package reason-react

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Reason bindings for React.js


Dune Dependency






  • Wrap the React library, exposing just a single top-level module (@anmonteiro in #783)

  • Re-organise toplevel modules (@davesnx in #794)

  • Require and adapt to Melange v3 (@anmonteiro in #821)


  • Remove legacy ReactDOMRe and ReasonReact modules (@anmonteiro in #782)

  • Fix React.useCallback* for callbacks with multiple arguments (@anmonteiro in #786)





Breaking change


0.9.0 (05/2020)

Breaking Changes




0.8.0 (04/2020)

v0.8.0 is a huge release that includes several breaking changes, maintenance fixes and documentation updates :tada:

Thanks to all of our contributors for making this happen!


Breaking Changes:



Small release with a couple additional bindings to existing React APIs.

New APIs:

  • React.cloneElement

  • React.Children

  • useEffect, useLayoutEffect, useMemo, useCallback, useImperativeHandle now are all consistently implemented up to useX7 to improve usability.

  • ReasonReactRouter.urlNotEqual

Also removes .cm* generated files from the release to clean up the build process.


Support for React Hooks API. Allows for writing function components with state and complex interactions.

You can now write components like

let make = (~className, ~text) => <div className> {text} </div>;

which will compile to a 0-cost React component that looks like

let make = ({className, text}) => <div className> {text} </div>;

These components use a new PPX shipped with BuckleScript 5.0.0+. Please read the documentation for more.

There is no breaking change here, but if you're interested in migrating some components the upgrade script is provided. It will wrap existing ReasonReact components as if they are Hooks components.


Mini release before a surprise release next time (shhhhh) =).


  • The DOM prop crossorigin is now correctly named crossOrigin. This is a tiny breaking change, thus the version bump (#290).


  • Fix issue with react-hot-loader (#298).

  • ReasonReact.Router.replace to replace state.


This release requires bs-platform 4.0.3.

Small breaking change (only if you haven't upgraded for a while). The migration/upgrade script, as always, is here.

Event Changes

ReactEventRe is now deprecated in favor of ReactEvent. They're similar but the latter comes with the big quality-of-life improvement of turning our old:




Aka, you can use the -> fast pipe now (|. in OCaml syntax), and we've changed the definition of target in the various ReactEvent modules to directly give you back a Js.t object instead of Dom.element. Same applies to other such attributes.

We've also changed things like ReactEventRe.Mouse._type into ReactEventRe.Mouse.type_ to abide by the Reason idiom.

Lastly, bs.send.pipe is informally deprecated, so we've removed the usage of those too. Instead of e |> ReactEventRe.Mouse.preventDefault, use either e->ReactEvent.Mouse.preventDefault or ReactEvent.Mouse.preventDefault(e). bs.send.pipe is, all things considered, the heaviest BuckleScript special annotation. If your library uses it, please consider removing it too. Thanks!


Fragment has landed! <> child1 child2 </>. For more info, check ReactJS' docs on Fragment. Note that we currently don't support:

  • Keyed fragment.

  • Fragment with ref.

  • Fragment spread (ReasonReact-specific): <> ...children </>.

The latter will be supported next. Fragment requires React 16.

Additionally, DOM component children spread <div></div> now works. No more need to use the ReasonReact.createDomElement fallback!

Removal of Previously Deprecated Features

  • The DOM props _type, _open, and others (see 0.4.2 release notes below) are officially removed. Use type_, open_, etc.

  • The field subscriptions was deprecated and is now removed completely. Please use the new subscriptions helper instead.

  • ReasonReact.stringToElement, nullElement, arrayToElement are also gone for good. Use ReasonReact.string, null, array. The previous migration script in 0.4.0 already took care of this.

  • ReasonReact.Callback module removed.


  • ReasonReact.createDomElement is changed in favor of ReactDOMRe.createElementVariadic. This is more consistent with ReactDOMRe.createElement. Both are used by the JSX transform; the latter, when it's a children spread for DOM elements (mentioned above) and has a small perf cost.

