# [Advent of Typescript 2024] 7/25 (const assertion)

###### tags `TypeScript`

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> created: 2025/01/23

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## 題目

https://www.adventofts.com/events/2024/7

## const assertion 常數斷言

const createRoute = <Route extends string[]>(author: string, route: Route) => ({
	author,
	route,
	createdAt: Date.now(),
});

// 希望丟入的陣列可以正確地得到裡面的型別，而不是其單純的表層型別
const oneMill = createRoute('💨Dasher', ['Atherton', "Scarsdale", "Cherry Hills Village"]).route;
type t0_actual = typeof oneMill; // => string[] => 希望可以得到 ['Atherton', "Scarsdale", "Cherry Hills Village"] 而不是表層的 string[]
type t0_expected = [           // => ['Atherton', "Scarsdale", "Cherry Hills Village"]
    'Atherton',
    "Scarsdale",
    "Cherry Hills Village"
];
type t0 = Expect<Equal<t0_actual, t0_expected>>;

這裡在 GitHub 找到其他人的答案才知道怎麼寫...竟然有這種寫法：

const createRoute = <const Route extends string[]>(author: string, route: Route) => ({
	author,
	route,
	createdAt: Date.now(),
});

然後根據這個 [stackoverflow: A "const" assertion error typescript with string array](https://stackoverflow.com/questions/72914885/a-const-assertion-error-typescript-with-string-array) 中的回覆：

> If you write

const colors = ["white", "red", "blue"] as const;

> then the compiler infers the type `readonly ["white", "red", "blue"]` for `colors`. The compiler knows the value and position of each member of that array, and it won't let you change any of that. [...]
> On the other hand, if you leave off `as const`,

const colors = ["white", "red", "blue"];

then the compiler infers `string[]` for `colors`. It's just an array of strings of unknown quantity, value, or order.

**所以想要讓 TypeScript 一定要從陣列裡面的值推導其型別，而不是上層的型別，就可以用 `as const`。**

從 [TS Playground](https://www.typescriptlang.org/play/?#code/MYewdgzgLgBFCm0AyBLaMC8MDaByAglABbwBOU4uANDAEQDKwAhqRACZMA28tNtAwiVKkAnjAASKTpwgwAalM5MA5jwC6AKFCRYCZGigAVAO4hMOAsTIUw1OoxbsuPPoLKiJi2Qukr1MJlltaC) 可以實際 hover `testList` 以及 `testListTwo`，可以看出兩個型別的不同。


## 參考資料

- [`const` assertions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#const-assertions)
- [讓 TypeScript 的 as const 救你一命](https://ngseke.me/blog/typescript-as-const)