Docs / ReasonReact / CreationPropsSelf

Creation, Props & Self

The Record API is in feature-freeze. For the newest features and better support going forward, please consider migrating to the new function components.

The documentation assumes relative familiarity with ReactJS.

ReasonReact doesn't use/need classes. The component creation API gives you a plain record, whose fields (like render) you can override.

The component template is created through ReasonReact.statelessComponent("TheComponentName"). The string being passed is for debugging purposes (the equivalent of ReactJS' displayName).

As an example, here's the file

let component = ReasonReact.statelessComponent("Greeting");

In ReactJS, you'd create a component class and call it through JSX which transforms into React.createElement(myClass, {prop1: 'hello'}) under the hood. In ReasonReact, instead of passing the whole "class" (aka component template) into a hypothetical ReasonReact.createElement function, you'd instead declare a make function:

/* still in */ let component = ReasonReact.statelessComponent("Greeting"); let make = (~name, _children) => { ...component, /* spread the template's other defaults into here */ render: _self => <div> {ReasonReact.string(name)} </div> };

The make function is what's called by ReasonReact's JSX, described later. For now, the JSX-less way of calling & rendering a component is:

ReasonReact.element(Greeting.make(~name="John", [||])) /* the `make` function in the module `Greeting` */ /* equivalent to <Greeting name="John" /> */

make asks you to return the component record created above. You'd override a few fields, such as the familiar render, initialState, didMount, etc., all described later.

Note: do not inline let component into the make function body like the following!

let make = _children => { ...(ReasonReact.statelessComponent("Greeting")), render: self => blabla }

Since make is called at every JSX invocation, you'd be accidentally creating a fresh new component every time.


Props are just the labeled arguments of the make function, seen above. They can also be optional and/or have defaults, e.g. let make = (~name, ~age=?, ~className="box", _children) => /* ... */.

The last prop must be children. If you don't use it, simply ignore it by naming it _ or _children. Names starting with underscore don't trigger compiler warnings if they're unused.

The prop name cannot be ref nor key. Those are reserved, just like in ReactJS.

Following that example, you'd call that component in another file through <Foo name="Reason" />. className, if omitted, defaults to "box". age defaults to None. If you'd like to explicitly pass age, simply do so: <Foo name="Reason" age=20 />.

Neat Trick with Props Forwarding

Sometimes in ReactJS, you're being given a prop from the owner that you'd like to forward directly to the child:

<Foo name="Reason" age={this.props.age} />

This is a source of bugs, because this.props.age might be accidentally changed to a nullable number while Foo doesn't expect it to be so, or vice-versa; it might be nullable before, and now it's not and Foo is left with a useless null check somewhere in the render.

In Reason, if you want to explicitly pass an optional myAge (whose type is option(int), aka None | Some(int)), the following wouldn't work:

<Foo name="Reason" age=myAge />

Because the age label expects a normal int when you do call Foo with it, not an option(int)! Naively, you'd be forced to solve this like so:

switch (myAge) { | None => <Foo name="Reason" /> | Some(nonOptionalAge) => <Foo name="Reason" age=nonOptionalAge /> }

Cumbersome. Fortunately, here's a better way to explicitly pass an optional value:

<Foo name="Reason" age=?myAge />

It says "I understand that myAge is optional and that I should either use the label age and pass an int, or not use the label at all. But I'd like to forward an optional value explicitly to avoid the verbose switch".

This isn't a JSX trick we've made up; it's just a language feature! See the section on "Explicitly Passed Optional" in the Reason docs.


You might have seen the render: (self) => ... part in make. The concept of JavaScript this doesn't exist in ReasonReact (but can exist in Reason, since it has an optional object system); the this equivalent is called self. It's a record that contains state, handle and send, which we pass around to the lifecycle events, render and a few others, when they need the bag of information. These concepts will be explained later on.