I’m trying to create a generic return type for this function, but I’m struggling hard. I’ve managed to get the keys but only if I changed QueryParam to QueryParam<T extends string> and use the generic type parameter T for name, but the values were all type of never.

So like this:

type QueryParam<T extends string> = {
  name: T
  type: 'string' | 'number' | 'boolean'
  isArray?: boolean
  transform?: (value: string | string[] | undefined) => unknown
}

This is how the original code looks like:

type QueryParam = {
  name: string
  type: 'string' | 'number' | 'boolean'
  isArray?: boolean
  transform?: (value: string | string[] | undefined) => unknown
}

const extractQueryParams = (query: Record<string, string | string[] | undefined>, params: QueryParam[]) => {
  const queryParams: Record<string, unknown> = {}

  for (const param of params) {
    const key = param.name
    if (!query[key]) continue

    const type = param.type
    const currentParam = query[key] as string | string[]

    if ((!param.isArray && Array.isArray(currentParam)) || (param.isArray && !Array.isArray(currentParam))) {
      if (!param.transform) continue
      queryParams[key] = param.transform(currentParam)
      continue
    }

    const parse = (x: string) => (type === 'number' ? Number(x) : type === 'boolean' ? Boolean(x) : x)

    if (!param.isArray && !Array.isArray(currentParam)) {
      queryParams[key] = parse(currentParam)
      continue
    }

    if (param.isArray && Array.isArray(currentParam)) {
      queryParams[key] = (currentParam as string[]).map(x => parse(x))
      continue
    }
  }

  return queryParams
}

This is how I call it:

const params = extractQueryParams(router.query, [{ name: 'page', type: 'number' }])

This is what I’d like to achive for the type of variable params with this input:

{ page: number }