React – Too many re-renders when using props

I have been doing a project where I present a graph on the screen. Now, the data state within the App.js is being passed as a prop and then causes an infinite loop error.

If the data variable is within the Graph.js file and used and defined within the useState, then there is no issue.

I was a bit uncertain why this was happening as I am relatively new to react and javascript.

The goal is to add the data to the graph, it loads, then when I change the data within the App.js file, the page will automatically load again with the new data.

Any help/advice will be much appreciated 🙂

App.js

function App() {
  const [data, setData] = useState(null)
  setData(
    {
    nodes:[
        {name:"Max"},
        {name:"George"},
        {name:"Jesus"},
        {name:"Ben"},
        {name:"James"},
        {name:"Sam"},
        {name:"Sassms"}
    ],

    edges:[
        {source:"Max", target:"George"},
        {source:"George", target:"Jesus"},
        {source:"Jesus", target:"Max"},
        {source:"Jesus", target:"Ben"},
        {source:"James", target:"Ben"},
        {source:"Sam", target:"Sassms"},
        {source:"Sam", target:"Ben"}
    ]
    })
    console.log(data)

  return (
    <Graph colour="Grey" data={data} setData={setData}/>
  );
}

Graph.js

function Graph(props) {
    const svgRef = useRef(null);
    useEffect(
        () => {
            let svg = d3.select(svgRef.current)
            if(svgRef.current) {// Check that svg element has been rendered
                let width = svg.attr("width")
                let height = svg.attr("height")

                d3.selectAll("svg > *").remove();

                let edge = svg
                    .append("g")
                    .selectAll("line")
                    .data(props.data.edges)
                    .enter()
                    .append("line")
                    .attr("stroke-width", function(d) {
                        return 3;
                    })
                    .style("stroke", "black")

                let node = svg
                    .append("g")
                    .selectAll("circle")
                    .data(props.data.nodes)
                    .enter()
                    .append("circle")
                    .attr("r", 7)
                    .attr("fill", function(d) {
                        return "grey";
                    })
                    .attr("stroke", "black")
            }

    }, [props.data])

    
    return (
        <svg ref={svgRef} id="svgID" width="1000" height="900"></svg>
    )
  }

Answer

It’s because you’re updating the state on every render again and again. If you just simply call setData() inside the function, it will call this setter that invokes a re-render and triggers React to call your App function again.

If you want to have a default state, you can do it like below. If you want to update the state, it should be done by other actions, like the click of a button, fetching some data, etc.

Check out the demo below where I added a button that adds a new name to your nodes array:

function App() {
  const [data, setData] = React.useState({
    nodes: [
      { name: 'Max' },
      { name: 'George' },
      { name: 'Jesus' },
      { name: 'Ben' },
      { name: 'James' },
      { name: 'Sam' },
      { name: 'Sassms' },
    ],
    edges: [
      { source: 'Max', target: 'George' },
      { source: 'George', target: 'Jesus' },
      { source: 'Jesus', target: 'Max' },
      { source: 'Jesus', target: 'Ben' },
      { source: 'James', target: 'Ben' },
      { source: 'Sam', target: 'Sassms' },
      { source: 'Sam', target: 'Ben' },
    ],
  });

  return <Graph colour="Grey" data={data} setData={setData} />;
}

function Graph(props) {
  function addRandomName() {
    props.setData({
      ...props.data,
      nodes: [
        ...props.data.nodes,
        { name: Math.random().toString(36).slice(2) },
      ],
    });
  }

  return (
    <React.Fragment>
      <button onClick={addRandomName}>Add new name</button>
      <h2>Names</h2>
      <p> {props.data.nodes.map((d) => d.name).join(', ')}
      </p>
      <h2>State</h2>
      <pre>{JSON.stringify(props.data, null, 2)}</pre>
    </React.Fragment>
  );
}

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

<div id="root"></div>
<script src="https://unpkg.com/react/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom/umd/react-dom.production.min.js"></script>