set.flatMap()

Create an Set by applying a Function to each Set element, then flatten the result by one level.

Lazy loading:

Yes

Signature

flatMap(function: (element: Any) => Set) => Set

Description

The flatMap() method creates an Set by invoking function on each element of the calling Set and flattening the resulting Set one level. The Set elements are passed as a parameter to function, sequentially.

The calling Set isn’t changed.

Iterator methods

FQL provides several methods for iterating over a Set. set.forEach(), set.map(), set.flatMap() are similar but used for different purposes:

Method Primary use Notes

set.forEach()

Perform in-place writes on Set elements.

Doesn’t return a value.

set.map()

Returns a new Set.

Can’t perform writes.

set.flatMap()

Similar to set.map(), but flattens the resulting Set by one level.

Can’t perform writes.

For examples, see:

Parameters

Parameter Type Required Description

function

Function

Yes

Function to invoke for each Set element that the Set represents. Each Set element is passed to function as an argument. function must return an Set

function parameters:

Parameter Type Required Description

element

Any

Yes

Set element to evaluate.

Return value

Type Description

Set

Set of the result of calling function on each element of the calling Set. The resulting Set is flattened by one level.

Examples

Basic example

// Returns a Set
let topCustomer = Customer.byEmail("alice.appleseed@example.com")

// Returns a flattened Set
topCustomer.flatMap((customer) => Order.byCustomer(customer))
{
  data: [
    {
      id: "410674590662000717",
      coll: Order,
      ts: Time("2099-10-02T22:37:39.583Z"),
      items: "hdW...",
      total: 5392,
      status: "cart",
      customer: Customer("111"),
      createdAt: Time("2099-10-02T22:37:39.434810Z"),
      payment: {}
    }
  ]
}

set.map() vs. set.flatMap()

set.flatMap() is similar to set.map(), except set.flatMap() also flattens the resulting Set by one level.

In the following example, set.map() returns a two-dimensional Set:

// Get a Set of all `Category` collection documents.
let categories = Category.all()

// Use `map()` to get a Set of `Product` documents
// for each category.
categories.map(category => {
  let category: Any = category
  Product.byCategory(category)
})
// Two-dimensional Set.
{
  data: [
    {
      data: [
        {
          id: "111",
          coll: Product,
          ts: Time("2099-10-02T22:37:39.583Z"),
          name: "cups",
          description: "Translucent 9 Oz, 100 ct",
          price: 698,
          stock: 100,
          category: Category("123")
        },
        ...
      ]
    },
    {
      data: [
        {
          id: "333",
          coll: Product,
          ts: Time("2099-10-02T22:37:39.583Z"),
          name: "pizza",
          description: "Frozen Cheese",
          price: 499,
          stock: 100,
          category: Category("456")
        }
      ]
    },
    {
      data: [
        {
          id: "444",
          coll: Product,
          ts: Time("2099-10-02T22:37:39.583Z"),
          name: "avocados",
          description: "Conventional Hass, 4ct bag",
          price: 399,
          stock: 1000,
          category: Category("789")
        },
        ...
      ]
    }
  ]
}

To flatten the result to a one-dimensional array, use set.flatMap() instead:

// Get a Set of all `Category` collection documents.
let categories = Category.all()

// Use `flatMap()` to get a Set of `Product` documents
// for each category. Then flatten the resulting Set.
categories.flatMap(category => {
  let category: Any = category
  Product.byCategory(category)
})
// One-dimensional Set.
{
  data: [
    {
      id: "111",
      coll: Product,
      ts: Time("2099-10-02T22:37:39.583Z"),
      name: "cups",
      description: "Translucent 9 Oz, 100 ct",
      price: 698,
      stock: 100,
      category: Category("123")
    },
    ...
    {
      id: "333",
      coll: Product,
      ts: Time("2099-10-02T22:37:39.583Z"),
      name: "pizza",
      description: "Frozen Cheese",
      price: 499,
      stock: 100,
      category: Category("456")
    },
    ...
    {
      id: "444",
      coll: Product,
      ts: Time("2099-10-02T22:37:39.583Z"),
      name: "avocados",
      description: "Conventional Hass, 4ct bag",
      price: 399,
      stock: 1000,
      category: Category("789")
    }
  ]
}

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