array.map()

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.

Apply a provided function to each element of an Array. Can’t perform writes.

Signature

map(mapper: (A => B)) => Array<B>

Description

Creates an Array by applying a mapper function to each element in the calling Array.

Writes are not permitted. 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

mapper

Function

Yes

Anonymous FQL function to call on each element of the calling Array. Each call returns a value that’s returned in the result Array.

Return value

Type Description

Array<Generic>

Array containing values returned by each mapper function call.

Examples

Basic

  1. Create an Array by mapping Array elements to subArrays that are constructed from the element value and the element value plus one, using an anonymous Function:

    [1, 2, 3].map(x => [x, x + 1])
    [
  [
    1,
    2
  ],
  [
    2,
    3
  ],
  [
    3,
    4
  ]
]

  2. Create an Array by passing a top-level object method as the mapping Function:

    [{name: "D"}, {name: "E"}].map(Collection.create)
    [
  {
    name: "D",
    coll: Collection,
    ts: Time("2099-02-18T20:06:25.620Z"),
    indexes: {},
    constraints: [],
    history_days: 0
  },
  {
    name: "E",
    coll: Collection,
    ts: Time("2099-02-18T20:06:25.620Z"),
    indexes: {},
    constraints: [],
    history_days: 0
  }
]

array.forEach() vs. array.map()

You can use both array.forEach() and array.map() to iterate through an Array. However, each method returns a different value type.

array.forEach() returns null:

// Create an Array of objects that contain category data.
let categories = [
  {
    "name": "Electronics",
    "description": "Bargain electronics!"
  },
  {
    "name": "Books",
    "description": "Bargain books!"
  }
]

// Use `forEach()` to create a `Category` collection document for each
// element of the previous Array.
categories.forEach(doc => Category.create({ doc }))
null

Although it returns null, array.forEach() still performed the requested operations. To verify:

// Get all `Category` collection documents
Category.all()
  // The results contain `Customer` documents created by
  // the previous `forEach()` call.
{
  data: [
    ...
    {
      id: "410665732340187209",
      coll: Category,
      ts: Time("2099-10-02T20:16:51.560Z"),
      products: "hdW...",
      name: "Electronics",
      description: "Bargain electronics!"
    },
    {
      id: "410665732340188233",
      coll: Category,
      ts: Time("2099-10-02T20:16:51.560Z"),
      products: "hdW...",
      name: "Books",
      description: "Bargain books!"
    }
  ]
}

array.map() returns an Array:

// Create an Array of objects that contain category data.
let categories = [
  {
    "name": "Movies",
    "description": "Bargain movies!"
  },
  {
    "name": "Music",
    "description": "Bargain music!"
  }
]

// Use `map()` to create a `Category` collection document for each
// element of the previous Array.
categories.map(doc => Category.create({ doc }))

In this case, the Array contains documents created by the array.map() call:

[
  {
    id: "410655308219678797",
    coll: Category,
    ts: Time("2099-10-02T17:31:10.366Z"),
    products: "hdW...",
    name: "Movies",
    description: "Bargain movies!"
  },
  {
    id: "410655308219679821",
    coll: Category,
    ts: Time("2099-10-02T17:31:10.366Z"),
    products: "hdW...",
    name: "Music",
    description: "Bargain music!"
  }
]

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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