TS 4.2
allows 前导/中间休息元素 https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/#non-trailing-rests在元组类型中。你现在可以写：
type WithLastQux = [...Bar[], Qux]
为了允许optional last Qux（感谢@last-child）：
type WithLastQux = [...Bar[], Qux] | Bar[]
Test:
// ok
const t11: WithLastQux = ["qux"]
const t12: WithLastQux = ["bar", "qux"]
const t13: WithLastQux = ["bar", "bar", "bar", "qux"]
const t14: WithLastQux = ["bar"]
const t15: WithLastQux = []
// error
const t16: WithLastQux = ["qux", "qux"]
const t17: WithLastQux = ["qux", "bar"]
作为扩展，定义一个通用函数助手来保留狭窄的、固定大小的元组类型：
function foo<T extends WithLastQux>(t: T): T { return t }
foo(["bar", "qux"])
// type: function foo<["bar", "qux"]>(t: ["bar", "qux"]): ["bar", "qux"]
请注意，一个optional last Qux元素通过[...Bar[], Qux?]不是直接可能的：
唯一的限制是剩余元素可以放置在元组中的任何位置，只要它后面没有另一个可选元素或剩余元素即可。 （docs https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/#non-trailing-rests)
操场 https://www.typescriptlang.org/play?#code/C4TwDgpgBAQghgJygXigIgEaLQbgFCiRQCKArgB4roCOFaB40A6gJbAAWAMnAM7BmVUAbQB0Y+AiEBdADQkKUqAB9YiaXjwB6TVAD2AazwBjXQDs+UYAEYrALiisO3PgKpC0tcminGzF6wBM9o5cvPwUbpjYch50PibmwJZWAMzBbKEuEcJRCGgxWHkF0TRxvonJACzpTmGuOYXe5f5WAKw1meGCUOpaOhAICLoIzUnWAGwdzl2RnvmlXoqjyQDsU3XZPbFexXk+fVAAggA2wAOmcMAsAG4Q9kYIEJfQAOYQpgMsRlAJfAhwLFMZwAJlAAGakUxGK5mKAHYC6KBgR48Aa3cEscgQYEAWh4LAAXtBgKQwMcIHJgWYAORJADuLGB70siMQ-xAeAhUJhpnBul0AB4ACpQCDkM6mYE8BwZaYCAB8AApgPYhQBKVVQADeUEeJIQvKSAF8NNo9IYwfzFdI1Zyre5GlJbZbdNbtt5nfbcvN3U67a6HSVvbsfXM-X0BkMRi63XMQ36Y+44wsPUA
TS 4.0
In JS, 其余参数 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters始终需要是最后一个函数参数。 TS 4.0 具有一个称为可变元组类型 https://github.com/microsoft/TypeScript/pull/39094，您可以在这里将其视为“类型的剩余参数”。
巧妙的是：可变元组类型可以放置在任何地方 https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#variadic-tuple-types在函数参数中，而不仅仅是在最后。我们现在可以定义任意数量的bar带有可选最后一个元素的项目qux.
例子：[bar, bar, <and arbitrary more>, qux]可以解释为可变参数元组[...T, qux], where T代表所有的bar items.
解决方案1：分开Assert type
type Assert<T extends readonly (Bar | Qux)[]> =
T extends readonly [] ? T :
T extends readonly [...infer I, infer U] ?
