# TypeScript眉角

## 使用enum當作object key

enum testType {
    a = 1,
    b = 2,
    c = 3,
}

:::info

object : { [key in testType] : string}

object = {
    [testType.a] : "A",
    [testType.b] : "B",
    [testType.c] : "C",
}

object 的型別

object : {
    1 : string,
    2 : string,
    3 : string,
}

let data = object[testType.a];
//data 型別會變成 any

:::

:::success
上述例子的 object key 型別, 建議設成 number

object : { [key : number] : string}

let data = object[testType.a];
//data 型別是 string

:::
[參考來源](https://stackoverflow.com/a/52700831)

---

## 判斷某個未引用的外部class型別
:::info

class TestA {}

namespace ABC {
    class TestB {}
}

:::

:::success

something instanceof (<any>window)["TestA"];
something instanceof (<any>window)["ABC"]["TestB"];

:::

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###### tags: `教學`