reduce()

Reduce a set to a single, accumulated value by applying a function to each element. Iterates through elements from left to right. Uses the first element as the initial value.

Signature

reduce(reducer: A, A) => A => A | Null

Description

The reduce() method iterates through each element in a set to perform a rolling operation. For example, you can use reduce() to calculate a rolling sum, concatenate elements, or perform complex transformations.

reduce() calls a reducer callback function on every element of the set from left to right. The reducer function takes two arguments:

  • The accumulator that holds the running result from previous iterations. The first element in the set serves as the initial accumulator.

  • The current element’s value.

The method returns the result of the last iteration. The calling set isn’t changed.

Because this method scans the full set, it returns an error if there are more than 16,000 documents in the set. This method can timeout for large sets under that limit.

Fold family methods

FQL supports several methods for folds, which iteratively reduce a set to a single value. These methods include:

The methods are similar but have the following differences:

Parameters

Parameter Type Required Description

reducer

Function

Yes

Anonymous FQL function to call on each element in the set.

reducer parameters:

Parameter Type Required Description

accumulator

Any

Yes

Value returned by the previous reducer function call. On the first call, seed is passed as the accumulator.

current

Any

Yes

The current element’s value.

Return value

Type Description

Any | Null

Result of the last reducer function call.

Examples

  1. Starting with the following product prices:

    // Gets a set of the first nine `Product` collection
// documents and projects the `price` field.
Product.all().take(9) { price }
    {
  data: [
    {
      price: 6.98
    },
    {
      price: 24.99
    },
    {
      price: 4.99
    },
    {
      price: 3.99
    },
    {
      price: 0.35
    },
    {
      price: 3.49
    },
    {
      price: 2.99
    },
    {
      price: 1.49
    },
    {
      price: 23.99
    }
  ]
}

  2. Use reduce() to find the maximum price in the set:

    Product.all().take(9).reduce((s, v) => {
  if (v.price > s.price) { v } else { s }
}) {price}
    {
  price: 24.99
}

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