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First of all, from a blog post in late March 2018, it was announced that the React lifecycle methods componentWillReceiveProps, componentWillMount, and componentWillUpdate will be deprecated in a future version of React. This is because of the eventual migration of React to async rendering; these lifecycle methods will become unreliable when async rendering is made default.
Example of replacing componentWillUpdate with componentDidUpdate
componentWillUpdate is invoked when a component is about to receive new props and the render method is definitely going to be called. Here’s an example of something we were doing previously:
componentWillUpdate(nextProps) {
if (!nextProps.user.isLogged && !nextProps.user.authenticating) {
this.context.router.history.push('/')
}
}
And, replacing that usage with componentDidUpdate:
componentDidUpdate(/*prevProps, prevState*/) {
if (!this.props.user.isLogged && !this.props.user.authenticating) {
this.context.router.history.push('/')
}
}
componentDidUpdate is similar to componentDidMount except that is caused after a change in state or props occurs instead of just on initial mount. As opposed to getDerivedStateFromProps, we have access to the context provided by this. Note that this method also has arguments for prevProps and prevState, which provides the previous versions of the component’s props and state for comparison to the current values.
componentWillMount
componentWillMount is executed just before the React Component is about to mount on the DOM. Hence, after this method the component will mount. All the things that you want to do before a component mounts has to be defined here. This method is executed once in a lifecycle of a component and before first render. Usage: componentWillMount is used for initializing the states or props, there is a huge debate going on to merge it with the constructor.
Difference between constructor and componentWillMount
Only thing you can’t achieve inside the constructor that you can with ComponentWillMount is to setState(). Also react throws a warning if anything inside your constructor modifies state even in another component.
