Passing Functions as React Props

I recently had a discussion with a coworker about the naming of a function property that will be passed to a React component. It got me thinking more thoroughly about the role of functions as properties. I find that they usually fall into one of two camps. The function either:

• Notifies the container of user or component action, or
• Provides some capability that the component needs.

Event Callbacks

Let's call the first an "Event Callback." This lines up well with the native event hooks provided by React. You, the component author, can provide a function (this.handleClick) via the prop (onClick) of your <button> component like so:

class MyComponent extends React.Component {
    handleClick = (ev) => {
        if (ev.keyCode === 13) {
            console.log('Enter!');
        }
    }
    
    render () {
        return (
            <button onClick={this.handleClick}>
            	Click it real good
            </button>
        );
    }
}

If <button> was a "regular" React component, you might expect it to have a propType defined that will declare that onClick expect a function.

onClick: PropTypes.func

What this means, intuitively, is that <button> accepts a function in the onClick prop, which will be called when the button is clicked. It doesn't, however, imply what it "means" to click the button. Does it submit a form? select an option? play a ninja cat video? That responsibility lies within the container and is the defining characteristic of an Event Callback:

With an Event Callback, a component is responsible for notifying the container that a user or component action has occurred, but not what the result of that action should be.

Dependency Injection

Sometimes, a component either requires a feature be implemented by its container or allows for its container to override a feature's default behavior.

An example of this comes from the popular react-router package.

NavLink#isActive accepts a function as a prop, which is used by the component to override the default behavior of marking a link active if it matches the current URL's pathname.

Here's an example lifted directly from the package docs:x`

// only consider an event active if its event id is an odd number
const oddEvent = (match, location) => {
  if (!match) {
    return false
  }
  const eventID = parseInt(match.params.eventID)
  return !isNaN(eventID) && eventID % 2 === 1
}

<NavLink
  to="/events/123"
  isActive={oddEvent}
>Event 123</NavLink>

In this example, the container doesn't need to be notified of anything; instead, it is injecting a new, custom behavior.

With Dependency Injection, the component is responsible for describing the contract for the feature (via the function's interface). The container will provide the feature.

Implementation Differences

A consistent API makes for a predictable API, which improves the overall developer experience. Below are a few implementation best practices for each type of function that leads to consistent components and happy consumers!

Event Callbacks should be:

• Optional: There are exceptions, but normally should be at the discretion of the consumer to handle an event
• Named consistently: Prefix all event callbacks with on, e.g. onSelectThing
• Single argument: Pass an object argument with relevant data for the event, e.g. onSelectThing({index: 1})
• Unidirectional: Only pass data via the callback and not rely on a return value from the callback

Dependency Injection functions should be:

• Unidirectional: Only pass behavior into a component and not expect feedback from the function (or its parameters!)
• Named appropriately: often beginning with a verb, e.g. isActive or generateId

Wrap Up

Hopefully this post helps you make consistent decisions when accepting functions as a prop to your React components. Have you encountered other use cases for functions as props? Do you disagree with the best practices above or have other recommendations? Please comment below

Also posted on Medium