使用字符串文字数组作为类型

Question

我想使用单个数组 const fruits = ['banana', 'apple', 'orange'] 作为普通数组，并作为 type.

我应该能够做到这一点： const x: fruits // => only accepts 'banana', 'apple' or 'orange'

并且还能够做到这一点： @IsIn(fruits)

我试过将数组声明为<const>，例如：

const fruits = <const>['banana', 'apple', 'orange']
type Fruits = typeof fruits[number] // this evaluates to type: "banana" | "apple" | "orange"

但是@IsIn(fruits)会return出现以下错误：

Argument of type 'readonly ["banana", "apple", "orange"]' is not assignable to parameter of type 'any[]'.
  The type 'readonly ["banana", "apple", "orange"]' is 'readonly' and cannot be assigned to the mutable type 'any[]'.ts(2345)

所以我想如果我创建两个数组，一个普通数组和一个只读数组，它应该可以工作。所以我尝试了这个：

const fruits = ['banana', 'apple', 'orange']
const fruits_readonly: <const>[...fruits]
type Fruits = typeof fruits_readonly[number]

但现在 Fruits 评估为 type: string 而不是 type: "banana" | "apple" | "orange"。

Answer 1

确实 const assertions 生成具有 readonly 个元素的对象和数组。如果您想获得 const 断言为您带来的字符串文字和元组类型的好处，同时还取消 readonly-ing 结果，您可以编写一个辅助函数来做到这一点。我将其命名为 mutable():

const mutable = <T>(t: T): { -readonly [K in keyof T]: T[K] } => t

const fruits = mutable(['banana', 'apple', 'orange'] as const);
// const fruits: ["banana", "apple", "orange"]

这将深入一个层次。如果你有嵌套的 object/array 类型，你可能想要创建一个 DeepMutable 类型和 deepMutable() 辅助函数：

type DeepMutable<T> =
    T extends object ? { -readonly [K in keyof T]: DeepMutable<T[K]> } : T

const deepMutable = <T>(t: T) => t as DeepMutable<T>;

上面的例子也是一样的，

const alsoFruits = deepMutable(['banana', 'apple', 'orange'] as const);
// const alsoFruits: ["banana", "apple", "orange"]

但嵌套对象的区别变得很重要：

const readonlyDeepFruits = {
    yellow: ["banana", "lemon"],
    red: ["cherry", "apple"],
    orange: ["orange", "mango"],
    green: ["lime", "watermelon"]
} as const;
/* const readonlyDeepFruits: {
    readonly yellow: readonly ["banana", "lemon"];
    readonly red: readonly ["cherry", "apple"];
    readonly orange: readonly ["orange", "mango"];
    readonly green: readonly ["lime", "watermelon"];
} */

const partiallyMutableDeepFruits = mutable(readonlyDeepFruits);
/* const partiallyMutableDeepFruits: {
    yellow: readonly ["banana", "lemon"];
    red: readonly ["cherry", "apple"];
    orange: readonly ["orange", "mango"];
    green: readonly ["lime", "watermelon"];
} */

const fullyMutableDeepFruits = deepMutable(readonlyDeepFruits);
/* const fullyMutableDeepFruits: {
    yellow: ["banana", "lemon"];
    red: ["cherry", "apple"];
    orange: ["orange", "mango"];
    green: ["lime", "watermelon"];
} */

好的，希望对您有所帮助。祝你好运！

Link to code

使用字符串文字数组作为类型

Use string literal array as type

typescript

class-validator