Thanks for the wait, and enjoy!


This release requires bs-platform 3.1.4.

  • DOM components now support aria-* attributes without needing hacks: <div ariaLabel="foo" />. The camelCase ariaStuff will compile to aria-stuff.

  • For DOM props, instead of _open, _type, _begin, _end, _in, _to, use the new trailing underscore version for consistency: open_, type_, etc. The former leading underscore versions are now deprecated.


  • Restore ReasonReact.Callback module for now and put a deprecation warning on it. This way, at least your third-party dependencies can compile further.

  • Relax wrapJsForReason and wrapReasonForJs types so that we can use bs.abstract for interop instead of Js.t objects.


Requires bs-platform >=3.0.0. Migration script is here.

Breaking Changes

  • Remove SilentUpdate and SilentUpdateWithSideEffects. These aren't used for a long time now.

  • didMount now returns unit, per our previous warnings in the docs. To trigger a state update, use self.send(MyAction).

  • Remove self.reduce for real; use self.send (not to be confused with reducer, which is still around).

  • Remove ReasonReact.Callback module (undocumented and unused).


  • ReasonReact.stringToElement, arrayToElement and nullElement are now deprecated in favor of ReasonReact.string, array and null!

  • Old subscriptions API is deprecated. Please use the new self.onUnmount.

  • willReceiveProps deprecated. We'll transition to React 16 very soon and release our binding to deriveStateFromProps.


  • Use bool instead of Js.boolean (thanks to BuckleScript 3.0.0).

  • Fix Router incompatibility with IE11 (#201).

  • Much more streamlined internals.

  • Prepare to transition to React 16.

  • Did we mention that ReasonReact.stringToElement is now ReasonReact.string, etc.?


This release requires bs-platform 2.2.2! If your app haven't upgraded to it, don't worry; you can still use ResonReact 0.3.2 just fine. Only two small changes.

  • Use the new bs-platform Js.Nullable.toOption. No more deprecation warnings when you use ReasonReact (main purpose of the release).

  • Add ReactDOMRe.hydrate, hydrateToElementWithId, hydrateToElementWithClassName (#184).


  • Portal support (#152)

  • Add Router.dangerouslyGetInitialUrl. Please see the corresponding docs on router.


No breaking change. The migration script is here.

  • New subscriptions helper.

  • Router is here!

  • self.reduce is now changed into self.send, with a simpler semantic that clears up the confusion on the immediate call case. More performant and fewer allocations too! The migration script will convert most of the code over for you.

Before: onClick={self.reduce(_event => Click)} After: onClick={_event => self.send(Click)}

Before: didMount: self => {self.reduce(() => Click, ()); NoUpdate} After: didMount: self => {self.send(Click); NoUpdate}


Technically a breaking change, but just because of the removal of a few deprecated things. No migration script this time; since the few breaking changes you encounter (if any) should have be spotted by the type system and fixed in a few secs anyway.


  • Loosen children's restriction. This unlocks huge potentials. See the blog post!

  • Fix a bug where side effects inside side effects were skipped (#98).

  • React 16 support.

  • All files upper-cased. This follows the new community idiom. Technically an internal change; doesn't affect your usage of ReasonReact.

  • More DOM props. There are like, more DOM props added per year than the number of releases we have. What a funny time to be alive.

Breaking Changes:

  • ref is now typed as Js.nullable(Dom.element), not Js.null(Dom.element). Js.nullable is the new community idiom from BuckleScript. Go through your codebase and change your state.myRef := Js.Null.to_opt(theRef) into state.myRef := Js.Nullable.to_opt(theRef). We suggest you to also remove all mentions of Js.null, Js.undefined, Js.Null and Js.Undefined. Js.Nullable checks for both JavaScript null and undefined, thus making the JS interop more robust. In the past, people type certain values (especially js objects fields) as e.g. null and forget they can be undefined.

