else<\/span> if<\/span> (typeof<\/span> x === \"string\"<\/span>) {<\/span>\n  const<\/span> z: string<\/span> = x; \n}<\/span> else<\/span> {<\/span>\n  throw<\/span> new<\/span> Error<\/span>(\"unexpected value\"<\/span>);)\n}<\/span>\n<\/pre>\n
\u3067\u306f\u3053\u306e\u6700\u5f8c\u306e else<\/code> \u306e\u4e2d\u3067 x<\/code> \u306e\u578b\u306f\u3069\u3046\u306a\u3063\u3066\u3044\u308b\u3067\u3057\u3087\u3046\u304b?
\nnumber<\/code> \u3067\u3082 string<\/code> \u3067\u3082\u306a\u304f\u4ed6\u306b\u53ef\u80fd\u6027\u304c\u306a\u3044\u3068\u3044\u3046\u3053\u3068\u306f, \u305d\u3046 never<\/code> \u3067\u3059\u306d.<\/p>\n
\u3053\u3046\u3044\u3063\u305f if<\/code> \u3084 switch<\/code> \u306e\u6761\u4ef6\u5206\u5c90\u304c\u5168\u3066\u306e\u30d1\u30bf\u30fc\u30f3\u3092\u7db2\u7f85\u3057\u3066\u3044\u308b\u304b\u3092\u30c1\u30a7\u30c3\u30af\u3059\u308b\u306e\u306b never<\/code> \u304c\u4f7f\u3048\u307e\u3059.
\n\u4f8b\u3048\u3070\u4ee5\u4e0b\u306e\u3088\u3046\u306b never<\/code> \u578b\u306e\u5909\u6570\u306b x<\/code> \u3092\u4ee3\u5165\u3059\u308b\u30b3\u30fc\u30c9\u3092\u66f8\u3044\u3066\u3042\u3052\u308b\u3068, x<\/code> \u306b\u5bfe\u3059\u308b\u6761\u4ef6\u304c\u7db2\u7f85\u3055\u308c\u3066\u4ed6\u306e\u53ef\u80fd\u6027\u304c\u6b8b\u3063\u3066\u3044\u306a\u3044\u3053\u3068\u3092\u78ba\u8a8d\u3067\u304d\u307e\u3059.<\/p>\n
if<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> if<\/span> (typeof<\/span> x === \"string\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> {<\/span>\n  const<\/span> _: never<\/span> = x; \n  throw<\/span> new<\/span> Error<\/span>(\"unexpected value\"<\/span>);\n}<\/span>\n<\/pre>\n
if<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> {<\/span>\n  const<\/span> _: never<\/span> = x; \n  throw<\/span> new<\/span> Error<\/span>(\"unexpected value\"<\/span>);\n}<\/span>\n<\/pre>\n
\u5f0f\u306e\u578b\u3092\u78ba\u8a8d\u3059\u308b\u305f\u3081\u306e\u69cb\u6587 satisfies<\/code> \u3092\u4f7f\u3063\u3066\u3082\u540c\u3058\u3053\u3068\u304c\u3067\u304d\u307e\u3059.<\/p>\n
if<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> if<\/span> (typeof<\/span> x === \"string\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> {<\/span>\n  x satisfies never;\n  throw<\/span> new<\/span> Error<\/span>(\"unexpected value\"<\/span>);\n}<\/span>\n<\/pre>\n
\u7d76\u5bfe\u306b\u547c\u3073\u51fa\u305b\u306a\u3044\u95a2\u6570<\/h3>\n
\u5148\u307b\u3069\u306e\u7d76\u5bfe\u306b\u5024\u3092\u8fd4\u3055\u306a\u3044\u95a2\u6570\u3068\u306f\u9006\u306b, \u95a2\u6570\u306e\u5f15\u6570\u306b never<\/code> \u3092\u4f7f\u3046\u3053\u3068\u3067, \u7d76\u5bfe\u306b\u547c\u3073\u51fa\u305b\u306a\u3044\u95a2\u6570\u304c\u4f5c\u308c\u307e\u3059.<\/p>\n
function<\/span> nonCallable<\/span>(x<\/span>: <\/span>never<\/span>): void<\/span> {}<\/span>\n\nnotCallable(42<\/span>); \n<\/pre>\n
function<\/span> unreachable<\/span>(value<\/span>: <\/span>never<\/span>): never<\/span> {<\/span>\n  console<\/span>.error(\"'unreachable' called\"<\/span>, value);\n  throw<\/span> new<\/span> Error<\/span>(\"'unreachable' called\"<\/span>);\n}<\/span>\n\ndeclare<\/span> const<\/span> x: number<\/span> |<\/span> string<\/span>;\n\nif<\/span> (typeof<\/span> x === \"number\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> if<\/span> (typeof<\/span> x === \"string\"<\/span>) {<\/span>\n  \n}<\/span> else<\/span> {<\/span>\n  unreachable(x);\n}<\/span>\n<\/pre>\n
\u307e\u305f\u578b\u30ec\u30d9\u30eb\u30d7\u30ed\u30b0\u30e9\u30df\u30f3\u30b0\u3068\u7d44\u307f\u5408\u308f\u305b\u308b\u3068, \u5f15\u6570\u304c\u7279\u5b9a\u306e\u6761\u4ef6\u3092\u6e80\u305f\u3059\u6642\u306e\u307f\u547c\u3073\u51fa\u305b\u308b\u95a2\u6570\u3092\u4f5c\u308b\u3053\u3068\u304c\u3067\u304d\u307e\u3059.
