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JSX

Reason comes with JSX syntax. Enables representation of HTML-like expressions within the language.

reason-react enables the ReactJS JSX transform in Reason.

Since reason-react.0.12.0, the JSX transformation currently supports the New JSX Transform. JSX functions are imported from react/jsx-runtime. Previous versions of reason-react used the legacy API React.createElement.

Install

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.

What the ppx does

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

Uncapitalized

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

transforms into

ReactDOM.jsxs(
  "div",
  ReactDOM.domProps(
    ~children=React.array([|child1, child2|]),
    ~foo=bar,
    (),
  )
)

which compiles to the JavaScript code:

React.jsx('div', {foo: bar, children: [ child1, child2 ] })

Prop-less <div /> transforms into

ReactDOM.jsx("div", ReactDOM.domProps());

Which compiles to

React.createElement('div', {})

Capitalized

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

transforms into

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

which compiles to

React.jsxs(
  MyReasonComponent.make,
  {
    ref: b,
    foo: bar,
    baz: qux,
    children: [ child1, child2 ],
  },
);

Prop-less <MyReasonComponent /> transforms into

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

which compiles to

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

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

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

Fragment

Fragment lets you group elements without a wrapper node, and return a single element without any effect on the DOM. More details about this in the react documentation: Fragments.

The empty JSX tag <></> is shorthand for <React.Fragment></React.Fragment>

<> child1 child2 </>;

transforms into

React.jsx(
  React.jsxFragment,
  ReactDOM.domProps(~children=React.array([|child1, child2|]), ()),
);

Which compiles to

React.jsx(React.Fragment, { children: [child1, child2] });

Children

reason-react 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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