  • DOM components (ReactDOMRe)'s open, end and in attributes are now changed to _open, _end and _in to avoid using reserved keywords. This is only breaking if you're using these attributes and are on the old syntax.

  • Remove old stuff: enqueue, statefulComponent and self.update. These have long been deprecated.


Major update, but again with no breaking changes, and again with a convenience migration script! =)

The big change in this release is the deprecation of statefulComponent and statefulComponentWithRetainedProps. statelessComponent stays the same.


Please first read the blog post.

After reading this migration guide, use the migration script (or not) like so: node node_modules/reason-react/migrate/from02xTo024.js

Migrate From StatefulComponent to ReducerComponent

There's no more need for ReasonReact.statefulComponent. Every state change is now controlled through a dedicated, centralized, react-fiber-ready, component-local mechanism called "reducer" (aka, the hype word for "state machine").

Reason-react-example is updated too. Go check the examples afterward!

In short:

  • Replace all ReasonReact.statefulComponent with ReasonReact.reducerComponent.

  • Replace e.g. self.update handleClick (where handleClick is fun self => ReasonReact.Update {...self.state, foo: bar}) with self.reduce (fun _ => Click). Click is just a variant constructor you've defined. Let's call it "action".

  • Add a reducer function to the body of your ...component spread:

reducer: fun action state =>
  switch action {
  | Click => ReasonReact.Update {...state, foo: bar}

We've also exposed new ReasonReact.SideEffects (aka ReasonReact.NoUpdate, with side-effect) and ReasonReact.UpdateWithSideEffects (ReasonReact.Update + side-effect).

The relevant section on actions, reducers and the new update additions are in the main docs.

If everything goes alright, we will be deprecating statefulComponent in the future

InstanceVars/React Ref Usage Changed

Before, we used to recommend using ReasonReact.SilentUpdate to deal with ReactJS' instance variables pattern (e.g. attaching properties onto the component class itself, like timer IDs, subscriptions, refs, etc.). Now we've moved to using a Reason ref cell (not the React ref, the mutative Reason ref). See the updated instance variables section.

The new recommendation also solves a corner-case bug with assigning more than one refs in the render.

LifeCycle: Future didMount and willReceiveProps Signature Change

The future ReactJS Fiber in ReasonReact won't work well with lifecycle events that return the new state, aka:

  • didMount's ReasonReact.Update {...state, foo: bar}

  • willReceiveProps state.

Please return ReasonReact.NoUpdate for the former (can't do much for the latter, willReceiveProps, for now. Keep it as it is). If you really need to trigger a state change, before the return, use a self.reduce (fun () => Bar) (), aka immediately apply a reduce.

We will make all lifecycles return unit in the future; it'll be an easy codemod to change ReasonReact.NoUpdate to nothing.

Miscellaneous Changes

  • Add defaultChecked, loop and others to DOM attribute (#29, #37, #44, #50).

  • Fix cloneElement binding (#49).

  • Fix stateless components's willReceiveProps's return value. It's now unit again.

  • Fix wrong version of retainedProps in willReceiveProps.

  • Remove the dependency on create-react-class. Now we're back to being dependency-free!

  • Bump react/react-dom to 16.

  • React/react-dom are now dependencies, rather than peerDependencies. This follows the Reason/BS idiom of making the bound library an implementation detail. NPM/Yarn will still dedupe multiple versions of react/react-dom correctly; no worries about that.



Breaking update (sorry!)

We've finally removed ReactRe. It's been deprecated since 0.1.4. And we've offered a comprehensive migration in the 0.1.4 section below.

We've given folks a bit of breathing room in terms of breaking changes; now we're shipping another one, this time with a small migration script. After installing reason-react, use node node_modules/reason-react/oldScriptCarefulMigrateFrom015To020.js

  • Instead of fun state self => ..., we've now rolled state into self, and now, you have fun {state, handle} => .... The whole record is self. Feel free to destructure and get whatever you need!

