activeMarker?.markerType as keyof typeof viewForm这行代码什么意思
时间: 2024-04-01 08:34:37 浏览: 14
这行代码的含义是:
- `activeMarker` 是一个对象(或者 `null` 或 `undefined`),它具有一个名为 `markerType` 的属性。
- `?.` 是可选链操作符,它表示如果 `activeMarker` 不为 `null` 或 `undefined`,则继续访问该对象的属性;否则返回 `undefined`。
- `as` 是类型断言操作符,它告诉编译器将表达式转换为指定的类型。在这里,它将表达式 `activeMarker?.markerType` 转换为类型 `keyof typeof viewForm`。
- `keyof typeof viewForm` 表示 `viewForm` 对象的所有属性名的联合类型。也就是说,它是一个字符串类型的集合,其中每个字符串都是 `viewForm` 对象的一个属性名。
因此,这行代码的作用是将 `activeMarker` 对象的 `markerType` 属性转换为 `viewForm` 对象中的某个属性名(或者返回 `undefined`)。这样做的目的可能是为了检查 `markerType` 是否是 `viewForm` 对象中的有效属性名,或者为了在后续的代码中使用该属性名进行相关操作。
相关问题
as keyof typeof
`as keyof typeof` is a TypeScript type assertion syntax that allows you to cast a value to a specific key of a type. It is often used to access a specific property of an object without hardcoding the property name as a string.
For example, consider the following code:
```typescript
const obj = {
foo: 'hello',
bar: 'world'
};
const key = 'foo';
const value = obj[key];
```
In this code, we have an object `obj` with two properties `foo` and `bar`. We want to access the value of the property whose name is stored in the variable `key`. However, TypeScript will give us an error because `key` is a string and not a valid key of `obj`.
To fix this, we can use `as keyof typeof`:
```typescript
const obj = {
foo: 'hello',
bar: 'world'
};
const key = 'foo' as keyof typeof obj;
const value = obj[key];
```
Now, TypeScript knows that `key` is a valid key of `obj` and we can safely access the value of the property.
Note that `typeof` is used here to get the type of `obj`. This allows us to avoid hardcoding the type of `obj` and makes the code more reusable.
keyof typeof
keyof typeof用于获取一个对象类型的所有属性名的联合类型。typeof操作符用于获取一个值的类型,而keyof操作符用于获取一个类型的所有属性名。因此,keyof typeof可以用于获取一个对象的所有属性名的联合类型。
举个例子,假设我们有一个对象person,其类型为Person:
```
type Person = {
name: string;
age: number;
gender: string;
};
const person: Person = {
name: "John",
age: 30,
gender: "male"
};
```
我们可以使用keyof typeof来获取person对象的所有属性名的联合类型:
```
type PersonKeys = keyof typeof person;
// PersonKeys的类型为 'name' | 'age' | 'gender'
```
这样,我们就可以使用PersonKeys来限制一个函数的参数类型,以确保只能传入person对象的属性名作为参数。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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