Typescript Conditional Types with "never"

conditional-type.ts

/**
A (contrived) example of conditional types with `never`.
*/

/** Gets keys that have string values from object T. */
type StringPropertyNames<T> = {
  [K in keyof T]: T[K] extends string ? K : never;
}[keyof T];

/**
Example usage recreating lodash `groupBy` function.
  
However, it only allows grouping by keys with string values.
This isn't super useful, but is a demonstration.
*/
export const groupBy = <T, K extends StringPropertyNames<T>>(
  list: T[],
  key: K
): { [k: string]: T[] } =>
  list.reduce<{ [k: string]: T[] }>((acc, item) => {
    const val = (item[key] as unknown) as string;
    (acc[val] || (acc[val] = [])).push(item);
    return acc;
  }, {});

const places = [
  { city: "San Francisco", state: "California", date: new Date() },
  { city: "Bakersfield", state: "California", date: new Date() },
  { city: "Cleveland", state: "Ohio", date: new Date() },
  { city: "Cincinnati", state: "Ohio", date: new Date() },
];

const byState = groupBy(places, "state");
 
/**
Won't compile:
  const byDate = groupBy(places, "date");

  "Argument of type '"date"' is not assignable to parameter of type 'StringPropertyNames<{ city: string; state: string; date: Date; }>'"
*/