Count the occurrences of array elements

Tally every distinct value into an object.

JavaScript Version 1.0.0 · MIT · .md

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

Input & expected output

[
  "delivered",
  "deferred",
  "delivered",
  "bounced"
]

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

The edge case

[
  "__proto__",
  "constructor"
]

{
  "__proto__": 1,
  "constructor": 1
}

Know the limits

• Keys become strings, so 1 and "1" share a count.
• A null-prototype accumulator keeps "__proto__" and "constructor" as ordinary keys; the original counted them wrongly.

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.