Typescript typing for a Multi Dimensional Array

Question

I am creating a matrix / 2d array of booleans and I want to infer a type that isn't simply "ANY" for dategrid.

let yearRange = [2000,2001,2002,2003,2004];
let monthRange = [0,1,2,3,4,5,6,7,8,9,10,11];
let dateGrid = any;

yearRange.forEach((year) => {
    monthRange.forEach((month) => {
        dateGrid[year][month] = true;
    });
});

How does one create an interface / type for dategrid that:

Infers the structure: e.g. dateGrid[yearIndex][possibleMonthValues]:boolean And restricts the months index to only the applicable months.

dateGrid[2000][0] = true
dateGrid[2000][1] = true
dateGrid[2000][2] = true
dateGrid[2000][3] = true
dateGrid[2000][4] = true
dateGrid[2000][5] = true
dateGrid[2000][6] = true
dateGrid[2000][7] = true
dateGrid[2000][8] = true
dateGrid[2000][9] = true
dateGrid[2000][10] = true
dateGrid[2000][11] = true
dateGrid[2001][0] = true

... and so on ...

Answer 1

Record based solution

To be totally strict we can infer the proper narrowed types from variables you have given by using const keyword. As I assume years are only example, so the sweet spot would be only restrict months, and leave years as number:

// use const in order to get proper types
let yearRange = [2000,2001,2002,2003,2004];
let monthRange = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] as const;
type Month = typeof monthRange[number];

let dateGrid: Record <number,Record<Month, boolean>>;

Record<Month, boolean> will be good idea if we restrict to have all months from 0 to 11. If we allow on only part of months in the year we can create non-exlusive Record:

let dateGrid: Record<number, Partial<Record<Month, boolean>>> = {
  [2000]: {
    "0" : false 
  }
} // valid as not all fields needs to be provided
const a = dateGrid[2000][4] // boolean | undefined

// in contrary exclusive type
let dateGridExclusive: Record<number, Record<Month, boolean>> = {
  [2000]: {
    "0" : false 
  }
} // error all months need to be provided
const b = dateGrid[2000][4] // boolean

Pay attention that I have used Partial utility type in order to loose the constraint and allow on part of months to be provided.

---

Array based solution

If we want to use this as array we can perform another array type consider:

type Months = [
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
  boolean,
]

let dateGrid: Months[] // array of 12 element tuples

The minus of using array at year level is that when we set start point of such from 2000, we have 1999 undefined values.