2y ago

type-safe GroupBy (key) function i wrote

i made a type-safe GroupBy function.

 typescript

    
/**
 * 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' &amp;&amp; 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:

 typescript

    
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:

ts

    
{
  "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.

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