\n\u8a73\u3057\u304f\u306f\u4ee5\u4e0b\u306e\u8a18\u4e8b\u3092\u53c2\u7167.<\/p>\n
\u3053\u308c\u3082\u4f7f\u3044\u6240\u304c\u3042\u3063\u3066, \u4e0a\u8a18\u306e\u3088\u3046\u306b\u7db2\u7f85\u6027\u30c1\u30a7\u30c3\u30af\u3092\u3057\u305f\u306f\u305a\u306a\u306e\u306b\u4e07\u304c\u4e00\u5909\u306a\u5024\u304c\u6df7\u5165\u3057\u3066\u3057\u307e\u3063\u305f\u6642\u306e\u30a8\u30e9\u30fc\u51e6\u7406\u3092\u5171\u901a\u5316<\/a>\u3059\u308b\u3068\u304d\u306a\u3093\u304b\u306b\u4fbf\u5229\u3067\u3059.
\n\u4e0b\u8a18\u306e unreachable<\/code> \u306f\u901a\u5e38\u306f\u7d76\u5bfe\u306b\u547c\u3073\u51fa\u305b\u306a\u3044\u306f\u305a\u3067\u3059\u304c, \u5909\u6570\u304c never<\/code> \u578b\u3068\u3044\u3046\u3053\u3068\u306b\u306a\u3063\u3066\u3044\u308b\u6642\u306b\u9650\u3063\u3066\u306f\u547c\u3073\u51fa\u3059\u3053\u3068\u304c\u3067\u304d\u307e\u3059.
\n\u3064\u3044\u3067\u306b\u3053\u308c\u306f\u7d76\u5bfe\u306b\u5024\u3092\u8fd4\u3055\u306a\u3044\u95a2\u6570\u3067\u3082\u3042\u308a\u307e\u3059\u306d.<\/p>\n
\u3082\u3057 if<\/code> \u304c\u6f0f\u308c\u3066\u3044\u308b\u3068\u3053\u306e\u4ee3\u5165\u6587\u306f\u30b3\u30f3\u30d1\u30a4\u30eb<\/a>\u30a8\u30e9\u30fc\u306b\u306a\u308b\u306e\u3067, \u5b9f\u884c\u524d\u306b\u8aa4\u308a\u306b\u6c17\u304c\u3064\u3051\u307e\u3059.<\/p>\n
\u307e\u305f\u95a2\u6570\u304c\u7d76\u5bfe\u306b\u5024\u3092\u8fd4\u3055\u306a\u3044\u3068\u3044\u3046\u3053\u3068\u306f, \u305d\u306e\u95a2\u6570\u3092\u547c\u3073\u51fa\u3057\u305f\u5f8c\u306e\u30b3\u30fc\u30c9\u306f\u7d76\u5bfe\u306b\u5b9f\u884c\u3055\u308c\u306a\u3044\u3068\u3044\u3046\u3053\u3068\u306a\u306e\u3067, \u30b3\u30f3\u30d1\u30a4\u30e9<\/a>\u304c\u6c17\u3092\u5229\u304b\u305b\u3066\u578b\u3092\u7d5e\u308a\u8fbc\u3093\u3067\u304f\u308c\u305f\u308a\u3057\u307e\u3059.<\/p>\n