Typing 小技巧

Custom Type Guard

interface Square {
    sideLength: number;
}
interface Circle {
    radius: number;
}

// Declare the custom type guard
function isSquare(obj: Square | Circle): obj is Square {
    return sideLength in Square;
}

// Use the custom type guard
if (isSquare()) // ...
else // ...

Mapped Type

type Getters<T> = {
    [K in keyof T as `get${Capitalize<string & K>}`]:  () => T[K];
};
type Client = {
    name: string;
    address: string;
};
type clientGetter = Getters<Client>;

// is equivalent to:

// type clientGetter = {
//     getName: () => string;
//     getAddress: () => string;
// }

Literal Tuple Type

const sizeOptions = ["s", "m", "l"] as const;
type SizeOption = (typeof sizeOptions)[number];

• 型別被定義為 SizeOption 的變數，其值只能是 "s" 或 "m" 或 "l"。

• ==重點是 as const==，如果 const sizeOptions = ["s", "m", "l"] 後面沒有 as const，那 type SizeOption 其實就只是 string 而已。