Question

33

Map of Maps in Typescript / ECMAScript

rated 0 times [ 34] [ 1] / answers: 1 / hits: 5831 / 4 Years ago, tue, october 20, 2020, 12:00:00

This is what i personally would consider correct in various languages, but here it is not. I must be ignorant of an aspect of Typescript, but I can not so easy determine where the gap is.

The implementation requires a map of maps. The top map is a string key, and the a map of string-keys and string-values.

class aClass

{

    myMap: Map<string, Map<string, string>>;



    constructor ()

    {

        this.myMap = new Map<string, Map<string, string>>([

            ["key1", new Map<string,string>()],

            ["key2", new Map<string,string>()]

        ]);

    }



    async aTask(map: Map<string,string>)

    {

        map.set("subKey1", "VALUE");

    }



    async someWork()

    {

        this.aTask(this.myMap["key1"]);

        this.aTask(this.myMap.get("key1"));

    }

}

How do I correctly access the Map<string, string>() in the function someWork() ?

The error for the first accessor : this.aTask(this.myMap["key1"]);

Element implicitly has an 'any' type because type 'Map<string, Map<string, string>>' has no index signature. Did you mean to call 'get'?

The error for the second accessor : this.aTask(this.myMap.get("key1"));

Argument of type 'Map<string, string> | undefined' is not assignable to parameter of type 'Map<string, string>'. Type 'undefined' is not assignable to type 'Map<string, string>'.

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answered 4 Years ago felix · Accepted Answer

Maps, despite being objects, don't put their values onto properties of the Map instance, which is why this.myMap["key1"] doesn't work.

TypeScript doesn't let you do

this.aTask(this.myMap.get("key1"));

because the .get does not guarantee that the key exists in the Map; it's warning you that it might have returned undefined. To be type-safe, do:

async someWork() {

    const val = this.myMap.get("key1");

    if (val) {

        this.aTask(val);

    }

}