arrays - 最后一项具有不同类型的元组(首先从其余元素开始)

Question

我有一个类型 Foo那是一个 Array可以包含任意数量的 Bar元素，带有可选的最后 Qux元素。

以下是一些有效数据的示例:

[]
[bar]
[qux]
[bar, qux]
[bar, bar, bar, bar, bar]
[bar, bar, bar, qux]

无效数据示例:

[qux, qux]
[qux, bar]
[bar, bar, qux, bar]
[bar, bar, qux, qux]

目前我把它作为 type Foo = Array<Bar | Qux> ，但这并没有捕捉到只有一个 Qux 的事实。是允许的，并且只能作为最后一项。

我不确定我是否应该期望 Typescript 能够表达这一点，或者如果可以实现它是否有任何实际的好处。

Answer 1

TS 4.2
允许 leading/middle rest elements在元组类型中。你现在可以写:

type WithLastQux = [...Bar[], Qux]

允许一个可选的最后 Qux (感谢@last-child):

type WithLastQux = [...Bar[], Qux] | Bar[]

测试:

// 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持窄的、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，最后一个可选的 Qux元素通过 [...Bar[], Qux?]不是直接可能的:

The only restriction is that a rest element can be placed anywhere in a tuple, so long as it’s not followed by another optional element or rest element. (docs)

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TS 4.0
在JS中， rest parameters始终需要是最后一个函数参数。 TS 4.0 附带一个名为 variadic tuple types 的功能，您可以将其视为“类型的其余参数”。
整洁的是:可变元组类型 can be placed anywhere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 .
示例: [bar, bar, <and arbitrary more>, qux]可以解释为可变参数元组 [...T, qux] ，其中 T代表所有 bar项目。
解决方案 1:分开 Assert类型

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"])

您可以替换 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

Playground
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 }

Playground

TS 3.9 及更早版本
如果您确实需要后跟最后一个元素的其余参数(规范只允许它们放在最后，请参阅@Nit 的回答)，这里有一个替代方案:
假设

存在并输入了具体的数组值(例如 as const )

数组大小最多 256 个元素(StringToNumber 硬编码大小限制)

您不想声明 Bar/Qux在 recursive manner 中声明类型:虽然技术上可行，但递归类型不受官方支持 till version 4.1 (注:更新)。

代码

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
};

测试

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是 mapped tuple type .每个键 K在 T是形式 "0" 的索引, "1" , "2" ， 等等。 。但这种类型是 string ，不是 number !
为了断言 Qux对于最后一个索引，我们需要转换 K返回 number为了与 T['length']进行比较，返回数组长度 number .由于编译器不支持动态 string to number conversion然而，我们已经使用 StringToNumber 创建了我们自己的转换器.
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