Javascript and Node Code Generators

There are two javascript code generators, node and javascript. There are two differences between the two: the javascript code generator when called to output an HTML file will also generate a basic HTML document with the generated code inside a <script> tag; the other distinction is on the ffi that will be explained below.

Javascript FFI Specifiers

There are three main kinds of javascript ffi specifiers javascript, node and browser. javascript is for foreigns that are available on node and the browser, node for foreigns that are only available on node and browser for browser only foreigns.

For node there are two ways of defining a foreign:

%foreign "node:lambda: n => process.env[n]"
prim_getEnv : String -> PrimIO (Ptr String)

here lambda means that we are providing the definition as a lambda expression.

%foreign "node:lambda:fp=>require('fs').fstatSync(fp.fd, {bigint: false}).size"
prim__fileSize : FilePtr -> PrimIO Int

require can be used to import javascript modules.

For completion below an example of a foreign available only with browser codegen:

%foreign "browser:lambda:x=>{document.body.innerHTML = x}"
prim__setBodyInnerHTML : String -> PrimIO ()

Short Example

An interesting example is creating a foreign for the setTimeout function:

%foreign "javascript:lambda:(callback, delay)=>setTimeout(callback, delay)"
prim__setTimeout : (PrimIO ()) -> Int -> PrimIO ()

setTimeout : HasIO io => IO () -> Int -> io ()
setTimeout callback delay = primIO $ prim__setTimeout (toPrim callback) delay

Note: Previous versions of the javascript backends treated Int as a 64 bit signed integer represented by BigInt in javascript land. This is no longer the case: Int is now treated as a 32 bit signed integer represented by Number. This should facilitate interop between Idris2 and the backend.

However, unless you have good reasons to do otherwise, consider using one of the other fixed precision integral types. They are supposed to behave the same across all backends. All signed and unsigned integrals of up to 32 bit precision (Int8, Int16, Int32, Bits8, Bits16, and Bits32) are represented by Number while Int64, Bits64, and Integer are represented by BigInt. The example above could therefore be improved by using Int32 instad of Int:

%foreign "javascript:lambda:(callback, delay)=>setTimeout(callback, delay)"
prim__setTimeout : (PrimIO ()) -> Int32 -> PrimIO ()

setTimeout : HasIO io => IO () -> Int32 -> io ()
setTimeout callback delay = primIO $ prim__setTimeout (toPrim callback) delay

Browser Example

To build JavaScript aimed to use in the browser, the code must be compiled with the javascript codegen option. The compiler produces a JavaScript or an HTML file. The browser needs an HTML file to load. This HTML file can be created in two ways

  • If the .html suffix is given to the output file the compiler generates an HTML file which includes a wrapping around the generated JavaScript.

  • If no .html suffix is given, the generated file only contains the JavaScript code. In this case manual wrapping is needed.

Example of the wrapper HTML:

 <head><meta charset='utf-8'></head>
  <script type='text/javascript'>
  JS code goes here

As our intention is to develop something that runs in the browser questions naturally arise:

  • How to interact with HTML elements?

  • More importantly, when does the generated Idris code start?

Starting point of the Idris generated code

The generated JavaScript for your program contains an entry point. The main function is compiled to a JavaScript top-level expression, which will be evaluated during the loading of the script tag and that is the entry point for Idris generated program starting in the browser.

Interaction with HTML elements

As sketched in the Short Example section, the FFI must be used when interaction happens between Idris generated code and the rest of the Browser/JS ecosystem. Information handled by the FFI is separated into two categories. Primitive types in Idris FFI, such as Int, and everything else. The everything else part is accessed via AnyPtr. The %foreign construction should be used to give implementation on the JavaScript side. And an Idris function declaration to give Type declaration on the Idris side. The syntax is %foreign "browser:lambda:js-lambda-expression" . On the Idris side, primitive types and PrimIO t types should be used as parameters, when defining %foreign. This declaration is a helper function which needs to be called behind the primIO function. More on this can be found in the FFI chapter.

Examples for JavaScript FFI


%foreign "browser:lambda: x => console.log(x)"
prim__consoleLog : String -> PrimIO ()

consoleLog : HasIO io => String -> io ()
consoleLog x = primIO $ prim__consoleLog x

String is a primitive type in Idris and it is represented as a JavaScript String. There is no need for any conversion between the Idris and the JavaScript.


%foreign "browser:lambda: (a,i)=>setInterval(a,i)"
prim__setInterval : PrimIO () -> Int32 -> PrimIO ()

setInterval : (HasIO io) => IO () -> Int32 -> io ()
setInterval a i = primIO $ prim__setInterval (toPrim a) i

The setInterval JavaScript function executes the given function in every x millisecond. We can use Idris generated functions in the callback as far as they have the type IO () .

HTML Dom elements

Lets turn our attention to the Dom elements and events. As said above, anything that is not a primitive type should be handled via the AnyPtr type in the FFI. Anything complex that is returned by a JavaScript function should be captured in an AnyPtr value. It is advisory to separate the AnyPtr values into categories.

data DomNode = MkNode AnyPtr

%foreign "browser:lambda: () => document.body"
prim__body : () -> PrimIO AnyPtr

body : HasIO io => io DomNode
body = map MkNode $ primIO $ prim__body ()

We create a DomNode type which holds an AnyPtr. The prim__body function wraps a lambda function with no parameters. In the Idris function body we pass an extra () parameter and the we wrap the result in the DomNode type using the MkNode data constructor.

Primitive values originated in JavaScript

As a countinuation of the previous example, the width attribute of a DOM element can be retrieved via the FFI.

%foreign "browser:lambda: n=>(n.width)"
prim__width : AnyPtr -> PrimIO Bits32

width : HasIO io => DomNode -> io Bits32
width (MkNode p) = primIO $ prim__width p

Handling JavaScript events

data DomEvent = MkEvent AnyPtr

%foreign "browser:lambda: (event, callback, node) => node.addEventListener(event, x=>callback(x)())"
prim__addEventListener : String -> (AnyPtr -> PrimIO ()) -> AnyPtr -> PrimIO ()

addEventListener : HasIO io => String -> DomNode -> (DomEvent -> IO ()) -> io ()
addEventListener event (MkNode n) callback =
  primIO $ prim__addEventListener event (\ptr => toPrim $ callback $ MkEvent ptr) n

In this example shows how to attach an event handler to a particular DOM element. Values of events are also associated with AnyPtr on the Idris side. To seperate DomNode form DomEvent we create two different types. Also it demonstrates how a simple callback function defined in Idris can be used on the JavaScript side.


The javascript code generators accepts three different directives about how compact and obfusacted the generated code should be. The following examples show the code generated for the putStr function from the prelude for each of the three directives. (--cg node is used in the examples below, but the behavior is the same when generating code to be run in browsers with --cg javascript).

With idris2 --cg node --directive pretty (the default, if no directive is given), a basic pretty printer is used to generate properly indented javascript code.

function Prelude_IO_putStr($0, $1) {
 return $0.a2(undefined)($7 => Prelude_IO_prim__putStr($1, $7));

With idris2 --cg node --directive compact, every toplevel function is declared on a single line, and unneeded spaces are removed:

function Prelude_IO_putStr($0,$1){return $0.a2(undefined)($7=>Prelude_IO_prim__putStr($1,$7));}

Finally, with idris2 --cg node --directive minimal, toplevel function names are (with a few exceptions like the ones from the static preamble) obfuscated to reduce the size of the generated javascript file:

function $R3a($0,$1){return $0.a2(undefined)($7=>$R3b($1,$7));}