I have this TypeScript playground:

export function isBlue<T extends Mesh>(
  object: unknown,
  type: T | Array<T>,
): object is BlueNodeType<T> {
  const array: Array<T> = Array.isArray(type) ? type : [type]
  return (
    object != null && typeof object === 'object' &&
    'type' in object &&
    typeof object.type === 'string' &&
    array.includes((object as BlueNodeType<T>).type)
  )
}

export enum Mesh {
  Assertion = 'mesh-assertion',
}

export type BlueBaseType = {
  color: 'blue'
}

export type BlueAssertionType = BlueBaseType & {
  type: Mesh.Assertion
}

export type BlueMappingType = {
  'mesh-assertion': BlueAssertionType
}

export type BlueNodeType<T extends Mesh> = BlueMappingType[T]

It is throwing this error:

Argument of type 'Mesh' is not assignable to parameter of type 'T'.
  'Mesh' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Mesh'.(2345)

How do I get this to work? In my real codebase I have a BlueMappingType with 40 types, so I would like for it to be able to pick the correct type based on the generic type parameter.

CodePudding user response：

Using Array.prototype.includes is tricky one. In order to make it work, you can try my custom includes:

const withTuple = <
  List extends string[]
>(list: readonly [...List]) =>
  (prop: string): prop is List[number] =>
    list.includes(prop)

export function isBlue<T extends Mesh>(
  object: unknown,
  type: T | Array<T>,
): object is BlueNodeType<T> {
  const array: Array<T> = Array.isArray(type) ? type : [type]
  const includes = withTuple(array)
  return (
    object != null && typeof object === 'object' &&
    'type' in object &&
    typeof object.type === 'string' &&
    includes(object.type)
  )
}

export enum Mesh {
  Assertion = 'mesh-assertion',
}

export type BlueBaseType = {
  color: 'blue'
}

export type BlueAssertionType = BlueBaseType & {
  type: Mesh.Assertion
}

export type BlueMappingType = {
  'mesh-assertion': BlueAssertionType
}

export type BlueNodeType<T extends Mesh> = BlueMappingType[T]

Playground

withTuple is just a curried version of Array.prototype.includes but it works with tuples.

You can check my article for more examples