Incorporate elements into an array using TypeScript's map method

Question

Incorporate elements into an array using TypeScript's map method

I'm working with a JS Map that's defined as

let mappy = new Map<string, string[]>()

I want to add elements to the array using the key. I could do it like this:

mappy.set(theKey, mappy.get(theKey).push('somevalue'));

This seems like a lot of code just for adding an element. If I used a simple object instead, I could easily achieve the same result:

let mappy = {};
mappy[theKey] = [];

Then later on, I can simply use mappy[theKey].push('somevalue'). However, this approach defeats the purpose of having typed objects in TypeScript.

Is there a way to add elements to the array within the map without needing to first "get" it?

typescript

Answer 1

Answer №1

To access the array in the Map object and add additional values to it, you can simply get a reference to the array. There is no requirement to re-insert it into the Map, as it remains there.

mappy.get(theKey)!.push('somevalue');

The presence of ! after mappy.get(theKey) is essential because TypeScript generates an error when the value returned by mappy.get() could potentially be undefined. By using the exclamation mark, we confirm that the value is indeed defined. This is known as a "definite assignment assertion"; a feature introduced in TypeScript 2.7.

You can test this code snippet here.

If preferred, you can utilize a regular object instead of a Map:

type MyMap = { [K: string]: string[] };
let mappy: MyMap = {};

mappy[theKey] = [];
mappy[theKey].push('somevalue');

Answer 2

To access the array in the Map object and add additional values to it, you can simply get a reference to the array. There is no requirement to re-insert it into the Map, as it remains there.

mappy.get(theKey)!.push('somevalue');

The presence of ! after mappy.get(theKey) is essential because TypeScript generates an error when the value returned by mappy.get() could potentially be undefined. By using the exclamation mark, we confirm that the value is indeed defined. This is known as a "definite assignment assertion"; a feature introduced in TypeScript 2.7.

You can test this code snippet here.

If preferred, you can utilize a regular object instead of a Map:

type MyMap = { [K: string]: string[] };
let mappy: MyMap = {};

mappy[theKey] = [];
mappy[theKey].push('somevalue');

Incorporate elements into an array using TypeScript's map method

Answer №1

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