Miscellaneous TypeScript helpers

getKeys.ts

// Wrapper for Object.keys which sets the correct type
export function getKeys<T>(obj: T): Array<keyof T> {
  return Object.keys(obj) as Array<keyof T>;
}

mapArrayToObject.ts

/**
 * Usage:
 *
 * const normalize = mapArrayToObject(obj => ({ [obj.id]: { ...obj, name: obj.name.toLowerCase() } }));
 * normalize(objects);
 */

export function mapArrayToObject<
  T,
  Key extends number | string | symbol,
  Value
>(fn: (t: T) => { [key in Key]: Value }) {
  return function(array: T[]): { [key in Key]: Value } {
    return Object.assign({}, ...array.map(fn));
  };
}

mapArrayToObjectWithPropAsKey.ts

function mapArrayToObjectWithPropAsKey<
  K extends string | number | symbol,
>(keyProp: K) {
  return function<T extends { [key in K]: string | number | symbol }>(array: T[]): { [key in T[K]]?: T } {
    return Object.assign({}, ...array.map((t: T) => ({ [t[keyProp]]: t })));
  };
}

interface Task {
  id: 1 | 2 | 3 | 4;
  text: string;
}

const tasks: Task[] = [
  { id: 1, text: 'To do 1' },
  { id: 2, text: 'To do 2' },
  { id: 3, text: 'To do 3' },
];

const normalize = mapArrayToObjectWithPropAsKey('id');
const state = normalize(tasks); // type: { [id: 1 | 2 | 3 | 4]?: Task }

mapObject.ts

type MapObjIndexedFn<T, TResult> = (
  value: T,
  key: string,
  obj?: {
    [key: string]: T;
  },
) => TResult;

export function mapObject<K extends string, T, TResult>(
  fn: MapObjIndexedFn<T, TResult>,
  obj: { [key in K]: T },
): { [key in K]: TResult } {
  return getKeys(obj).reduce(
    (acc, key) => {
      acc[key] = fn(obj[key], key.toString(), obj);
      return acc;
    },
    {} as { [key in K]: TResult },
  );
}

const tasks = {
  todo1: { id: 1, text: 'To do 1' },
  todo2: { id: 2, text: 'To do 2' },
  todo3: { id: 3, text: 'To do 3' },
};

const taskTexts = mapObject(task => tast.text, tasks);
// { todo1: 'To do 1', todo2: 'To do 2', todo3: 'To do 3' }

prop.ts

function prop<K extends string | number | symbol>(key: K) {
  return function<V>(obj: { [key in K]: V }): V {
    return obj[key];
  }
}

const tasks = [
  { id: 1, text: 'To do 1' },
  { id: 2, text: 'To do 2' },
  { id: 3, text: 'To do 3' },
];

const ids = tasks.map(prop('id'));
const texts = tasks.map(prop('text'));