Use parameter as a generic type, and also give it a default assignment

Question

I have a simple class

Board

with parameters

width

,

height

, and

types

in the constructor. Height and width will always have the type

number

, but

types

is an array of any type.

What I would like to do is to capture the type of the

types

parameter using a generic type

<T>

. Then

T

can be used for the return of another function that uses

this.types

. The problem is that I also would like to set the default types to an array of numbers.

This is what I thought might work, but I currently get the error

Type 'number[]' is not assignable to Type 'T[]' Type 'number' is not assignable to Type 'T'

.

export class Board<T> {

    constructor(width: number = 8, height: number = 8, types: T[] = [0, 1, 2, 3, 4, 5, 6]) {

        this.width = width;

        this.height = height;

        this.types = types;

    }

    getTypes(): T[] {

        return this.types;

    }

}

What am I doing wrong? Or is this setup even possible?

What I ultimately want is for the

T

to be assigned to whatever type that

this.types

ends up being after the class is instantiated. If I call

new Board(8, 8, ['a', 'b', 'c', 'd']);

, I'd like it to know that

T

should be

string

. AND if I just call

new Board()

it will know that

T

should be

number

.

Answer 1

Better approach using advanced types:

class Board<T> {
    private width:number;
    private height:number;
    private types:T[]|number[];

    constructor(width:number = 8, height:number = 8, types:T[]|number[] = [1, 2]) {
        this.width = width;
        this.height = height;
        this.types = types;
    }

    getTypes():T[]|number[] {
        return this.types;
    }
}

https://jsfiddle.net/r8uqn760/2/

Old answer using type alias:

This approach will always allow numbers in the this.types array.

For example new Board(0, 0, [0, "foo"]) will pass at compilation, and I think it's not what you want to happen.

type X<Y> = number | Y;

class Board<T> {
    private width:number;
    private height:number;
    private types:X<T>[];

    constructor(width:number = 8, height:number = 8, types = [1,2] as X<T>[]) {
        this.width = width;
        this.height = height;
        this.types = types;
    }

    getTypes():X<T>[] {
        return this.types;
    }
}

Fiddle

Use parameter as a generic type, and also give it a default assignment

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