Building a Custom React Renderer

React renderers are powerful. In this post, we'll dive deep into the Reconciler API and build a custom renderer from scratch. It's not magic, it's just trees and mutations.

1. The Reconciler API

To start, you need to import the react-reconciler package and define your HostConfig object. This object contains the methods React will call to mutate your target environment.

import ReactReconciler from 'react-reconciler'
 
const HostConfig = {
    // Create an instance of your target element
    createInstance(type, props, rootContainerInstance, hostContext, internalInstanceHandle) {
        if (type === 'custom-div') {
            return { type: 'div', props }
        }
        throw new Error(`Cannot create element of type: ${type}`)
    },
 
    // ... other required methods
    supportsMutation: true,
}
 
const reconciler = ReactReconciler(HostConfig)

The createInstance function is the core of it all. It dictates what primitive element your renderer outputs when React asks for a specific tag.

Understanding Mutability

Notice we set supportsMutation: true. This tells React that our target environment supports in-place updates (like the DOM).