i made a type-safe GroupBy function.

/**
 * Groups array of objects by a given key
 * @param arr array of objects to group
 * @param key must be present on every object, and it's values must be string|number
 * @author telepresence
 * @license CC-BY-4.0
 */
function groupBy(arr: T[], key: keyof T, defaultAcc: Record = {}) {
    return arr.reduce((acc, val, i) => {
        const compValue = val[key];
        if (typeof compValue !== 'string' && typeof compValue !== 'number') {
            throw new Error(`key ${key.toString()} has values other than string/number. can only group by string/number values`);
        }
        if (!acc[compValue]) acc[compValue] = []
        acc[compValue].push(val);
        return acc;
    }, defaultAcc);
}

• like lodash's groupBy, but by key and not function
  • group an array of objects which all have a key in common into an object with keys matching all the different possible values of your common key
• type-safe, no unknown's no any's
• does not copy arrays ([...array]), uses push
• supports selecting by keys, where the key values are string / number (although you can easily add symbol support)
• shared for free under the CC BY 4.0 license - only attribution is requred (link to this post is fine)
• custom default accumulator support, if you already know the groups beforehand and would rather have an empty array than undefined.

example:

const data = [{
  "name": "jim",
  "color": "blue",
  "age": "22"
}, {
  "name": "Sam",
  "color": "blue",
  "age": "33"
}, {
  "name": "eddie",
  "color": "green",
  "age": "77"
}];

groupBy(data, 'color') 

would result into:

{
  "blue": [
    {
      "name": "jim",
      "color": "blue",
      "age": "22"
    },
    {
      "name": "Sam",
      "color": "blue",
      "age": "33"
    }
  ],
  "green": [
    {
      "name": "eddie",
      "color": "green",
      "age": "77"
    }
  ]
} 

TL;DR i've sucessfully wrote something using generics in typescript for the first time, and i think it's pretty epic.