Docs / Language Manual / Bind to JS Function


Binding a JS function is like binding any other value:

ReScriptJS Output
// Import nodejs' path.dirname
@module("path") external dirname: string => string = "dirname"
let root = dirname("/User/github") // returns "User"

We also expose a few special features, described below.

Labeled Arguments

ReScript has labeled arguments (that can also be optional). These work on an external too! You'd use them to fix a JS function's unclear usage. Assuming we're modeling this:

// MyGame.js function draw(x, y, border) { // suppose `border` is optional and defaults to false } draw(10, 20) draw(20, 20, true)

It'd be nice if on ReScript's side, we can bind & call draw while labeling things a bit:

ReScriptJS Output
external draw: (~x: int, ~y: int, ~border: bool=?, unit) => unit = "draw"

draw(~x=10, ~y=20, ~border=true, ())
draw(~x=10, ~y=20, ())

We've compiled to the same function, but now the usage is much clearer on the ReScript side thanks to labels!

Note: in this particular case, you need a unit, () after border, since border is an optional argument at the last position. Not having a unit to indicate you've finished applying the function would generate a warning.

Note that you can freely reorder the labels on the ReScript side; they'll always correctly appear in their declaration order in the JavaScript output:

ReScriptJS Output
external draw: (~x: int, ~y: int, ~border: bool=?, unit) => unit = "draw"

draw(~x=10, ~y=20, ())
draw(~y=20, ~x=10, ())

Object Method

Functions attached to a JS objects (other than JS modules) require a special way of binding to them, using send:

ReScriptJS Output
type document // abstract type for a document object
@send external getElementById: (document, string) => Dom.element = "getElementById"
@val external doc: document = "document"

let el = getElementById(doc, "myId")

In a send, the object is always the first argument. Actual arguments of the method follow (this is a bit what modern OOP objects are really).


Ever used foo().bar().baz() chaining ("fluent api") in JS OOP? We can model that in ReScript too, through the pipe operator.

Variadic Function Arguments

You might have JS functions that take an arbitrary amount of arguments. ReScript supports modeling those, under the condition that the arbitrary arguments part is homogenous (aka of the same type). If so, add variadic to your external.

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

let v = join(["a", "b"])

module will be explained in Import from/Export to JS.

Modeling Polymorphic Function

Apart from the above special-case, JS function in general are often arbitrary overloaded in terms of argument types and number. How would you bind to those?

Trick 1: Multiple externals

If you can exhaustively enumerate the many forms an overloaded JS function can take, simply bind to each differently:

ReScriptJS Output
@module("MyGame") external drawCat: unit => unit = "draw"
@module("MyGame") external drawDog: (~giveName: string) => unit = "draw"
@module("MyGame") external draw: (string, ~useRandomAnimal: bool) => unit = "draw"

Note how all three externals bind to the same JS function, draw.

Trick 2: Polymorphic Variant + unwrap

If you have the irresistible urge of saying "if only this JS function argument was a variant instead of informally being either string or int", then good news: we do provide such external features through annotating a parameter as a polymorphic variant! Assuming you have the following JS function you'd like to bind to:

function padLeft(value, padding) { if (typeof padding === "number") { return Array(padding + 1).join(" ") + value; } if (typeof padding === "string") { return padding + value; } throw new Error(`Expected string or number, got '${padding}'.`); }

Here, padding is really conceptually a variant. Let's model it as such.

ReScriptJS Output
external padLeft: (
  @unwrap [
    | #Str(string)
    | #Int(int)
  => string = "padLeft"
padLeft("Hello World", #Int(4))
padLeft("Hello World", #Str("Message from ReScript: "))

Obviously, the JS side couldn't have an argument that's a polymorphic variant! But here, we're just piggy backing on poly variants' type checking and syntax. The secret is the @unwrap annotation on the type. It strips the variant constructors and compile to just the payload's value. See the output.

Constrain Arguments Better

Consider the Node fs.readFileSync's second argument. It can take a string, but really only a defined set: "ascii", "utf8", etc. You can still bind it as a string, but we can use poly variants + string to ensure that our usage's more correct:

ReScriptJS Output
external readFileSync: (
  ~name: string,
  @string [
    | #utf8
    | @as("ascii") #useAscii
) => string = "readFileSync"

readFileSync(~name="xx.txt", #useAscii)
  • Attaching @string to the whole poly variant type makes its constructor compile to a string of the same name.

  • Attaching a @as("bla") to a constructor lets you customize the final string.

And now, passing something like "myOwnUnicode" or other variant constructor names to readFileSync would correctly error.

Aside from string, you can also compile an argument to an int, using int instead of string in a similar way:

ReScriptJS Output
external testIntType: (
  @int [
    | #onClosed
    | @as(20) #onOpen
    | #inBinary
  => int = "testIntType"

onClosed compiles to 0, onOpen to 20 and inBinary to 21.

Unknown for type safety

It is best practice to inspect data received from untrusted external functions to ensure it contains what you expect. This helps avoid run-time crashes and unexpected behavior. If you're certain about what an external function returns, simply assert the return value as string or array<int> or whatever you want it to be. Otherwise use unknown. The ReScript type system will prevent you from using an unknown until you first inspect it and "convert" it using JSON parsing utilities or similar tools.

Consider the example below of two external functions that access the value of a property on a JavaScript object. getPropertyUnsafe returns an 'a, which means "anything you want it to be." ReScript allows you to use this value as a string or array or any other type. Quite convenient! But if the property is missing or contains something unexpected, your code might break. You can make the binding more safe by changing 'a to string or option<'a>, but this doesn't completely eliminate the problem.