  • self now contains a new prop, retainedProps. This is a new (non-breaking) feature that solves the previous slightly inconvenient way of forwarding props to state, as described in the old API's lifecycle methods. Now there's a dedicated API for it! The docs describes this in detail.


Non-breaking update. Works better with bs-platform >=1.8.0, which comes with the following ReasonReact JSX fixes:

  • JSX ppx now recursively transforms component's props.

  • JSX ppx now reports the correct location for some errors.

  • JSX ppx now correctly transforms some corner case with ref and key (ref=?foo).

Our own release contains the following improvements:

  • Adjust ReactDOMRe's props and style to include more accurate DOM and style attributes and styles (#9, #15, #17).

  • Add ReactDOMRe.Style.unsafeAddProp to unsafely add a prop to an existing style. Make sure you know what you're doing!

  • Fix reactRef's type in various locations. A React ref is actually always nullable; we've previously only acknowledged it for DOM ref, now we do for custom (composite) components ref too. A more detailed explanation is here. This is documented in our docs in the ref section as well.

  • Add cloneElement (solves adding otherwise invalid keys like aria-label and data-foo).

  • Add shouldUpdate.


Major update! Though this is 100% backward compatible, so no major version bump. We've revamped the whole API based on all you awesome folks' feedback, and we've provided a gradual migration path.

Requirements & Self-Congratulations

  • New BuckleScript. bs-platform >=1.7.5

  • This repo. reason-react >=0.1.4

Upon installing the new dependencies, your existing code still works. Isn't that great? You can incrementally convert things over. The old modules will stay around until the next or next next version. No rush!

ReactDOMRe, ReactDOMServerRe and ReactEventRe stay as-is. ReactRe is now deprecated (but again, is staying around) in favor of the new implementation, ReasonReact.

Small overview:

  • No more modules/functors needed for the API (don't go crazy with the new function composition power please!)

  • No more include

  • 10-20% smaller code on average, both input and output

  • Corner-cases with children and empty props mostly gone

  • Unused props now warn

Forwarded Definitions

The following definitions are carried over from ReactRe into ReasonReact, unchanged. A simple search-and-replace fixes all of them:

  • ReactRe.reactElement -> ReasonReact.reactElement

  • like wise, reactClass

  • reactRef, refToJsObj

  • nullElement, stringToElement, arrayToElement

  • createElement (if you recall, this isn't the pervasive ReactJS React.createElement). It's only used raw in escape hatch situations. If you've never used it: great!


Lowercase JSX <div> whatever </div> didn't change. Uppercase <Foo ref=a key=b bar=baz> hello goodbye </Foo> used to translate to Foo.createElement ref::a key::b bar::baz [hello, goodbye]. It now translates to ReasonReact.element ref::a key::b (Foo.make bar::baz [|hello, goodbye|]). We've pulled out ref and key into a dedicated call for good measures, and instead of using list as children, we now use array. More idiomatic ReactJS, list <-> array conversion churn.

To use the new JSX, change bsconfig.json's {"reason": {"react-jsx": true}} to {"reason": {"react-jsx": 2}}. Although you probably won't do that at the beginning, since that'd change all JSX in the whole project and cause type errors everywhere. Instead, keep your old bsconfig.json unchanged and for the JSX you'd like to selectively convert over, put a [@@@bs.config {jsx: 2}]; at the top of the file. Once you've converted everything over, switch to {"react-jsx": 2} in bsconfig.json and remove those @@@bs.config at the top of every file.

Alternatively, you can go straight to {"react-jsx": 2} in bsconfig.json, and put a [@@@bs.config {jsx: 1}] at the top of files where you'd like to use the old uppercase JSX transform.

Before starting the sections below, please briefly go through the new API on the documentation page.

Foo.createElement (Jargon Change)

Not to be confused with the ReactRe.createElement in the previous section.

Foo.createElement is now referred to as Foo.make. make is a more idiomatic term in Reason/OCaml, and is shorter to type!


