Docs / rescript-react / Beyond JSX

Beyond JSX

JSX is a syntax sugar that allows us to use React components in an HTML like manner. A component needs to adhere to certain interface conventions, otherwise it can't be used in JSX. This section will go into detail on how the JSX transformation works and what React APIs are used underneath.

Note: This section requires knowledge about the low level apis for creating elements, such as React.createElement or ReactDOMRe.createDOMElementVariadic.

Note: This page assumes your bsconfig.json to be set to "reason": { "react-jsx": 3 } to apply the right JSX transformations.

Component Types

A plain React component is defined as a ('props) => React.element function. You can also express a component more efficiently with our shorthand type React.component('props).

Here are some examples on how to define your own component types (often useful when interoping with existing JS code, or passing around components):

// Plain function type type friendComp = ({"name": string, "online": bool}) => React.element; // Equivalent to // ({"padding": string, "children": React.element}) => React.element type containerComp = React.component({ "padding": string, "children": React.element });

The types above are pretty low level (basically the JS representation of a React component), but since ReScript React has its own ways of defining React components in a more language specific way, let's have a closer look on the anatomy of such a construct.

JSX Component Interface

A ReScript React component needs to be a (sub-)module with a make and makeProps function to be usable in JSX. To make things easier, we provide a @react.component decorator to create those functions for you:

module Friend = {
  let make = (~name: string, ~children) => {

In the expanded output:

  • makeProps: A function that receives multiple labeled arguments (according to prop names) and returns the value that is consumed by make(props)

  • make: A converted make function that complies to the component interface (props) => React.element

Note: The makeProps function will also always contain a ~key prop.

Special Case React.forwardRef

The @react.component decorator also works for React.forwardRef calls:

module FancyInput = {
  let make = React.forwardRef((~className=?, ~children, ref_) =>
      // use ref_ here

As shown in the expanded output above, our decorator desugars the function passed to React.forwardRef in the same manner as a typical component make function. It also creates a makeProps function with a ref prop, so we can use it in our JSX call (<FancyInput ref=.../>).

So now that we know how the ReScript React component transformation works, let's have a look on how ReScript transforms our JSX constructs.

JSX Under the Hood

Whenever we are using JSX with a custom component ("capitalized JSX"), we are actually using React.createElement to create a new element. Here is an example of a React component without children:

<Friend name="Fred" age=1 />

As you can see, it uses Friend.make and Friend.makeProps to call the React.createElement API. In case you are providing children, it will use React.createElementVariadic instead (which is just a different binding for React.createElement):

JSXWithout JSXJS Output
<Container width=200>

Note that the ~children=React.null prop has no relevance since React will only care about the children array passed as a third argument.

Dom Elements

"Uncapitalized JSX" expressions are treated as DOM elements and will be converted to ReactDOMRe.createDOMElementVariadic calls:

JSXWithout JSXJS Output
<div title="test"/>

The same goes for uncapitalized JSX with children:

JSXWithout JSXJS Output
<div title="test">