JavaScript to ReasonML transpiler for small code examples
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JavaScript to ReasonML Transpiler

Check it out Live on GitHub Pages. Tested with:

  • Chrome 65
  • Firefox 59
  • Edge 41

Table of contents


This is a helper script to port small examples from JS to ReasonML. It should be mainly for generating a baseline for externals, unless it grows to something bigger :p . Requires to actually run the code with eval because it will inspect the types for declaration at runtime. Code path coverage in example must be 100% for now (TODO: implement partial transpilation with unresolvedAnyT if requested).

Why eval and not flow? I tried flow and it wasn't able to infer much, while evaling the code gets all types without much effort. Conditional code paths may present complications though, so try to avoid them in example code, or if used you must ensure that all conditional paths are evaluated at least once for types to resolve. See "Manual labour" for more information.

I am using :

  • Esprima (reading js)
  • Escodegen (writing js)

Example transpilation


var app = new PIXI.Application(800, 600, { backgroundColor: 0x1099bb });

// Scale mode for all textures, will retain pixelation

var sprite = PIXI.Sprite.fromImage('');

// Set the initial position
sprite.x = app.screen.width / 2;
sprite.y = app.screen.height / 2;

// Opt-in to interactivity
sprite.interactive = true;

// Shows hand cursor
sprite.buttonMode = true;

/* PS: Functions are not automatically lifted and must be in order for
 * generated script to use them correctly. */
function onClick(evt) {
  /* PS: Make sure your event handlers are called at least once by script */
  sprite.scale.x *= 1.25;
  sprite.scale.y *= 1.25;
  return 0;

// Pointers normalize touch and mouse
sprite.on('pointerdown', onClick);

/* PS: Make sure your event handlers are called at least once by script, and
 * done after function passed around, like in the following line: */
onClick(new Event('test'));

// Alternatively, use the mouse & touch events:
// sprite.on('click', onClick); // mouse-only
// sprite.on('tap', onClick); // touch-only


if (true) {
} else {

var x = 3;

x *= 2;


ReasonML syntax, reformatted with refmt.

type appApplicationT;

type appHTMLDocumentT;

type appHTMLBodyElementT;

type appHTMLCanvasElementT;

type usageFunSpriteT;

type appSpriteT;

type appObservablePointT;

type appRectangleT;

type appEventT;

type appMemoryInfoT;

type appContainerT;

type smallObject0T = {. "backgroundColor": int};

type smallObject21T = {. "RESOLUTION": int};

type smallObject42T = {
  "LINEAR": int,
  "NEAREST": int

type smallObject65T = {. "memory": appMemoryInfoT};

type usageFunOnClickT = appEventT => int;

[] [@bs.module "pixi.js"]
external newApplication3 : (int, int, smallObject0T) => appApplicationT =

[@bs.val] external document : appHTMLDocumentT = "document";

[@bs.get] external getBody : appHTMLDocumentT => appHTMLBodyElementT = "body";

[@bs.get] external getView : appApplicationT => appHTMLCanvasElementT = "view";

external appendChild1 :
  (appHTMLBodyElementT, appHTMLCanvasElementT) => appHTMLCanvasElementT =

[@bs.val] [@bs.module "pixi.js"]
external getSettings : smallObject21T = "settings";

external setSCALE_MODE : (smallObject21T, int) => unit = "SCALE_MODE";

[@bs.val] [@bs.module "pixi.js"] [@bs.scope "settings"]
external getSCALE_MODE : int = "SCALE_MODE";

[@bs.val] [@bs.module "pixi.js"]
external getSCALE_MODES : smallObject42T = "SCALE_MODES";

[@bs.val] [@bs.module "pixi.js"] [@bs.scope "SCALE_MODES"]
external getNEAREST : int = "NEAREST";

[@bs.val] [@bs.module "pixi.js"]
external getSprite : usageFunSpriteT = "Sprite";

[@bs.val] [@bs.module "pixi.js"] [@bs.scope "Sprite"]
external fromImage1 : string => appSpriteT = "fromImage";

[@bs.get] external getAnchor : appSpriteT => appObservablePointT = "anchor";

[@bs.send] external set1 : (appObservablePointT, float) => unit = "set";

[@bs.set] external setX : (appSpriteT, int) => unit = "x";

[@bs.get] external getX : appSpriteT => int = "x";

[@bs.get] external getScreen : appApplicationT => appRectangleT = "screen";

[@bs.get] external getWidth : appRectangleT => int = "width";

[@bs.set] external setY : (appSpriteT, int) => unit = "y";

[@bs.get] external getY : appSpriteT => int = "y";

[@bs.get] external getHeight : appRectangleT => int = "height";

external setInteractive : (appSpriteT, Js.boolean) => unit = "interactive";

[@bs.get] external getInteractive : appSpriteT => Js.boolean = "interactive";

external setButtonMode : (appSpriteT, Js.boolean) => unit = "buttonMode";

[@bs.get] external getButtonMode : appSpriteT => Js.boolean = "buttonMode";

[@bs.val] external console : smallObject65T = "console";

[@bs.get] external getScale : appSpriteT => appObservablePointT = "scale";

external setXappObservablePointTfloat : (appObservablePointT, float) => unit =

external getXappObservablePointT : appObservablePointT => float = "x";

external setYappObservablePointTfloat : (appObservablePointT, float) => unit =

external getYappObservablePointT : appObservablePointT => float = "y";

external on2 : (appSpriteT, string, usageFunOnClickT) => appSpriteT = "on";

[] external newEvent1 : string => appEventT = "Event";

