API / Core / Map

Map

t

RESCRIPT

type t<'k, 'v> = Js.Map.t<'k, 'v>

Type representing an instance of Map.

make

RESCRIPT

let make: unit => t<'k, 'v>

Creates a new, mutable JavaScript Map. A Map can have any values as both keys and values.

See Map on MDN.

Examples

RESCRIPT
`make()`
// You can annotate the type of your map if you want to
let myMap: Map.t<string, int> = Map.make()

// Or you can let ReScript infer what's in your map
let map = Map.make()
map->Map.set("lang", "ReScript") // Inferred as Map.t<string, string>

Alternatives

A JavaScript Map is mutable. If you're looking for an immutable alternative, check outBelt.Map.

fromArray

RESCRIPT

let fromArray: array<('k, 'v)> => t<'k, 'v>

Turns an array of key/value pairs into a Map.

Examples

RESCRIPT
type languages = ReScript | JavaScript | TypeScript
let languageRank = [(ReScript, 1), (JavaScript, 2), (TypeScript, 3)]

let map = Map.fromArray(languageRank) // Map.t<languages, int>

switch map->Map.get(ReScript) {
| Some(1) => Console.log("Yay, ReScript is #1!")
| _ => Console.log("Uh-oh, something is _terribly_ wrong with this program... abort.")
}

fromIterator

RESCRIPT

let fromIterator: Core__Iterator.t<('k, 'v)> => t<'k, 'v>

Turns an iterator in the shape of ('key, 'value) into a Map.

Examples

RESCRIPT
// Let's pretend we have an interator in the correct shape
@val external someIterator: Iterator.t<(string, int)> = "someIterator"

let map = Map.fromIterator(someIterator) // Map.t<string, int>

size

RESCRIPT

let size: t<'k, 'v> => int

Returns the size, the number of key/value pairs, of the map.

Examples

RESCRIPT
let map = Map.make()

map->Map.set("someKey", "someValue")

let size = map->Map.size // 1

clear

RESCRIPT

let clear: t<'k, 'v> => unit

Clears all entries in the map.

Examples

RESCRIPT
let map = Map.make()

map->Map.set("someKey", "someValue")
let size = map->Map.size // 1

map->Map.clear
let size = map->Map.size // 0

forEach

RESCRIPT

let forEach: (t<'k, 'v>, 'v => unit) => unit

Iterates through all values of the map.

Please note that this is without the keys, just the values. If you need the key as well, use Map.forEachWithKey.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
map->Map.set("someKey2", "someValue2")

map->Map.forEach(value => {
  Console.log(value)
})

forEachWithKey

RESCRIPT

let forEachWithKey: (t<'k, 'v>, ('v, 'k) => unit) => unit

Iterates through all values of the map, including the key for each value.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
map->Map.set("someKey2", "someValue2")

map->Map.forEachWithKey((value, key) => {
  Console.log2(value, key)
})

get

RESCRIPT

let get: (t<'k, 'v>, 'k) => option<'v>

Returns the value for a key, if a value exists at that key.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")

switch map->Map.get("someKey") {
| None => Console.log("Nope, didn't have it.")
| Some(value) => Console.log2("Yay, had the value, and it's:", value)
}

has

RESCRIPT

let has: (t<'k, 'v>, 'k) => bool

Checks whether the map has a specific key.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")

switch map->Map.has("someKey") {
| false => Console.log("Nope, didn't have it.")
| true => Console.log("Yay, we have the value!")
}

set

RESCRIPT

let set: (t<'k, 'v>, 'k, 'v) => unit

Sets the provided value to the provided key.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")

delete

RESCRIPT

let delete: (t<'k, 'v>, 'k) => bool

Deletes the provided key and its value from the map. Returns a bool for whether the key existed, and was deleted.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
let didDeleteKey = map->Map.delete("someKey")
Console.log(didDeleteKey) // Logs `true` to the console, becuase the map had the key, so it was successfully deleted

let didDeleteKey = map->Map.delete("someNonExistantKey")
Console.log(didDeleteKey) // Logs `false` to the console, becuase the key did not exist

keys

RESCRIPT

let keys: t<'k, 'v> => Core__Iterator.t<'k>

Returns an iterator that holds all keys of the map.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
map->Map.set("anotherKey", "anotherValue")

let keys = map->Map.keys

// Logs the first key
Console.log(Iterator.next(keys).value)

// You can also turn the iterator into an array.
// Remember that an iterator consumes values. We'll need a fresh keys iterator to get an array of all keys, since we consumed a value via `next` above already.
Console.log(map->Map.keys->Iterator.toArray)

values

RESCRIPT

let values: t<'k, 'v> => Core__Iterator.t<'v>

Returns an iterator that holds all values of the map.

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
map->Map.set("anotherKey", "anotherValue")

let values = map->Map.values

// Logs the first value
Console.log(Iterator.next(values).value)

// You can also turn the iterator into an array.
// Remember that an iterator consumes values. We'll need a fresh values iterator to get an array of all values, since we consumed a value via `next` above already.
Console.log(map->Map.values->Iterator.toArray)

entries

RESCRIPT

let entries: t<'k, 'v> => Core__Iterator.t<('k, 'v)>

Returns an iterator that holds all entries of the map. An entry is represented as a tuple of ('key, 'value),

Examples

RESCRIPT
let map = Map.make()
map->Map.set("someKey", "someValue")
map->Map.set("anotherKey", "anotherValue")

let entries = map->Map.entries

// Logs the first value
Console.log(Iterator.next(entries).value)

// You can also turn the iterator into an array.
// Remember that an iterator consumes entries. We'll need a fresh entries iterator to get an array of all entries, since we consumed a value via `next` above already.
Console.log(map->Map.entries->Iterator.toArray)