The getPropertySafe function returns an unknown, which could be null or a string or anything else. But ReScript prevents you from using this value inappropriately until it has been safely parsed.

@get_index external getPropertyUnsafe: ({..}, string) => 'a = "" @get_index external getPropertySafe: ({..}, string) => unknown = "" let person = {"name": "Bob", "age": 12} let greeting1 = "Hello, " ++ getPropertyUnsafe(person, "name") // works (this time!) // let greeting2 = "Hello, " ++ getPropertySafe(person, "name") // syntax error

Special-case: Event Listeners

One last trick with polymorphic variants:

ReScriptJS Output
type readline

external on: (
    @string [
      | #close(unit => unit)
      | #line(string => unit)
  => readline = "on"

let register = rl =>
  ->on(#close(event => ()))
  ->on(#line(line => Js.log(line)));

Fixed Arguments

Sometimes it's convenient to bind to a function using an external, while passing predetermined argument values to the JS function:

ReScriptJS Output
external processOnExit: (
  @as("exit") _,
  int => unit
) => unit = "process.on"

processOnExit(exitCode =>
  Js.log("error code: " ++ Js.Int.toString(exitCode))

The @as("exit") and the placeholder _ argument together indicates that you want the first argument to compile to the string "exit". You can also use any JSON literal with as: @as(json`true`), @as(json`{"name": "John"}`), etc.

Ignore arguments

You can also explicitly "hide" external function parameters in the JS output, which may be useful if you want to add type constraints to other parameters without impacting the JS side:

ReScriptJS Output
@val external doSomething: (@ignore 'a, 'a) => unit = "doSomething"

doSomething("this only shows up in ReScript code", "test")

Note: It's a pretty niche feature, mostly used to map to polymorphic JS APIs.

Curry & Uncurry

Curry is a delicious Indian dish. More importantly, in the context of ReScript (and functional programming in general), currying means that function taking multiple arguments can be applied a few arguments at time, until all the arguments are applied.

See the addFive intermediate function? add takes in 3 arguments but received only 1. It's interpreted as "currying" the argument 5 and waiting for the next 2 arguments to be applied later on. Type signatures:

let add: (int, int, int) => int let addFive: (int, int) => int let twelve: int

(In a dynamic language such as JS, currying would be dangerous, since accidentally forgetting to pass an argument doesn't error at compile time).


Unfortunately, due to JS not having currying because of the aforementioned reason, it's hard for ReScript multi-argument functions to map cleanly to JS functions 100% of the time:

  1. When all the arguments of a function are supplied (aka no currying), ReScript does its best to to compile e.g. a 3-arguments call into a plain JS call with 3 arguments.

  2. If it's too hard to detect whether a function application is complete*, ReScript will use a runtime mechanism (the Curry module) to curry as many args as we can and check whether the result is fully applied.

  3. Some JS APIs like throttle, debounce and promise might mess with context, aka use the function bind mechanism, carry around this, etc. Such implementation clashes with the previous currying logic.

* If the call site is typed as having 3 arguments, we sometimes don't know whether it's a function that's being curried, or if the original one indeed has only 3 arguments.

ReScript tries to do #1 as much as it can. Even when it bails and uses #2's currying mechanism, it's usually harmless.

However, if you encounter #3, heuristics are not good enough: you need a guaranteed way of fully applying a function, without intermediate currying steps. We provide such guarantee through the use of the "uncurrying" syntax on a function declaration & call site.

Solution: Use Guaranteed Uncurrying

Uncurried function annotation also works on external:

ReScriptJS Output
type timerId
@val external setTimeout: ((. unit) => unit, int) => timerId = "setTimeout"

let id = setTimeout((.) => Js.log("hello"), 1000)

Extra Solution

The above solution is safe, guaranteed, and performant, but sometimes visually a little burdensome. We provide an alternative solution if:

  • you're using external

  • the external function takes in an argument that's another function

  • you want the user not to need to annotate the call sites with the dot

Then try @uncurry:

ReScriptJS Output
@send external map: (array<'a>, @uncurry ('a => 'b)) => array<'b> = "map"
map([1, 2, 3], x => x + 1)

In general, uncurry is recommended; the compiler will do lots of optimizations to resolve the currying to uncurrying at compile time. However, there are some cases the compiler can't optimize it. In these cases, it will be converted to a runtime check.

Modeling this-based Callbacks

Many JS libraries have callbacks which rely on this (the source), for example:

x.onload = function(v) { console.log(this.response + v) }

Here, this would point to x (actually, it depends on how onload is called, but we digress). It's not correct to declare x.onload of type (. unit) -> unit. Instead, we introduced a special attribute, this, which allows us to type x as so:

ReScriptJS Output
type x
@val external x: x = "x"
@set external setOnload: (x, @this ((x, int) => unit)) => unit = "onload"
@get external resp: x => int = "response"
setOnload(x, @this ((o, v) => Js.log(resp(o) + v)))

this has its first parameter is reserved for this and for arity of 0, there is no need for a redundant unit type.

Function Nullable Return Value Wrapping

For JS functions that return a value that can also be undefined or null, we provide @return(...). To automatically convert that value to an option type (recall that ReScript option type's None value only compiles to undefined and not null).

ReScriptJS Output
type element
type dom

@send @return(nullable)
external getElementById: (dom, string) => option<element> = "getElementById"

let test = dom => {
  let elem = dom->(getElementById("haha"))
  switch (elem) {
  | None => 1
  | Some(_ui) => 2

return(nullable) attribute will automatically convert null and undefined to option type.

Currently 4 directives are supported: null_to_opt, undefined_to_opt, nullable and identity.

identity will make sure that compiler will do nothing about the returned value. It is rarely used, but introduced here for debugging purpose.