I extends [] ? T :
I extends Bar[] ? T : readonly [...{ [K in keyof I]: Bar }, U] :
never
function assert<T extends readonly (Bar | Qux)[] | readonly [Bar | Qux]>(
a: Assert<T>): T { return a as T }
// OK
const ok1 = assert([])
const ok2 = assert(["bar"])
const ok3 = assert(["qux"])
const ok4 = assert(["bar", "qux"])
const ok5 = assert(["bar", "bar", "bar"])
const ok6 = assert(["bar", "bar", "bar", "qux"])
// errors
const err1 = assert(["qux", "qux"])
const err2 = assert(["qux", "bar"])
const err3 = assert(["bar", "bar", "qux", "bar"])
const err4 = assert(["bar", "bar", "qux", "qux"])
你可以替换assert具有某种“哨兵”类型的辅助函数：
type Sentinel<T, U extends T> = U
const arr1 = ["bar", "bar"] as const
const arr2 = ["qux", "bar"] as const
type Arr1Type = Sentinel<Assert<typeof arr1>, typeof arr1> // OK
type Arr2Type = Sentinel<Assert<typeof arr2>, typeof arr2> // error
操场 https://www.typescriptlang.org/play?#code/C4TwDgpgBAQghgJygXigIgEaLQKFJKARQFcAPFdARzNxwHo6oEI4ATAewDsAbEKYgM4RWUABYRmUAJacoAYzhCocKMGJhu0KQKhhFQkQDcpKgAaL5XAcFN5w0AIIChCYAB4AKlAilgETqw6zGxcvFAAFPBIAD5EZACUANoAugB8KDhQWVBePn4BQSwcPHwpUAD8OVAAXJnZub7%20gUxFoaUAdJ0yAGYSUACSADTSnL1IAKrJFXVZ-d6NBVBllV612QPz%20c1Ry1XVLSElS53tAN5LANIjUADWECDs3QPJ%201FQAL7DkzUzUJwQhgkODsBA8AAYKE4XO4UqkQdAPABGSHOCQwzDYNLwnIAJhR0LciTQ1FIaCx%20ARAGZ8WjCRiEGhhsSaOT7DkACw01x0rAMpm8xnoAWs0EAVi56IF-Ow0r5VBZcPojAAShAALZwMCqdSaKDGFSJE7nYLFMKJK4yW73R7PV6ID5fKbddhIDAQYB%20JASBAuqCsbQaOAgbFIgD6AFEEEhUFDaUSSYLmaSRQicRGoxK6QnZWS4RScpT09GoLHuUSpUKZfLSTmUxyi5ny1X6Yns9Xc8CGAdTaU3rESKQnS65BAdB4AMqqdgjMbajSj4aWhRKB7EP1cADkwH4SnMOjkVhsOG6xE4cmAUi4ylR3IaW0KhzCkXt-YSZViJraSz7cUHqXCcD7KW7geKk8T7F4py-MwagILIKgWB4ODvJ2jAAPIXDgB6cNYUDsDcyKoPoaLhCk8RYYeeE3HiRE3sApEtsk5HYbh%20HUrR0IMQmTEUTh274ZyHEkU2cpJmSzGUfh4pCa4DEVi2tYSXxVEAGwUMRskiYm8nydx5FKt4UYugIvG4d6hHXpx8Y0Eyemmdu3o0ZZwliYp9mGQg7HOZpCmVqJbaMUpZlRoJ3n0VpOa2TZ7Y8di47%20Be-zcJ4XybE0Y7pKg4zuYgCAWRFflktelh8TlUZOdZNaFVMFgscA2IOFGiIeGyqDxZwiUQMlwFuBSNq5YiqTDH1TwDXmbKNQgOItQQbUJTIXVuD1I3KOVQ2qPY-VrUAA
解决方案2：函数重载
type Bar = "bar"
type Qux = "qux"
function assert<T extends Bar[] = []>(
arg: (Qux | Bar)[] extends [...T, Qux] ? never : [...T, Qux]): [...T, Qux]
function assert<T extends Bar[] = []>(arg: [...T]): [...T]
function assert<T extends Bar[] = []>(arg: [...T, Qux?]) { return arg }
操场 https://www.typescriptlang.org/play?#code/C4TwDgpgBAQghgJygXigIgEaLQKFJKARQFcAPFdARzNxwDNiA7AY2AEsB7RqOAZ14gJgAHgAqUCKWARGAE16xEAbQC6FVQD4AFDih6eCAOYAuKFpLkAPooQBKVRKkz5UJQDoPogDREyagPxQjBAAboJQpu6ePhYqtpEebt6+pCr0TKyc3HwCQmKO0nIK8AgOqJpaiCauiaJxCZ5pDCzsXDz8giLikoUuJWWuKtpVDUkxZP5xUADeUAgQwMQI2UZQAL44OAD0W1AA8gDSOMxcvMBQHADWAIwUOZ1aqrbHp+dXAEx3HUKPmNhxL0YZwulwAzF9csBftRSGgAScgW9LgAWCEPJR-BBoHxoGFw54I4FXACsaJ+GKwWJxlOx6Bp8NeIIAbGSoRTsNSOXSubiaADtrtBAgOAheIDgULbqh7uTebCcXiGYiJAgEJ9pd82XLaZj8eLzkLwRrIb8aZyqVQaOa9YSDarUcb0brrQqrZbYXEgA
TS 3.9 及更早版本
如果您确实需要剩余参数后跟最后一个元素（规范只允许它们位于最后，请参阅@Nit的答案），这里有一个替代方案：
假设
- 存在并键入具体的数组值（例如
as const)
- 数组大小最多 256 个元素（
StringToNumber硬编码的大小限制）
- 你不想声明
Bar/Qux断言 a 中的类型递归方式 https://github.com/microsoft/TypeScript/issues/26223：虽然技术上可行，但递归类型不受官方支持直到4.1版本 https://github.com/microsoft/TypeScript/pull/40002 (注：已更新).