The concept of componentBag is now called self. We thought it'd be a more appropriate name. The new self doesn't contain props, state, setState and instanceVars anymore; these are no longer needed in the new ReasonReact.


Replaced with the new make (previously createElement) call which takes in labeled arguments. See more in this section.

How to access props in the update/handle callbacks now? You'd move these callback definitions into the make function body.


Now passed to you as an argument in callbacks and lifecycle events.


No longer needed. In ReactJS, attaching instance variables onto a component has always been a sly way of introducing 1. mutative state that 2. doesn't trigger re-render. This whole concept is now replaced by putting your value into state and using ReasonReact.SilentUpdate (doesn't trigger re-render, but does update state) in callbacks & lifecycles.


Not to be confused with the ReactJS setState you often use (though if you're reading this migration guide, you probably know this already). This was an escape hatch designed for times when you can't, for example, return an option state from an updater, because you want to setState imperatively and asynchronously. The new idiom is to just use self.update myhandler (). Notice update myhandler returns a callback just like before, but now you're immediately applying the callback.


updater and handler are now called update and handle. They should be an easy search-and-replace.

The return type of update, instead of option state, is now update state.

The signature of the callback they take has changed. Instead of e.g. updater (fun {props, state, instanceVars} event => Some {...state, foo: true}), it's now update (fun event state self => ReasonReact.Update {...state, foo: true}). Formal, simplified type of the new callback: payload => state => self => update state.

update and handle don't memoize anymore; this reduces confusion and potential memory leaks.


render, previously in your component module, is now inside make. See the example here.

Ref & Other Mutable Instance Variables

ref now lives inside state, as described in componentBag.instanceVars just above. Instead of componentBag.handler+ mutating instanceVars, you'd now use update and returning ReasonReact.SilentUpdate {...state, ref: theRef}. refs and others probably default to None in your initial state.


We've decided to finally drop the component prefix to lifecycle methods! componentDidMount is now just called didMount, etc. The signatures changed slightly; see them in the new lifecycle events section.


The children helpers & types reactJsChildren, listToElement and jsChildrenToReason are all gone from ReasonReact, since we now use array instead of list.

New Reason <-> JS Interop

wrapPropsShamelessly (Reason Calling JS)

It's not clear why we called it this way in the old API. Please tell @_chenglou that he should name things more seriously on a serious project. The new name is wrapJsForReason. The signature hasn't changed much; arguments are labeled now and explicitly accept an unlabeled children at the last position. Example:


external myJSReactClass : ReasonReact.reactClass = "myJSReactClass" [@@bs.module];

let createElement name::(name: string) age::(age: option int)=? =>
  ReactRe.wrapPropsShamelessly myJSReactClass {"name": name, "age": Js.Nullable.from_opt age};


external myJSReactClass : ReasonReact.reactClass = "myJSReactClass" [@@bs.module];

let make name::(name: string) age::(age: option int)=? children =>
    props::{"name": name, "age": Js.Nullable.from_opt age}

Don't forget that once these are converted over, the callsites of these components will need to use the new JSX transform described above. Otherwise they'll generate type errors.

jsPropsToReasonProps (JS Calling Reason)

Now called wrapReasonForJs. The file-level include that served this interop is gone; it used to magically export the backing component comp for JS consumption. You now have to manually export comp through the new wrapReasonForJs. Continuation of the previous example:

let component = ...;
let make ...;

let comp =
    (fun jsProps => make name::jsProps##name age::?(Js.Nullable.to_opt jsProps##age) [||]);

The function takes in the labeled reason component, and a function that, given the js props, asks you to call make while passing in the correctly converted parameters.

Aaaand that's it! Enjoy!


DOM ref is now typed as Js.null Dom.element, instead of just Dom.element ( Trivial migration:


Add defaultValue prop for input DOM component (


Correct type of dangerouslySetInnerHTML (#76)


Lots of great people working together.


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