What does it signify when it is stated that "it is not a descendant of the indexer"?

Currently, I am diving into Typescript with the help of this informative guide on indexer types.

There is a specific piece of code that has me puzzled:

interface NumberDictionary {
    [index: string]: number;
    length: number;    // okay, length should be a number
    name: string;      // error, 'name' type is not compatible with indexer
}

To start, why include 'length' in the index type?

let myDict : NumberDictionary;
myDict[10] = 23;

I'm confused about why there is a "length" property in the NumberDictionary when it's supposed to have an index like a[10]. Is an index type essentially an array, considering JavaScript arrays have lengths? If so, why define a length property in this example? Is it necessary? Apologies for my confusion – you can tell I'm trying to wrap my head around it.

Secondly,

name: string;// error, the type of 'name' is not a subtype of the indexer

The comment here has me scratching my head. Why does 'name' need to be a subtype of the indexer? Isn't an indexer something like a[10] = "Tom"? So, what exactly is a subtype of the indexer in this scenario?

typescript

Answer №1

I am confused by the comment made about this particular line. Why does the name have to be a subtype of the indexer?

This is because when accessing a string in TypeScript, it will automatically assume the type is number (based on `[index: string]: number;`). If `name: string;` was allowed, then the following code snippets would assume `number`, even though you may expect it to be a `string`: 

let x: NamedDictionary = Object.create(null);
let n = "name";
let y = x[n]; // TypeScript assumes `number`
let z = x["name"]; // Do you want `string` here?

Notice the inconsistency between `y` and `z` ^

Answer №2

Addressing the initial query, NumberDictionary serves as an object interface rather than an array interface. The index signature allows for referencing any index within the object.

A NumberDictionary object must include two essential values: length and name. Additional attributes can be included only if they adhere to the index signature (index and value type).

Object Interface

    interface NumberDictionary {
        [index: string]: number | string;
        length: number;
        name: string;
    }
    
    let nd: NumberDictionary = { length: 10, name: "Ten", lang: "en" };

Index Signature

    [index: string]: number | string;

This indicates that additional property like lang: string can be added to the object without triggering type errors.

If it's known that lang: string should be optional in the object, then it needs to be specified in the interface definition.

    interface NumberDictionary {
        [index: string]: number | string;
        length: number;
        name: string;
        lang?: string;
    }
    
    let nd: NumberDictionary = { length: 10, name: "Ten", lang: "en" };

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