Code
type Bar = { bar: string };
type Qux = { qux: number };
// asserts Qux for the last index of T, other elements have to be Bar
type AssertQuxIsLast<T extends readonly any[]> = {
[K in keyof T]: StringToNumber[Extract<K, string>] extends LastIndex<T>
? T[K] extends Qux
? Qux
: never
: T[K] extends Bar
? Bar
: never;
};
// = calculates T array length minus 1
type LastIndex<T extends readonly any[]> = ((...t: T) => void) extends ((
x: any,
...u: infer U
) => void)
? U["length"]
: never;
// TS doesn't support string to number type conversions ????,
// so we support (index) numbers up to 256
export type StringToNumber = {
[k: string]: number;
0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9, 10: 10, 11: 11, 12: 12, 13: 13, 14: 14, 15: 15, 16: 16, 17: 17, 18: 18, 19: 19, 20: 20, 21: 21, 22: 22, 23: 23, 24: 24, 25: 25, 26: 26, 27: 27, 28: 28, 29: 29, 30: 30, 31: 31, 32: 32, 33: 33, 34: 34, 35: 35, 36: 36, 37: 37, 38: 38, 39: 39, 40: 40, 41: 41, 42: 42, 43: 43, 44: 44, 45: 45, 46: 46, 47: 47, 48: 48, 49: 49, 50: 50, 51: 51, 52: 52, 53: 53, 54: 54, 55: 55, 56: 56, 57: 57, 58: 58, 59: 59, 60: 60, 61: 61, 62: 62, 63: 63, 64: 64, 65: 65, 66: 66, 67: 67, 68: 68, 69: 69, 70: 70, 71: 71, 72: 72, 73: 73, 74: 74, 75: 75, 76: 76, 77: 77, 78: 78, 79: 79, 80: 80, 81: 81, 82: 82, 83: 83, 84: 84, 85: 85, 86: 86, 87: 87, 88: 88, 89: 89, 90: 90, 91: 91, 92: 92, 93: 93, 94: 94, 95: 95, 96: 96, 97: 97, 98: 98, 99: 99, 100: 100, 101: 101, 102: 102, 103: 103, 104: 104, 105: 105, 106: 106, 107: 107, 108: 108, 109: 109, 110: 110, 111: 111, 112: 112, 113: 113, 114: 114, 115: 115, 116: 116, 117: 117, 118: 118, 119: 119, 120: 120, 121: 121, 122: 122, 123: 123, 124: 124, 125: 125, 126: 126, 127: 127, 128: 128, 129: 129, 130: 130, 131: 131, 132: 132, 133: 133, 134: 134, 135: 135, 136: 136, 137: 137, 138: 138, 139: 139, 140: 140, 141: 141, 142: 142, 143: 143, 144: 144, 145: 145, 146: 146, 147: 147, 148: 148, 149: 149, 150: 150, 151: 151, 152: 152, 153: 153, 154: 154, 155: 155, 156: 156, 157: 157, 158: 158, 159: 159, 160: 160, 161: 161, 