array.groupBy in TypeScript

Question

The basic array class has .map, .forEach, .filter, and .reduce, but .groupBy i noticably absent, preventing me f

Answer 1

You can use the following code to group stuff using Typescript.

const groupBy = (list: T[], getKey: (item: T) => K) =>
  list.reduce((previous, currentItem) => {
    const group = getKey(currentItem);
    if (!previous[group]) previous[group] = [];
    previous[group].push(currentItem);
    return previous;
  }, {} as Record);

So, if you have the following structure and array:

type Person = {
  name: string;
  age: number;
};

const people: Person[] = [
  {
    name: "Kevin R",
    age: 25,
  },
  {
    name: "Susan S",
    age: 18,
  },
  {
    name: "Julia J",
    age: 18,
  },
  {
    name: "Sarah C",
    age: 25,
  },
];

You can invoke it like:

const results = groupBy(people, i => i.name);

Which in this case, will give you an object with string keys, and Person[] values.

There are a few key concepts here:

1- You can use function to get the key, this way you can use TS infer capabilities to avoid having to type the generic every time you use the function.

2- By using the K extends keyof any type constraint, you're telling TS that the key being used needs to be something that can be a key string | number | symbol, that way you can use the getKey function to convert Date objects into strings for example.

3- Finally, you will be getting an object with keys of the type of the key, and values of the of the array type.