groupBy.md

groupBy

Used to group a collection of objects by a given property or set of properties.

Syntax:
groupBy(keySelector, valueSelector)
Usage in a reducer:
array.reduce(groupBy(keySelector, valueSelector))

Arguments:

• keySelector is a function used to extraxt the key values.
  The keySelector can return any type of value, even an object or an array.
  • For objects, groupBy will do a deep comparisson, comparing each property of the two objects, to determine if the represent the same group.
  • For arrays, groupBy will do a deep comparisson, comparing each element in the two arrays, to determine if the represent the same group.
• valueSelector is a function used to determine object values to put in the groups.
  This paramter is optional, and if it is skipped, the whole item is selected as the value

Usage:

import { groupBy } from "@aeinbu/groupby"

const people = [
    { name: "Tony", residence: "Rome" },
    { name: "Mary", residence: "Rome" },
    { name: "Peter", residence: "London" },
    { name: "Peter", residence: "Rome" },
    { name: "Elisabeth", residence: "London" },
    { name: "Francois", residence: "Paris" }
]

const results = people.reduce(groupBy(x => x.residence, x => x.name), [])

// Result (Note how the output is a new array):
// [
//     {"key": "Rome", values: ["Tony", "Mary", "Peter"],}
//     {"key": "London", values: ["Peter", "Elisabeth"],}
//     {"key": "Paris", values: ["Francois"]}
// ]

Remarks:
If you want the results of a groupBy operation behave like a lookup, you can use toMap (or toDictionary), like show in the following example.

import { groupBy, toMap } from "@aeinbu/groupby"

const people = [/* same array as in previous sample*/ ]

const results = people
                    .reduce(groupBy(x => x.residence, x => x.name), [])
                    .reduce(toMap(), new Map())

// Result are now a Map, and you can select a group by its key:
results.get("Rome") // ["Tony", "Mary", "Peter"]

See also:

• toMap
• toDictionary
• unit tests for groupBy