UNSAFE_componentWillMount() (for componentWillMount() deprecation process since v16.3).React throws an error in render() and React.lazy() cases and outputs a warning message in the other cases.Īlso, this rule checks the following which need no return value:

In the above cases, since a specific value should be returned, it might be a programmer's mistake returning a wrong value or forgetting a return statement. The callback function passed as the first argument of eEffect(): undefined or function.The callback function passed as the first argument of React.lazy(): dynamic import().The callback function passed as the first argument of React.forwardRef(): React element, array, fragment, portal, string, number, boolean, or null.getSnapshotBeforeUpdate(): any value except undefined.getDerivedStateFromProps(): any value except undefined.shouldComponentUpdate(): truthy or falsy value except undefined.render(): React element, array, fragment, portal, string, number, boolean, or null.The lifecycle methods and callback functions which cause problem for an invalid return value are listed below with the valid return values: Even when no immediate problem occurs, the value becomes meaningless. If an invalid value is returned from a React lifecycle method or callback function, it may cause a problem when React uses the value. This rule applies when the return value of a React lifecycle method or callback function is invalid. Return values of React lifecycle methods and API callback functions should have valid types

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