[@bs.get] external getStage : appApplicationT => appContainerT = "stage";

external addChild1 : (appContainerT, appSpriteT) => appSpriteT = "addChild";

let app = newApplication3(800, 600, {"backgroundColor": 0x1099bb});

document |. getBody |. appendChild1(app |. getView);

/* Scale mode for all textures, will retain pixelation */
setSCALE_MODE(getSettings, getNEAREST);

let sprite =

/* Set the initial position */
sprite |. getAnchor |. set1(0.5);

setX(sprite, (app |. getScreen |. getWidth) / 2);

setY(sprite, (app |. getScreen |. getHeight) / 2);

/* Opt-in to interactivity */
setInteractive(sprite, Js.true_);

/* Shows hand cursor */
setButtonMode(sprite, Js.true_);

/* PS: Functions are not automatically lifted and must be in order for
 * generated script to use them correctly.  */
let onClick = evt => {
  /* PS: Make sure your event handlers are called at least once by script  */
    sprite |. getScale,
    (sprite |. getScale |. getXappObservablePointT) *. 1.25
    sprite |. getScale,
    (sprite |. getScale |. getYappObservablePointT) *. 1.25

/* Pointers normalize touch and mouse */
sprite |. on2("pointerdown", onClick);

/* PS: Make sure your event handlers are called at least once by script, and
 * done after function passed around, like in the following line:  */

/* Alternatively, use the mouse & touch events:
   sprite.on('click', onClick); // mouse-only
   sprite.on('tap', onClick); // touch-only */
app |. getStage |. addChild1(sprite);

if (Js.to_bool(Js.true_)) {
} else {

let xRef = ref(3);

xRef := xRef^ * 2;

Manual labour

This section lists workarounds for unimplemented features and transpilations.

  • It is up to you to ensure that all branches of if statements are evaluated at least once to get the types.
  • Control flow keywords like continue and break are not supported yet. Early return is not supported either: there should only be a single return statement at end of function.
  • Initialize all (non-int) variables. Default value is set to 0 if not initialized and a TODO comment is added.
  • Reorder functions so that they are called after they are declared.
  • Tweak integer values that should be floats, e.g. write 0.5 + 0.5 instead of 1 if you want the value to be considered float.
  • Call event handlers manually at least once, after they've been passed around to event registers. Alternatively, set a high timeout and trigger events manually in browser.
  • By default there is a 10 second timeout to allow for resource load, timer updates etc when evaluating example code. If all types are resolved before the grace period is done, transpilation proceeds at once. Adjust period in index.js if needed. TODO: create option for this on web page.

After transpile:

  • Reorder type definitions if necessary. There is a TODO issue on sorting them topologically.
  • Add |> ignore or let _ = to avoid warnings. I don't do it automatically because it will be hard to detect when it's necessary and too many of them clutters the code.

Testing examples

Node example

Node support is basic so far, but an example can be found in src/example-node.txt.

To run the example:

npm run webpack # to compile dist/node.js
node dist/node.js # will transpile src/example-node.txt to src/
npm start # to compile src/
node src/

Browser examples

node src/puppet.js examples/pixiBasics/example_001.js >| src/
npm start


  • Example 4. Need to add a |> ignore at end of function.
  • Example 9. Need to add a |> ignore at end of function.

Go to http://localhost:8080/test .

Running tests

# run tests script
npm run test

Supported syntax

This list of ESprima syntax nodes is from here and annotated with whether transpilation is implemented or not yet. Create an issue if you want a syntax implemented.

  • YES: YES
  • NO: NO
Feature Supported
AssignmentExpression YES
AssignmentPattern NO
ArrayExpression YES
ArrayPattern NO
ArrowFunctionExpression NO
AwaitExpression NO
BlockStatement YES
BinaryExpression YES
BreakStatement NO
CallExpression YES
CatchClause NO
ClassBody NO
ClassDeclaration NO
ClassExpression NO
ConditionalExpression YES
ContinueStatement NO
DoWhileStatement NO
DebuggerStatement NO
EmptyStatement NO
ExportAllDeclaration NO
ExportDefaultDeclaration NO
ExportNamedDeclaration NO
ExportSpecifier NO
ExpressionStatement YES
ForStatement YES
ForOfStatement NO
ForInStatement NO
FunctionDeclaration YES
FunctionExpression YES
Identifier YES
IfStatement YES
Import NO
ImportDeclaration NO
ImportDefaultSpecifier NO
ImportNamespaceSpecifier NO
ImportSpecifier NO
Literal YES
LabeledStatement NO
LogicalExpression NO
MemberExpression YES
MetaProperty NO
MethodDefinition NO
NewExpression YES
ObjectExpression YES
ObjectPattern NO
Program YES
Property YES
RestElement NO
ReturnStatement YES
SequenceExpression NO
SpreadElement NO
Super NO
SwitchCase NO
SwitchStatement NO
TaggedTemplateExpression NO
TemplateElement NO
TemplateLiteral NO
ThisExpression NO
ThrowStatement NO
TryStatement NO
UnaryExpression YES
UpdateExpression YES
VariableDeclaration YES
VariableDeclarator YES
WhileStatement NO
WithStatement NO
YieldExpression NO


  • Jeason is another approach using Flow. Differences to my project are that flow cannot really infer many types at all without annotations. The code Jeason generates is also full of dynamic lookups, not using typed getters and setters as my project does. Also it is not as convenient to copy paste and quickly transpile live examples in browser.
  • ReasonablyTyped I tried this a while ago to convert WebGL2 bindings (and three.js if I remember correctly) but I couldn't get it to work. Well, seems like it's a work in progress with last commit 4 days ago per 03.04.2018.