array.flatMap()

This method operates on an array. You typically fetch documents from a collection as a set, not an Array. For the equivalent Set method, see set instance methods.

For differences between Sets and Arrays, see Sets vs. Arrays.

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

Signature

flatMap(function: (element: T) => Array<U>) => Array<U>

Description

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

The calling Array isn’t changed.

Array iteration methods

FQL provides several methods for iterating over an Array. array.forEach(), array.map(), and array.flatMap() are similar but return different values.

Method Return value

array.forEach()

Null.

array.map()

Array.

array.flatMap()

Array, flattened by one level.

For examples, see:

Parameters

Parameter Type Required Description

function

Function

Yes

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

function parameters:

Parameter Type Required Description

element

Any

Yes

Array element to evaluate.

Return value

Type Description

Array

Array with the results of calling function on each element of the calling Array. The resulting Array is flattened by one level.

Examples

Basic example

[1, 2, 3].flatMap((val) => [val, val * 2, val * 3])
[
  1,
  2,
  3,
  2,
  4,
  6,
  3,
  6,
  9
]

array.map() vs. array.flatMap()

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

In the following example, array.map() returns a two-dimensional Array:

// Create an Array of product names.
let products = [
  "limes",
  "avocados"
]

// Use `map()` to get a Set of matching `Product` collection
// documents for each name. Convert each Set to an Array.
products.map(product => {
  Product.byName(product).toArray()
})
// Two-dimensional Array.
[
  [
    {
      id: "777",
      coll: Product,
      ts: Time("2099-10-02T19:37:36.357Z"),
      name: "limes",
      description: "Conventional, 16 oz bag",
      price: 299,
      stock: 30,
      category: Category("789")
    }
  ],
  [
    {
      id: "444",
      coll: Product,
      ts: Time("2099-10-02T19:37:36.357Z"),
      name: "avocados",
      description: "Conventional Hass, 4ct bag",
      price: 399,
      stock: 1000,
      category: Category("789")
    }
  ]
]

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

// Create an Array of product names.
let products = [
  "limes",
  "avocados"
]

// Use `flatMap()` to get a Set of matching `Product` collection
// documents for each name. Convert each Set to an Array.
// Then flatten the resulting Array by one level.
products.flatMap(product => {
  Product.byName(product).toArray()
})
// One-dimensional Array.
[
  {
    id: "777",
    coll: Product,
    ts: Time("2099-10-02T19:37:36.357Z"),
    name: "limes",
    description: "Conventional, 16 oz bag",
    price: 299,
    stock: 30,
    category: Category("789")
  },
  {
    id: "444",
    coll: Product,
    ts: Time("2099-10-02T19:37:36.357Z"),
    name: "avocados",
    description: "Conventional Hass, 4ct bag",
    price: 399,
    stock: 1000,
    category: Category("789")
  }
]

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