count

Counts items in a nested array.

Signature

count<ArrayName extends string, TPropName extends string>(
  arrayName: ArrayName,
  outputProperty: TPropName
): PipelineBuilder</* type with outputProperty: number */>

Parameters

Parameter	Type	Description
arrayName	string	Name of the array to count
outputProperty	string	Name of the new count property

Returns

A new PipelineBuilder with the count property added to the parent type.

Example

const builder = createPipeline<Sale>()
  .groupBy(["category"], "items")
  .count("items", "itemCount");

Result:

{
  key: "hash_Electronics",
  value: {
    category: "Electronics",
    itemCount: 3,  // number of items in the array
    items: [...]
  }
}

Incremental Updates

• Add: count = count + 1
• Remove: count = count - 1

Both operations are O(1).

With Scoping

.groupBy(["region"], "regions")
.in("regions").groupBy(["city"], "cities")
.in("regions", "cities").count("items", "venueCount")
.in("regions").count("cities", "cityCount")

See Also

• sum - Sum numeric values
• average - Compute average