Use Map constructor in Typescript with initialized values

Question

The type definition for Map is as follows

interface MapConstructor {
    new <K, V>(): Map<K, V>;
    new <K, V>(iterable: IterableShim<[K, V]>): Map<K, V>;
    prototype: Map<any, any>;
}

I want to use the 2nd constructor new <K, V>(iterable: IterableShim<[K, V]>): Map<K, V>;

I have a type I've defined type ChangeMap = { [key: string]: Array<string> }; which is used as follows

let changeMap: ChangeMap = { A: ["Car", "Boat"], B: ["Plane", "Rocket"] };

I want to create a Map<number, ChangeMap> which I can do easily enough but I'm stuck as to how to structure the data I want to initialize my Map with

public changeOptions = new Map<number, ChangeMap>( ... what goes here ... ?)

Is this possible or do I need to initialize my Map without values and then call changeOptions.set(1, changeMap); to give it values?

--- EDIT #1

To answer @Paarth's question below, the number keys in the changeOptions Map are identifiers for other objects in the domain.

Assume I have a class of Option defined as follows

class Option {
    id: number;
    name: string;
}

The id of a given Option instance would be the key for the changeOptions Map. There are a limited predefined set of Options based on business rules for the domain. Assume for this question there are 3 Option instances with id values of 2, 10, 34, so I want those to be the keys of the changeOptions Map

Where do the numbers (first parameter in your set call) come from? — Paarth
– Paarth, Commented Sep 29, 2016 at 15:38
@Paarth The number keys for the changeOptions Map are IDs for other objects in the domain. Depending on that ID I want to use a different ChangeMap — seangwright
– seangwright, Commented Sep 29, 2016 at 16:37

MasterKiller · Accepted Answer · 2023-04-19 19:40:54Z

7

The syntax is:

  let changeOptions = new Map<number, ChangeMap>([[2, changeMap], [10, anotherChangeMap], [34, thirdChangeMap]]);

answered Apr 19, 2023 at 19:40

MasterKiller

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seangwright Over a year ago

Confirmed - example here tsplay.dev/WkEyDN Thanks! 🙏

Paarth · Accepted Answer · 2016-09-29 16:01:51Z

2

Where do the numbers come from? Are they just an order based on an array?

If so...

let changeMaps = ...

public changeOptions = new Map<number, ChangeMap>(changeMaps.map((v,i) => [i+1,v] as [number, ChangeMap]));

You don't actually need the <number, ChangeMap> anymore because the expression ends up as a Array<[number, ChangeMap]> thanks to the as [number, ChangeMap] but it doesn't hurt to have and it's good for documentation.

Breaking it down: If we want to have an iterable set of changes to pass into the map constructor, we need to store them in a collection. That's changeMaps up above, the ... is for you to fill it out how you wish. The naive form with one item is just [changeMap]. We then map that collection to a collection of tuples, including the indices. I used i+1 because it seemed like you wanted to start with the number 1. That's what gets passed into the map constructor.

edited Sep 29, 2016 at 16:01

answered Sep 29, 2016 at 15:51

Paarth

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1 Comment

seangwright Over a year ago

I can't use auto-incrementing indexes as explained in EDIT #1 in my question

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