How to interface with Javascript

There are a number of ways to talk to Javascript code and objects from OCaml:

External functions

You can call Javascript functions in the same way you call C primitives in ordinary OCaml, by declaring them external. Put the external functions in a file foo.js, and add foo.js to the link line. The names of the primitives are read out of the Javascript file; any name after var or function at the beginning of the line is included.

You can also use special external declarations to avoid having to write trivial wrapper functions for Javascript field accessors and method calls. The following special prefixes to the function name are recognized:

prefixcompiles to
#method call
.read property
=assign property
@function call for builtin function, not checked against primitives

The object-related prefixes expect the object as the first argument. For example, if you declare external foo : bar -> unit = "#foo" and call foo b, that compiles to

Finally, there are several special external symbols which produce Javascript syntax when compiled. They can also help you to avoid trivial wrappers in some cases. They expect particular OCaml syntax as arguments (they are recognized at compile time). But see below for inline Javascript, which is a better way to do it for larger fragments.

symbolexpects argscompiles to
$assign[lvalue; expression]lvalue = expression
$fieldref[expression; id]
$function[function]Javascript function
$hashref[exp1; exp2]exp1[exp2]
$newid::argsnew id(args)
$new idargsnew id(args)
$objalist of ids and expsobject literal
$throw[expression]throw expression

Identifiers are given as strings. The $new id symbol uses the native name argument to external, e.g. external "$new" "Foo" : unit -> foo. See comp_ccall in src/jscomp/ to see exactly what these do.

Most of these are available in the Ocamljs library, or you can declare them yourself with external (sometimes that’s more convenient if you want more accurate types).

Object wrappers

It’s possible to call methods on Javascript objects using the OCaml method call syntax. For example, document#getElementById "foo" becomes document.getElementById("foo"). You can also access the properties of an object with specially-named methods, e.g. style#_get_background and style#_set_background "#ffffff" become style.background and style.background = "#ffffff".

There are some other special cases: a leading underscore is dropped so you can use capitalized method names (e.g. obj#_QueryInterface); a trailing underscore or _.*_ is dropped so you can give multiple types to the same method when the Javascript interface is more dynamically-typed than can be accomodated in OCaml (e.g. canvas#createPattern and canvas#createPattern_canvas_ both become canvas.createPattern); you can use either of these to deal with reserved words (e.g. _open or end_).

To get these uses to typecheck, of course, you need to operate on an object of the appropriate type. It’s easiest to write down an interface as a class type, then magic your Javascript object to that type, or get the object using a function or method with a polymorphic return value (such as Ocamljs.var or Dom.document#getElementById) and ascribe the type. See the Dom and Mozilla libraries for examples.

Types given this way are only advisory in two senses: first, you have to ascribe the correct types; second, if an object is given as input to a Javascript method or function, it may not be the type that’s expected even if it has the right shape (e.g. a DOM object must be an actual DOM object, not just an object with the same methods). In both cases there can be a dynamic failure at the Javascript level that’s not caught at compile time.

A final caveat is that partial application does not work for native methods; a partial application causes an immediate call to the method with the arguments you provide. (Partial application does work for functions called via external, because the OCaml compiler wraps an OCaml function around them.)

Using OCaml objects from Javascript

Objects defined in OCaml can be used from Javascript; they are compiled to Javascript objects with the same method names (along with some other stuff to support OCaml’s object system—see here for details). There is no way to define properties, however, only methods. (XXX should implement _get_foo to generate a property foo.)

Inline Javascript

A final way to talk to Javascript is to use inline code. This is implemented with Camlp4 and Jslib; you need to compile with Camlp4 and the jslib.inline package, then open Ocamljs.Inline to get the AST types. To get an inline expression use an <:exp< >> or << >> quotation; for an inline statment use <:stmt< >>. You can use antiquotations to refer to OCaml expressions. For example:

  let concat s1 s2 = << $s1$ + $s2$ >>

There is no typechecking of quotations and antiquotations. You can see some other examples in src/stdlib. Caveat: the Jslib parser is incomplete.


OCaml functions are compiled to Javascript functions, so you can use them directly as callbacks. For example, Dom.window#setTimeout takes a (unit -> unit).

You can use the usual Callback.register function in the standard library to register a value for use from Javascript. A linked ocamljs program exports a function ocamljs$caml_named_value which you can use to look up registered values. You can also get access to OCaml values by stashing them in a global from inline code.

Strings, null

See Jscomp for details about how OCaml is compiled. In particular, when interfacing with Javascript you may need to be aware how values are represented, particularly strings, which are not always Javascript strings.

Most Javascript code can return null, and this can cause a runtime failure at the Javascript level that isn’t checked at compile time. You should use Ocamljs.is_null before using a value that might be null.