Elements & JSX

Note: This page assumes your bsconfig.json to be set to "reason": { "react-jsx": 3 }, otherwise your JSX will not be transformed to its React specific form.

Element Basics

Let's start out by creating our first React element.

RES
let element = <h1> {React.string("Hello World")} </h1>

The binding element and the expression {React.string("Hello World")} are both of type React.element, the fundamental type for representing React elements within a React application. An element describes what you see on the screen whenever you render your application to the DOM.

Let's say you want to create a function that handles another React element, such as children, you can annotate it as React.element:

RES
let wrapChildren = (children: React.element) => {
  <div>
    <h1> {React.string("Overview")} </h1>
    children
  </div>
}

wrapChildren(<div> {React.string("Let's use React with ReScript")} </div>)

Understanding the definition of a React.element is essential since it is heavily used within the React APIs, such as ReactDOM.render(element, ...), etc. Be aware that JSX doesn't do any automatic string to React.element conversion for you (ReScript forces explicit type conversion). For example <div> Hello World </div> will not type-check (which is actually a good thing because it's also a huge source for subtle bugs!), you need to convert your "Hello World" with the React.string function first.

Fortunately our React bindings bring all necessary functionality to represent all relevant data types as React.elements.

Using Elements within JSX

You can compose elements into more complex structures by using JSX:

RES
let greeting = React.string("Hello ")
let name = React.string("Stranger");


// element is also of type React.element
let element = <div className="myElement"> greeting name </div>

JSX is the main way to express your React application as a tree of elements.

Sometimes, when doing a lot of interop with existing ReactJS codebases, you'll find yourself in a situation where you can't use JSX syntax due to syntactic restrictions. Check out the Escape Hatches chapter later on for workarounds.

Creating Elements

Creating Elements from string, int, float, array

Apart from using JSX to create our React elements or React components, the React module offers various functions to create elements from primitive data types:

RES
React.string("Hello") // new element representing "Hello"

React.int(1) // new element representing "1"

React.float(1.0) // new element representing "1.0"

It also offers React.array to represent multiple elements as one single element (useful for rendering a list of data, or passing children):

RES
let element = React.array([
  React.string("element 1"),
  React.string("element 2"),
  React.string("element 3")
])

Note: We don't offer a React.list function because a list value would impose runtime overhead. ReScript cares about clean, idiomatic JS output. If you want to transform a list of elements to a single React element, combine the output of Belt.List.toArray with React.array instead.

Creating Null Elements

ReScript doesn't allow element || null constraints due to it's strongly typed nature. Whenever you are expressing conditionals where a value might, or might not be rendered, you will need the React.null constant to represent Nothingness:

let name = Some("Andrea")

let element = switch name {
  | Some(name) => <div> {React.string("Hello " ++ name)} </div>
  | None => React.null
}

<div> element </div>

Escape Hatches

Note: This chapter features low level APIs that are used by JSX itself, and should only be used whenever you hit certain JSX syntax limitations. More infos on the JSX internals can be found in our Beyond JSX section.

Creating Elements from Component Functions

Note: Details on components and props will be described in the next chapter.

Sometimes it's necessary to pass around component functions to have more control over React.element creation. Use the React.createElement function to instantiate your elements:

RES
type props = {"name": string};

let render = (myComp: props => React.element) => {
  <div>
    {React.createElement(myComp, {"name": "Franz"})}
  </div>
}

This feature is often used when interacting with existing JS / ReactJS code. In pure ReScript React applications, you would rather pass a function that does the rendering for you (also called a "render prop"):

RES
let render = (renderMyComp: (~name: string) => React.element) => {
  <div> {renderMyComp(~name="Franz")} </div>
}

Pass Variadic Children

There is also a React.createElementVariadic function, which takes an array of children as a third parameter:

type props = {"title": string, "children": React.element};

let render = (article: props => React.element) => {
  let children = [React.string("Introduction"), React.string("Body")];

  let props = {"title": "Article #1", "children": React.null};

  {React.createElementVariadic(article, props, children)}
}

Note: Here we are passing a prop "children": React.null to satisfy the type checker. React will ignore the children prop in favor of the children array.

This function is mostly used by our JSX transformations, so usually you want to use React.createElement and pass a children prop instead.

Creating DOM Elements

To create DOM elements (<div>, <span>, etc.), use ReactDOMRe.createDOMElementVariadic:

RES
ReactDOMRe.createDOMElementVariadic("div", ~props=ReactDOM.domProps(~className="card", ()), []);

The function above requires the ReactDOM.domProps constructor function, so ReScript can make sure that we are only passing valid dom props. You can find an exhaustive list of all available props in the ReactDOM module.

Warning: The ReactDOMRe module will be removed in the next major rescript-react release, so use sparsely!

Cloning Elements

Note: This is an escape hatch feature and will only be useful for interoping with existing JS code / libraries.

Sometimes it's required to clone an existing element to set, overwrite or add prop values to a new instance, or if you want to set invalid prop names such as data-name. You can use React.cloneElement for that:

let original = <div className="hello"/>

// Will return a new React.element with className set to "world"
React.cloneElement(original, {"className": "world", "data-name": "some name"});

The feature mentioned above could also replicate props spreading, a practise commonly used in ReactJS codebases, but we strongly discourage the usage due to its unsafe nature and its incorrectness (e.g. adding undefined extra props to a component doesn't make sense, and causes hard to find bugs).

In ReScript, we rather pass down required props explicitly to leaf components or use a renderProp instead. We introduced JSX punning syntax to make the process of passing down props more convenient.