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JSX

Reason comes with the JSX syntax! ReasonReact works very similar to how the ReactJS JSX transform does.

To use it, you would need to install reason-react-ppx and add (preprocess (pps reason-react-ppx)) in melange.emit or library stanzas in your dune file.

Here's a list of transformations made by the ppx:

Uncapitalized

<div foo={bar}> {child1} {child2} </div>

transforms into

ReactDOM.createDOMElementVariadic(
  "div",
  ~props=ReactDOM.domProps(~foo=bar, ()),
  [|child1, child2|]
);

which compiles to the JavaScript code:

React.createElement('div', {foo: bar}, child1, child2)

Prop-less <div /> transforms into

ReactDOM.createDOMElementVariadic(
  "div",
  ~props=ReactDOM.domProps(),
  [||]
);

Which compiles to

React.createElement('div', {})

Capitalized

<MyReasonComponent key={a} ref={b} foo={bar} baz={qux}> {child1} {child2} </MyReasonComponent>

transforms into

React.createElementVariadic(
  MyReasonComponent.make,
  MyReasonComponent.makeProps(
    ~key=a,
    ~ref=b,
    ~foo=bar,
    ~baz=qux,
    ~children=React.null,
    ()
  ),
  [|child1, child2|]
);

which compiles to

React.createElement(
  MyReasonComponent.make,
  {
    key: a,
    ref: b,
    foo: bar,
    baz: qux,
    children: null,
  },
  child1,
  child2,
);

Prop-less <MyReasonComponent /> transforms into

React.createElement(MyReasonComponent.make, MyReasonComponent.makeProps());

which compiles to

React.createElement(MyReasonComponent.make, {});

The make above is exactly the same make function you've seen in the previous section.

ref and key are reserved in ReasonReact, just like in ReactJS. Don't use them as props in your component!

Fragment

<> child1 child2 </>;

transforms into

ReactDOMRe.createElement(ReasonReact.fragment, [|child1, child2|]);

Which compiles to

React.createElement(React.Fragment, undefined, child1, child2);

Children

ReasonReact children are fully typed, and you can pass any data structure to it (as long as the receiver component permits it). When you write:

<MyReasonComponent> <div /> <div /> </MyReasonComponent>

You're effectively passing the array [| <div />, <div /> |] to MyReasonComponent's children. If you pass a single child like so:

<MyReasonComponent> <div /> </MyReasonComponent>

We unwrap this for you automatically to just <div /> instead of an array of a single element.

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