ppx_expjs

A ppx for exporting OCaml values to JavaScript using type annotations.

Example

my_module.ml:

let concat (s1 : string) (s2 : string) : string = s1 ^ s2
[@@expjs]

node:

Welcome to Node.js v16.13.0.
Type ".help" for more information.
> const my_module = require("./_build/default/my_module.bc.js");
undefined
> x.concat("Hello ", "World")
"Hello World"
>

Usage

Basic

If you want to convert between JS and OCaml values of "standard" types (i.e. integers, strings, optional values), simply add the [@@expjs] attribute to any value binding with type annotations, i.e.

let my_int : int = 10 [@@expjs]

Custom conversions

You can specify a custom convertor function to/from JS using the [@expjs.conv] attribute:

let f (x [@expjs.conv int_of_js]) = ... [@@expjs]

Strict mode

You can require that all values have conversions by enabling strict mode. For example, the following block will fail since there are no type annotations for x:

let f x = ... [@@expjs { strict = true }]

Without strict mode, values that don't have known or explicit conversion functions are passed directly as JS values.

Named arguments

Named arguments use the standard JavaScript convention of having the first argument be an object with fields corresponding to the argument names. For example, the following OCaml function:

let f ~x = ... [@@expjs]

can be called in JS as:

const foo = f({x: y});

Default type mappings

OCaml	JavaScript
option : Some x	x
option : None	null
int	number
float	number
string	String
unit	undefined

lists are not yet supported, but are planned to be.

License

This project is licensed under the MIT License.