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JavaScript Array Group

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Managing, sorting, and manipulating data with JavaScript is a skill we've often delegated to third party libraries like lodash. As the JavaScript language progresses, however, these features eventually get. added to the JavaScript specification. Two such APIs for grouping of Array data are `Array.prototype.group and Array.prototype.groupToMap.

Array.prototype.group

To group an array of objects by a given property, call the group method with function that returns the grouping string:

const teams = [
  { name: "Arsenal", origin: "London", tier: "legendary" },
  { name: "Manchester United", origin: "Manchester", tier: "legendary" },
  { name: "Liverpool", origin: "Liverpool", tier: "legendary" },
  { name: "Newcastle United", origin: "Newcastle", tier: "mid" },
  // Lol, awful club
  { name: "Tottenham", origin: "London", tier: "lol" },
];

const tieredTeams = teams.group(({ tier }) => tier);

The result of the array's group is an object with keys that match the grouping key:

{
  legendary: [
    {name: "Arsenal", origin: "London", tier: "legendary"},
    {name: "Manchester United", origin: "Manchester", tier: "legendary"},
    {name: "Liverpool", origin: "Liverpool", tier: "legendary"}
  ], 
  mid: [
    {name: "Newcastle United", origin: "Newcastle", tier: "mid"}
  ], 
  lol: [
    {name: "Tottenham", origin: "London", tier: "lol"}
  ]
}

Array.prototype.groupToMap

groupToMap returns a Map instance instead of an object literal:

const tieredTeamsMap = teams.group(({ tier }) => tier);

tieredTeamsMap.has('lol') // true

tieredTeamsMap.get('lol') // [{name: "Tottenham", origin: "London", tier: "lol"}]

As of the time of publish, group and groupToMap are only available in Safari. These two methods are crucial to data management moving forward. Whether you're manipulating data on client or server side, these newly added native methods are much welcomed.

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Discussion

  1. Ben S

    The groupToMap() example might have an error – it’s still calling group()

    I didn’t know about these functions though, very useful!

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