方括号是什么意思字段应该在打字稿中的位置? [英] What do square brackets mean where a field should be in typescript?

问题描述

我在三个 d.ts 中遇到了这条线:

dispatchEvent(event: { type: string; [attachment: string]: any; }): void;

并想知道这是什么意思.
我知道这意味着一个名为 dispatchEvent 的函数，它接受一个类型为成员类型的参数，但我不确定是什么:

[附件:字符串]:任意；

意思.

解决方案

那是一个索引签名.来自 TypeScript 文档:

<块引用><块引用>

可索引类型有一个索引签名，它描述了我们可以用来索引对象的类型，以及索引时相应的返回类型.

因此，例如，您可以为可索引对象定义一个接口，例如:

interface IArrayOfStrings {[索引:数字]:字符串；}

这告诉编译器，对于 IArrayOfStrings 类型的任何对象，通过数字索引访问的任何成员都将是 string 类型.

所以，这将编译没有错误:

interface IArrayOfStrings {[索引:数字]:字符串；}让的话: IArrayOfStrings = ["foo","bar"];让单词:字符串=单词[0]；

但这不会:

interface IArrayOfStrings {[索引:数字]:字符串；}让的话: IArrayOfStrings = ["foo","bar"];让 myNumber: number = words[0];

在您的示例中，这一行:

dispatchEvent(event: { type: string; [attachment: string]: any; }): void;

正在描述一种方法 dispatchEvent，它接受一个 { type: string; 类型的参数；[附件:字符串]:任何；}.

为了使该类型更易于理解，请查看定义该类型的接口:

interface IEvent {类型:字符串；[附件:字符串]:任何；}

这告诉编译器 IEvent 类型的对象将有一个名为 type 的字符串属性，以及一个 IEvent 对象的元素，通过访问字符串索引将为 any 类型.

所以，像这样编译不会出错:

interface IEvent {类型:字符串；[附件:字符串]:任何；}让我的事件:IEvent = {类型:'一些事件类型'};让 eventType: string = myEvent["type"];

I came across this line in three.d.ts:

dispatchEvent(event: { type: string; [attachment: string]: any; }): void;

and was wondering what it meant.
I understand that this would mean a function called dispatchEvent which takes an argument of a type with a member type but I am not sure what:

[attachment: string]: any;

means.

解决方案

That is an index signature. From the TypeScript documentation:

Indexable types have an index signature that describes the types we can use to index into the object, along with the corresponding return types when indexing.

So, for example, you could define an interface for an indexable object like:

interface IArrayOfStrings {
    [index: number]: string;
}

This tells the compiler, that for any object of type IArrayOfStrings, any member accessed by the numerical index will be of type string.

So, this will compile without error:

interface IArrayOfStrings {
    [index: number]: string;
}

let words: IArrayOfStrings = ["foo","bar"];

let word: string = words[0];

But this will not:

interface IArrayOfStrings {
    [index: number]: string;
}

let words: IArrayOfStrings = ["foo","bar"];

let myNumber: number = words[0];

In your example, this line:

dispatchEvent(event: { type: string; [attachment: string]: any; }): void;

is describing a method dispatchEvent that accepts one parameter of type { type: string; [attachment: string]: any; }.

To make that type easier to understand, look at an interface that defines this type:

interface IEvent {
    type: string;
    [attachment: string]: any;
}

This tells the compiler that objects of type IEvent will have a string property called type, and elements of an IEvent object, accessed by the string index will be of any type.

So, something like this would compile without error:

interface IEvent {
    type: string;
    [attachment: string]: any;
}

let myEvent: IEvent = {
    type: 'some-event-type'
};

let eventType: string = myEvent["type"];

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