Group objects by a key

Collect records by a string or numeric field without prototype-key collisions.

JavaScript Version 1.0.0 · MIT · .md

function groupBy({ values, key }) { const groups = new Map(); for (const value of values) { const group = String(value[key]); groups.set(group, [...(groups.get(group) || []), value]); } return Object.fromEntries(groups); }

Input & expected output

{
  "values": [
    {
      "recipient": "[email protected]",
      "status": "delivered"
    },
    {
      "recipient": "[email protected]",
      "status": "deferred"
    },
    {
      "recipient": "[email protected]",
      "status": "delivered"
    }
  ],
  "key": "status"
}

{
  "delivered": [
    {
      "recipient": "[email protected]",
      "status": "delivered"
    },
    {
      "recipient": "[email protected]",
      "status": "delivered"
    }
  ],
  "deferred": [
    {
      "recipient": "[email protected]",
      "status": "deferred"
    }
  ]
}

The edge case

{
  "values": [
    {
      "team": "__proto__"
    }
  ],
  "key": "team"
}

{
  "__proto__": [
    {
      "team": "__proto__"
    }
  ]
}

Know the limits

• Every record must own the grouping field, with a string or number value.
• Group keys become strings, so numeric 1 and string "1" share a group.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Clone an array — Create a new array without copying the objects inside it.
• Remove duplicate values — Keep the first occurrence of each value, in its original order.
• Flatten a nested array — Turn a nested array into one sequence of values.