Why does a simple React Component render twice?

user_3376065 0
Viewed 2148

I just started a new create-react-app project and noticed that react is rendering components twice! My react version in package.json is "react": "^16.13.1"

import React, { useRef } from "react";

const App = () => {
  const renders = useRef(0);
  console.log("renders: ", renders.current++);

  return (
    <div>
      Hello
    </div>
  );
};

This produces on first render:

renders: 0
renders: 0

Now if I add a button to increment state, each state change also produces two additional renders:

import React, { useRef } from "react";

const App = () => {
  const [count, setCount] = useState(0);
  const renders = useRef(0);
  console.log("renders: ", renders.current++);

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>increment</button>
      <div>count: {count}</div>
    </div>
  );
};

This will result in:

//--- initial render
renders: 0
renders: 0
//--- first click
renders: 1
renders: 2
//--- second click
renders: 3
renders: 4
//--- third click
renders: 5
renders: 6

Is this normal or is this a bug in the latest version of react?

reactjs javascript
2 Answers

OK looks like I found out the reason. After inspecting index.js I found the following:

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

Looks like create-react-app now includes React.StrictMode which double invokes certain methods in development mode (not in production).

user_33760650

In addition to the StrictMode issue you found, I think when you're using a ref like that it creates a side-effect so you'd typically need to put it in useEffect to prevent it from rendering twice:

import React, { useState, useEffect, useRef } from "react";

const App = () => {
  const [count, setCount] = useState(0);
  const renders = useRef(0);
  useEffect(() => {
    // Every time the component has been re-rendered,
    // the counter is incremented
    console.log("renders: ", renders.current++);
  }); 


  return (
    <div>
      <button onClick={() => setCount(count + 1)}>increment</button>
      <div>count: {count}</div>
    </div>
  );
};

export default App;
user_86740940
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