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Interop Cheatsheet

This is a glossary with examples. All the features are described by later pages.

List of Decorators

Note: In ReScript < 8.3, all our attributes started with the bs. prefix. This is no longer needed and our formatter automatically removes them in newer ReScript versions.

Attributes

Extension Points

Raw JS

ReScriptJS Output
let add = %raw("(a, b) => a + b")
%%raw("const a = 1")

Global Value

ReScriptJS Output
@val external setTimeout: (unit => unit, int) => float = "setTimeout"

Global Module's Value

ReScriptJS Output
@val @scope("Math")
external random: unit => float = "random"

let someNumber = random()

@val @scope(("window", "location", "ancestorOrigins"))
external length: int = "length"

Nullable

ReScriptJS Output
let a = Some(5) // compiles to 5
let b = None // compiles to undefined

Handling a value that can be undefined and null, by ditching the option type and using Nullable.t:

ReScriptJS Output
let jsNull = Nullable.null
let jsUndefined = Nullable.undefined
let result1: Nullable.t<string> = Nullable.make("hello")
let result2: Nullable.t<int> = Nullable.fromOption(Some(10))
let result3: option<int> = Nullable.toOption(Nullable.make(10))

JS Object

Function

Object Method & Chaining

ReScriptJS Output
@send external map: (array<'a>, 'a => 'b) => array<'b> = "map"
@send external filter: (array<'a>, 'a => 'b) => array<'b> = "filter"
[1, 2, 3]
  ->map(a => a + 1)
  ->filter(a => mod(a, 2) == 0)
  ->Console.log

Variadic Arguments

ReScriptJS Output
@module("path") @variadic
external join: array<string> => string = "join"

Tagged template functions

ReScriptJS Output
// see https://bun.sh/docs/runtime/shell
type result = {exitCode: int}
@module("bun") @taggedTemplate
external sh: (array<string>, array<string>) => promise<result> = "$"

let filename = "index.res"
let result = await sh`ls ${filename}`

Polymorphic Function

ReScriptJS Output
@module("Drawing") external drawCat: unit => unit = "draw"
@module("Drawing") external drawDog: (~giveName: string) => unit = "draw"
ReScriptJS Output
@val
external padLeft: (
  string,
  @unwrap [
    | #Str(string)
    | #Int(int)
  ])
  => string = "padLeft"

padLeft("Hello World", #Int(4))
padLeft("Hello World", #Str("Message from ReScript: "))

JS Module Interop

See here

Dangerous Type Cast

Final escape hatch converter. Do not abuse.

ReScriptJS Output
external convertToFloat: int => float = "%identity"
let age = 10
let gpa = 2.1 +. convertToFloat(age)
Scoped Polymorphic TypesEmbed Raw JavaScript