162: 162, 163: 163, 164: 164, 165: 165, 166: 166, 167: 167, 168: 168, 169: 169, 170: 170, 171: 171, 172: 172, 173: 173, 174: 174, 175: 175, 176: 176, 177: 177, 178: 178, 179: 179, 180: 180, 181: 181, 182: 182, 183: 183, 184: 184, 185: 185, 186: 186, 187: 187, 188: 188, 189: 189, 190: 190, 191: 191, 192: 192, 193: 193, 194: 194, 195: 195, 196: 196, 197: 197, 198: 198, 199: 199, 200: 200, 201: 201, 202: 202, 203: 203, 204: 204, 205: 205, 206: 206, 207: 207, 208: 208, 209: 209, 210: 210, 211: 211, 212: 212, 213: 213, 214: 214, 215: 215, 216: 216, 217: 217, 218: 218, 219: 219, 220: 220, 221: 221, 222: 222, 223: 223, 224: 224, 225: 225, 226: 226, 227: 227, 228: 228, 229: 229, 230: 230, 231: 231, 232: 232, 233: 233, 234: 234, 235: 235, 236: 236, 237: 237, 238: 238, 239: 239, 240: 240, 241: 241, 242: 242, 243: 243, 244: 244, 245: 245, 246: 246, 247: 247, 248: 248, 249: 249, 250: 250, 251: 251, 252: 252, 253: 253, 254: 254, 255: 255
};
Tests
const arr1 = [{ bar: "bar1" }, { bar: "foo1" }, { qux: 42 }] as const
const arr2 = [{ bar: "bar2" }, { bar: "foo2" }] as const
const arr3 = [{ qux: 42 }] as const
const typedArr1: AssertQuxIsLast<typeof arr1> = arr1
const typedArr2: AssertQuxIsLast<typeof arr2> = arr2 // error (OK)
const typedArr3: AssertQuxIsLast<typeof arr3> = arr3
说明
AssertQuxIsLast is a 映射元组类型 https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-1.html#mapped-types-on-tuples-and-arrays。每个键K in T是以下形式的索引"0","1","2"， ETC。 。但这种类型是一个string, not a number!
为了断言Qux对于最后一个索引，我们需要转换K回到number为了与它进行比较T['length']，返回数组长度number。由于编译器不支持动态字符串到数字的转换 https://github.com/microsoft/TypeScript/issues/26223然而，我们已经创建了自己的转换器StringToNumber.
操场 https://www.typescriptlang.org/play/#code/C4TwDgpgBAQghgJygXigbygI0QLigZ2AQEsA7AcygF8BuAWAChRIoBFAVwA8V0oBHLnlLsAtpghJajRgHoZUOPnwTg+NlygAzAPZJgAC2gAbRcChkAJhG7bNUACoAaKNoMSoEIxBERSqqPpwAG7QwNpY0PAIjMzQAIJKKhycAJL4ADKmADz2HpzAvhZqCBBwFtqkRiAKpCAA2gC6AHw8aIxQUHUA0uakUADWECC2Dg14AMpEZOT22gByouIIdQCi+QhwAMbAWV3OhCQUTQ15BaRFUJmEKefWOU3tHVAA-A7dJ9ZnF8mPTy-qnF+TyEEBC0QYfzw9nep0KaiiQNeCIhwKgpFBEnoDCkDFk8lQmzgRk27BMBTUuUQG2qXgoBigIjI7DUAEYYuBoFdgDcrJwcrDzsVSuVKtU4LVGi1UAAKaUAOgVwChAEoUC0gtpiBZVZ84VBZY9OHhxSBHI8FXL2HgyJp3ABVRiq5DqzXax6vO11ABEtPIBi9DUeILBWLxDnGUHKEHwpAA5GZ8OwwGBdAmphQoGE0Yt3LEoJsKmD8MQKmpALwbgFI9s0MOQEcIAd2gieTqf1lmsquEYgkaiTmfCACYAKwANkY1hTCDMecmhxm8xzSFQbRRdX6eAO0zG2e7CCxHQADHgD84WXgWc4B3gB84AMx4W-OAAseCfziHeCHzhHeBHzgA7Hg-7OAAHHgIHOAAnHgkGnkeUAsieCFnshp5XghN4IfeWGni+CFvghH6EaeP4IX+CGARRp5gQhEEIdB9GXvBA5IQOKFsZe6EDphA7Ybxl54QOBHDteX5QAOpESZelEDsB4k0QOdEDgxyl3vBt5IbeKFaXe6G3pht7YYZd54beBG3kRFl3qRt7kbelH2XeNG3nRt4MW5z7wU+SFPihvnPuhT6YU+2Ehc+eFPgRT5EdFz6kU+5FPpRSXPjRT50U+DGZe+8FDkhQ4oQV77oUOmFDth5XvnhQ4EUORF1e+pGju+lFDnJQ40R174MUOsFQCO8EDd+KEjhe-XoSOmEjth03fnhI4ESORFLd+pEjuRI6UZt340SOdEjgxB0AfB-5If+KHnQB6H-ph-7YXdAF4f+BH-kRr0AaR-7kf+lE-QBNH-nR-4McDoHwSBSEgShUOgehIGYSB2GI6BeEgQRIFERjoGkSB5EgZR+OgTRIF0SBDFk1B8GQUhkEobTUHoZBmGQdhLNQXhkEEZBRHc1BpGQeRkGUULUE0ZBdGQQxktwfBiFIYhKEK3B6GIZhiHYercF4YhBGIURetwaRiHkYhlGm3BNGIXRiEMTbp6IeeiH24rLJja7KssmrLIa979vayyussvrQf20bLImyyZuR-blsstbLK2wnaGyyxaGKxxGEq9xaEa-xGHa0JaH68OaFG1JGFm7JaGW4paG26pWGyxpp5aeeOlYSr+ktxrxlYdrZkt-rVlYUbtkt2bjlYZbLkt7bHn4bL3m4Yr-n4SrQW4RrYX4drkW4frsX4UbCW4WbKX4Zb6W4bb2WEbLeWngV55FYRKulY-GuVYR2s1Y-+sNYRI2zVCJmzao-S2XVCK216iRWWQ0yKK1GiRFWk0SIa1mmRbWC0SL6xWmRI260SJm22mRS2e0SK2yOhRWWp1TznXPJdCiKsbp0I1g9Ci2tnp0P1u9CiRsvp0LNn9CiltAZ0NtqDWissIbUUVjDWiKt4bUQ1sjWi2s0bUX1ljWiRtcbUTNoTWilsSbUVthTeistqanlpueem9EVZM2sRrNm9Ftac2sfrXm9EjYC2sWbEW9FLbi2sbbaW4kDzMQiUxdiB4xosS4geHiB4+LJKYoJA8wkDxERYmJFikkDzkRYjJA8ckWIKQPEpA8KkqmXgduJJ29T2Ku1qVxT2tS+K+3qYJAOtTskh3qZJcOtSZLR3qQpOOtSVJJ3Eixa8qcZnsQztxLi2cZl8TztxQShcZnZJLjMyS5duIySrjMhStcZkqQbrxZizdxKtzuXE-S14u53L4r3XigkB53OycPXikkx53JkpPXiCkZ53JUvPISzEl7iV8teVeQkuIb1hXxbeQlBJ71hdkw+QlJIn1hTJc+QkFJX1hSpW+w5mIP3Ek-GlcTSqiR4uVUSj4aWCV-jS7JdVGA4kYAWUghAFAIAQCyHgdQMDYAQHgL0kqWRemoM4CVuAoBeh0NoOVCreACCNFAIK1ATiKHzKWYAfLjVCoQAOMVSqpUqslQOeVVBFVYGVaq7Q2h7X6oUGoflhBTUCrMFSW8Vr+CCF1ZaqgBrvXGukAwH104OQWDiMKlCCRlBTmSGkLkWRYgjCpCyKU5q2SxrNbERNwr0KpqSFwTN2Qc12CpAOAtDaoC1gkAgXQ+oADyXRlR+sFaWpNCBsKVvTdWjItaOS5uFbeJt07GBAA
