Avoid unnecessary rerendering

As you write your code, be aware of situations where React components may be rerendered unnecessarily.

Common situations to avoid include:

• Rendering a component multiple times when a page is first loaded.
• Rerendering a component even though the props have not changed.

Avoid rerendering widgets when a page is initially loaded

When debugging a widget’s render() method, check how many times the widget is rendered when a page is loaded. Ideally, a widget should be rendered only once when the page is first loaded. If a widget is rendered more than once, try to identify why and rewrite your code to rectify this. For example, you might be modifying a state variable or props in the useEffect() hook, resulting in the widget rendering again.

In the following example, a state variable is declared and then its value is set in useEffect():

const [quantity, setQuantity] = useState(null); 

useEffect(() => {
  ...
  setQuantity(1);
  ... 
}, []);

This results in the component being rendered twice. After the first render, useEffect() is executed and the quantity is changed from null to 1. The rerendering can be avoided by setting the value when the variable is declared. For example:

const [quantity, setQuantity] = useState(1);

Avoid rerendering with the same props

Although React updates only the DOM nodes that have changed, rerendering still takes some time. If your widget is rerendered frequently with the same set of props, you can use the React.memo() higher-order component to reduce unnecessary rendering.

In the following example, the Child() function would be forced to render by the Parent() function, but React.memo() is used to prevent this:

function Parent() {
 const currentDate = 10/10/2020; 
 const dynValue = "Test";
  return(
    <Child dynValue ={dynValue} currentValue = {currentDate }/>
  ); 
} 

function Child({ someFn, currentDate }) { ... };

export default React.memo(Child);

Note: If the component is rerendered due to changes in a Context object, using React.memo() may not improve performance.