Re-rendering using React.StrictMode and fabric.js's Canvas instance

Question

I'm creating a fabric canvas and buttons that instantiate shapes that should be selectable. I don't understand why my component is re-rendered twice in the following situation. Therefore, my fabric shape cannot be selected. However, when I remove the from my index.tsx file, the rendering only happens once and my shape is selectable. I could remove the , but I don't think that's the best solution. Here is a demo:

const { Fragment, StrictMode, useEffect, useRef } = React;
const { createRoot } = ReactDOM;

const styles = {};

const CanvasComponent = ({ id }) => {
    const canvasRef = useRef(null);

    useEffect(() => {
        console.log('init canvas'); // displayed twice with <React.StrictMode>
        canvasRef.current = initCanvas();
    }, []);

    const initCanvas = () => (
        canvasRef.current = new fabric.Canvas(`canvas-${id}`, {
            width: 800,
            height: 400,
        })
    );

    const addShape = (shapeType: string) => {
        let shape: fabric.Object;
        switch (shapeType) {
            case 'circle':
                shape = new fabric.Circle({ radius: 30, fill: 'red', left: 100, top: 100 });
                break;
            case 'rectangle':
                shape = new fabric.Rect({ width: 60, height: 70, fill: 'green', left: 100, top: 100 });
                break;
            default:
                return;
        }
        canvasRef.current.add(shape);
    };

    return (
        <div>
            <button onClick={() => addShape('circle')}>Add Circle</button>
            <button onClick={() => addShape('rectangle')}>Add Rectangle</button>
            <div className={styles.canvasContainer}>
                <canvas id={`canvas-${id}`}></canvas>
            </div>
        </div>
    );
}

function StrictModeEnabled() {
    return <StrictMode><h1>Strict Mode Enabled</h1><CanvasComponent id={1} /></StrictMode>;
}

function StrictModeDisabled() {
    return <Fragment><h1>Strict Mode Disabled</h1><CanvasComponent id={2} /></Fragment>;
}

const strictModeEnabledRoot = createRoot(document.getElementById("strict-mode-enabled"));
strictModeEnabledRoot.render(<StrictModeEnabled />);

const strictModeDisabledRoot = createRoot(document.getElementById("strict-mode-disabled"));
strictModeDisabledRoot.render(<StrictModeDisabled />);

<script crossorigin src="https://www.unpkg.com/fabric@5.3.0/dist/fabric.js"></script>
<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="strict-mode-enabled"></div>
<div id="strict-mode-disabled"></div>

Answer 1

question

Why useEffect runs twice in React and how to deal with it? This question has a good answer describing why this happens and a general solution.

solution

In your case you need to clean up the instantiated canvas. I'm not familiar with Fabric, but from reading the documentation, the dispose method seems appropriate:

You need to return a function from useEffect that calls the above method. As you can see from the linked question, it's good practice to return a function that does the cleanup from useEffect. There is also a working example below:

const { Fragment, StrictMode, useEffect, useRef } = React;
const { createRoot } = ReactDOM;

const styles = {};

const CanvasComponent = ({ id }) => {
    const canvasRef = useRef(null);

    useEffect(() => {
        console.log('init canvas'); // displayed twice with <React.StrictMode>
        canvasRef.current = initCanvas();
        
         return () => canvasRef.current.dispose();
    }, []);

    const initCanvas = () => (
        canvasRef.current = new fabric.Canvas(`canvas-${id}`, {
            width: 800,
            height: 400,
        })
    );

    const addShape = (shapeType: string) => {
        let shape: fabric.Object;
        switch (shapeType) {
            case 'circle':
                shape = new fabric.Circle({ radius: 30, fill: 'red', left: 100, top: 100 });
                break;
            case 'rectangle':
                shape = new fabric.Rect({ width: 60, height: 70, fill: 'green', left: 100, top: 100 });
                break;
            default:
                return;
        }
        canvasRef.current.add(shape);
    };

    return (
        <div>
            <button onClick={() => addShape('circle')}>Add Circle</button>
            <button onClick={() => addShape('rectangle')}>Add Rectangle</button>
            <div className={styles.canvasContainer}>
                <canvas id={`canvas-${id}`}></canvas>
            </div>
        </div>
    );
}

function StrictModeEnabled() {
    return <StrictMode><h1>Strict Mode Enabled</h1><CanvasComponent id={1} /></StrictMode>;
}

const strictModeEnabledRoot = createRoot(document.getElementById("strict-mode-enabled"));
strictModeEnabledRoot.render(<StrictModeEnabled />);

<script crossorigin src="https://www.unpkg.com/fabric@5.3.0/dist/fabric.js"></script>
<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="strict-mode-enabled"></div>