可索引类型-TypeScript [英] Indexable type - TypeScript

问题描述

语法如下，

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

指出，当用number索引StringArray时，它将返回string.例如-let myArray: StringArray;myArray = ["Bob", "Fred"];let myStr: string = myArray[0];

states that when a StringArray is indexed with a number, it will return a string. For example - let myArray: StringArray;myArray = ["Bob", "Fred"];let myStr: string = myArray[0];

因此，通过将myArray声明为StringArray类型，将myArray的类型限制为存储为string类型的值.尽管键(index)被称为number

So, myArray is type constrained on values stored as string type, by declaring it with type StringArray. Key(index) is always string type, under the hood(JavaScript), despite mentioning it as number

根据以下语法，

class Animal{
  name: string;
}

class Dog extends Animal{
  breed: string;
}

interface NotOkay {
  [x: number]: Animal;
  [x: string]: Dog;   
}

1)NotOkay语法说明什么?

根据以下语法，

interface NumberDictionary {
    [index: string]: number;
    length: number;    // ok, length is a number
    name: string;      // error, the type of 'name' is not a subtype of the indexer
}

2)NumberDictionary表示什么?

3)为什么这两种语法是错误的? error TS2413& error: TS2411

3) Why these two syntax are wrong? error TS2413 & error: TS2411

推荐答案

(如@betadeveloper所指出，这些示例摘自

(as @betadeveloper points out, the examples are taken from the typescript documentation at https://www.typescriptlang.org/docs/handbook/interfaces.html)

1 + 3)文档指出的一点是此声明不一致.

1+3) The point the documentation is making is that this declaration is not consistent.

该声明似乎表明此代码有效:

The declaration would seem to say that this code works:

let no: NotOkay

function byString(key: string): Dog {
   return no[key]
}

function byNumber(key: number): Animal {
   return no[key]
}

但是问题是，例如no[0]的确与no['0']相同(这就是JavaScript的工作方式).因此，byString('0')实际上会 只会产生动物，而不一定是狗.

But the problem is that a lookup of e.g. no[0] is really the same things as no['0'] (it's how JavaScript works). So byString('0') will actually just result in an Animal, not necessarily a Dog.

因此，为防止上述代码-看起来正确，但实际上对于某些字符串键(那些恰好是字符串形式的数字)是错误的-TypeScript不允许声明.

So to prevent the code above – that looks correct but is in fact wrong for certain string keys (those that happen to be numbers in string form) – TypeScript does not allow the declaration.

2 + 3)[index: string]: number;表示您可以将任何字符串用作NumberDictionary的索引，并且您会得到一个数字.

2+3) [index: string]: number; says that you can use any string as an index of a NumberDictionary and you will get a number.

另一方面，name: string;表示属性name是字符串.但是属性也可以作为索引访问，例如做的时候

On the other hand, name: string; says that the property name is a string. But a property can also be accessed as an index, so e.g. when doing

let nd: NumberDictionary

let a = nd['name']
let b = nd.name

a和b表示完全相同，但是第一个声明似乎说a是一个数字，而第二个声明似乎说b是一个字符串.这是矛盾的，因此也不允许此声明.

a and b mean exactly the same thing, but the first declaration seemed to say a was a number, whereas the second declaration seems to say b is a string. That's a contradiction and therefore this declaration is also not allowed.

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