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

tags TypeScript

created: 2025/01/23

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 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 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
• 讓 TypeScript 的 as const 救你一命