Count by the properties of an array of objects

Tally records by the value of one field.

JavaScript Version 1.0.0 · MIT · .md

function countBy({ values, key }) { return { ...values.reduce((counts, item) => ((counts[item[key]] = (counts[item[key]] || 0) + 1), counts), Object.create(null)) }; }

Input & expected output

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

{
  "delivered": 2,
  "deferred": 1
}

The edge case

{
  "values": [],
  "key": "status"
}

{}

Know the limits

• Records without the field are counted under "undefined".
• A null-prototype accumulator keeps special keys as data.

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.