How to create a Javascript/Typescript Map dynamically without for loop?

Question

I want to dynamically create a Typescript Map<string, Person> object where Person is a class.

I can use for loop to create a map like this:

function getInitializedPersonMap(size){
  const personMap = new Map<string, Person>();
  for (let index = 0; index < size; index++) {
    personMap.set(index.toString(), {} as Person);
  }
  return personMap;
}

However, I am trying to build a habit of moving away from 'for' loops and use functions like map, filter, reduce.

My attempt without loop looks like this which is not quite as readable as one with for loop:

function getInitializedPersonMap(size){
   return new Map<string, Person>(
      Array.from(new Array(size), (_, index) => [
        index.toString(),
        {} as Person,
      ])
    );
}

For scenarios like this is this an overkill to avoid 'for' loops?

I have edited my answer

WSD
– WSD

2020-06-06 13:57:45 +00:00
Commented Jun 6, 2020 at 13:57 — WSD
– WSD, Commented Jun 6, 2020 at 13:57

Navoneel Talukdar · Accepted Answer · 2020-06-06 13:01:45Z

1

You can use Map, notice the type has been explicitly mentioned.

var result = sizeArr.map((i): [string, Person] => [i, {} as Person]);

As @VLAZ mentioned that size is an number, so use Array.from to flatten the array from target size.

let sizeArr = Array.from(Array(n).keys());

edited Jun 6, 2020 at 13:01

answered Jun 6, 2020 at 12:45

Navoneel Talukdar

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3 Comments

VLAZ Over a year ago

size seems to be a number, not an array.

Navoneel Talukdar Over a year ago

In the first example OP provided a working code, which is based on loop. Seems from there size is an array. OP needs to clarify more.

dushyg Over a year ago

@NavoneelTalukdar Just to understand what you are suggesting is to use map over Array.from() overload that takes a mapping function to make it more readable. Yes this is much better. But I was trying to arrive at a way which doesnt include creating additional Array to loop over. Now to get the desired map I will still have to pass the result to a Map constructor. var result = sizeArr.map((i): [string, Person] => [i, {} as Person]); const resultToBeReturned = new Map<string, Person>(result);

WSD · Accepted Answer · 2020-06-06 13:45:31Z

0

If from the very beginning you know the values, then is pretty straightforward:

  const personMap = new Map<string, Person>([
    ['value1', new Person()],
    ['value2', new Person()],
    ['value3', new Person()]
  ]);

If what you want is to create the Map based on some existing generated Array, then you can:

const arrayLength = 5;
const inputArray = Array.from({ length: arrayLength }, (_, index) => [index + '', new Person()]);

const theMap = new Map<string, Person>(
  inputArray.map(x => [x[0], x[1]] as [string, Person])
);

edited Jun 6, 2020 at 13:45

answered Jun 6, 2020 at 12:44

WSD

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2 Comments

VLAZ Over a year ago

OP's code shows that the size can be anything and should be dynamic.

dushyg Over a year ago

Thanks @JoseGuzman, looks like with your solution, we end up using both Array.from overload that takes a mapping function along with another call to map in the Map constructor.

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