In this post, I'll show you how to write ReasonReact bindings for React.js components.

For all of you who don't know about ReasonML: ReasonML is syntactic sugar on top of the OCaml toolchain.

ReasonML/OCaml can be compiled into optimized Javascript with Bucklescript.

You can learn more about it by checking out this article by Dr. Axel Rauschmayer: What is ReasonML?

#The official way according to the docs

ReasonReact provides a great way to inteprop with React.js components using ReasonReact.wrapJsForReason. Here is an example from the ReasonReact docs:

/* PersonalInformation.re */
[@bs.module] external jsPersonalInformation : ReasonReact.reactClass = "./PersonalInformation.js";

let make = (~name: string, ~age: option(int)=?, children) =>
  ReasonReact.wrapJsForReason(
    ~reactClass=jsPersonalInformation,
    ~props={
      "name": name,
      "age": Js.Nullable.from_opt(age)
    },
    children
  );

Now we'll use the above component like this:

<PersonalInformation name="Khoa Nguyen" age=Some(24) />

PersonalInformation.js should be called with these props:

{
  name: "Khoa Nguyen";
  age: 24;
}

Everything is working as expected. Now let's try with slightly different props:

<PersonalInformation name="Khoa Nguyen" age=None />

PersonalInformation.js should be called with these props:

{
  name: "Khoa Nguyen";
  age: undefined;
}

age now has an undefined value. It's not so bad in most cases, but it could problematic.

Here is how I implemented my PersonalInformation.js component:

// PersonalInformation.js
import React from "react";

const PersonalInformation = props => {
  let hasAge = props.hasOwnProperty("age");

  if (hasAge) {
    return (
      <p>
        My name is {props.name}. I'm {props.age} years old
      </p>
    );
  }

  return <p>My name is {props.name}</p>;
};

export default PersonalInformation;

I've created a CodeSandbox here for you to play around with.

With the the first example (~age=Some(24)), it renders My name is Khoa Nguyen. I'm 24 years old as expected.

With the second example (~age=None), however, it renders My name is Khoa Nguyen. I'm years old.

What is going on?

Let's open the node repl and try it out:

❯ node
> let firstExample = {name: "Khoa Nguyen", age: 24};
undefined
> firstExample.hasOwnProperty("age")
true
> let secondExample = {name: "Khoa Nguyen", age: undefined};
undefined
> secondExample.hasOwnProperty("age")
true
> "age" in secondExample
true  

Ah. This totally makes sense. The age property has a value of undefined.

Quick note:

I know that the above code is not idiomatic React code. I can re-implement the React compponent like this to fix it:

<p>
  My name is {props.name}.
  {props.age && <span>I'm {props.age} years old</span>}
</p>

This should work even with age = undefined, but the fact that I have to change the original component to write a binding isn't ideal. The pattern I use here (props.hasOwnPropty("age")) is common for switching between controlled/uncontrolled mode of a component.

#"The right way" of writing ReasonReact bindings

Now we've identified our problem, let's fix it.

We need to find a way for not defining age in props.

Lucky for us, Bucklescript provides a function called the Special Creation Function

The idea is simple:

[@bs.obj] external makeProps : (~name: string, ~age: int=?, unit) => _ = "";

let props1 = makeProps(~name="Khoa Nguyen", ~age=24, ());
let props2 = makeProps(~name="Khoa Nguyen", ());

Tips: You can try this on the playground at https://reasonml.github.io/en/try.html

This is the compiled code from Bucklescript:

var props1 = {
  name: "Khoa Nguyen",
  age: 24,
};

var props2 = {
  name: "Khoa Nguyen",
};

As you can see, Bucklescript creates an object for us so no intermediate step is needed, and the age property is not in props2.

With this knowledge, we can rewrite our binding like this:

/* PersonalInformation.re */
[@bs.module] external jsPersonalInformation : ReasonReact.reactClass = "./PersonalInformation.js";

[@bs.obj] external makeProps : (~name: string, ~age: int=?, unit) => _ = "";

let make = (~name, ~age=?, children) =>
  ReasonReact.wrapJsForReason(
    ~reactClass=jsPersonalInformation,
    ~props=makeProps(~name, ~age?, ()),
    children
  );

As you can see, I removed the type annotations in make and put them in makeProps. It's not necessary to annotate types everywhere with a great compiler. Now, our binding works as expected.

#Transform props

The above is a simple case. We sometimes need to apply some transformations to the props before passing them to React.js. A common case might be transforming between Js.boolean and ReasonML's boolean.

Here is a small snippet on how you can do that:

/* PersonalInformation.re */
[@bs.module] external jsPersonalInformation : ReasonReact.reactClass = "./PersonalInformation.js";

[@bs.obj] external makeProps : (
 ~name: string,
 ~age: int=?,
 ~showAge: Js.boolean=?,
 unit
) => _ = "";

let make = (~name, ~age=?, showAge=?, children) =>
 ReasonReact.wrapJsForReason(
   ~reactClass=jsPersonalInformation,
   ~props=makeProps(
     ~name,
     ~age?,
     ~showAge=?Js.Option.map([@bs] (a => Js.Boolean.to_js_boolean(a)), showAge),
     ()
    ),
   children
 );

You can use this to make your code more idiomatic ReasonML by replacing string enums with polymorphic variants:

[@bs.deriving jsConverter]
type theme = [
  | `dark
  | `light
];

[@bs.obj] external makeProps : (
 ~name: string,
 ~age: int=?,
 ~theme: string=?,
 unit
) => _ = "";

let make = (~name, ~age=?, theme=?, children) =>
 ReasonReact.wrapJsForReason(
   ~reactClass=jsPersonalInformation,
   ~props=makeProps(
     ~name,
     ~age?,
     ~theme=?Js.Option.map([@bs] (a => themeToJs(a)), theme),
     ()
    ),
   children
 );

I think this is enough for this blog post. If you want to see more example of the pattern on writing ReasonReact binding, checkout my Ant Design binding here: https://github.com/thangngoc89/bs-ant-design

This is my first time writing a blog post in English, so please let me know If I wrote anything that's incorrect.


Update 3/16/2018: If you're having troubles running the code in the example, you could try with this repository


You can reach me via Twitter(@thangngoc89) or @thangngoc89 on Discord channel.