Warning: componentWillMount has been renamed, and is not recommended for use

The Problem

Warning: componentWillMount has been renamed, and is not recommended for use

Initializing state

// Before
class AppComponent extends React.Component {
  state = {};

  componentWillMount() {
    this.setState({
      selected: this.props.selected,
      color: 'red',
    });
  }
}

Fetching external data

// Before
class AppComponent extends React.Component {
  state = {
    data: null,
  };

  componentWillMount() {
    fetch('https://sentry.io/data').then(res => {
      this.setState({ data: res.json() });
    });
  }
}

The Solution

Starting in React version 16.3, the following component lifecycle methods are being phased out.

componentWillMount
componentWillReceiveProps
componentWillUpdate

If you want to use these methods, prefix the methods with UNSAFE_ . These methods are considered “unsafe” because the React team expect code that depends on these methods to be more likely to have bugs in future versions of React.

Depending on the objective of the code, you can remove the use of componentWillMount entirely with other lifecycle methods.

The solution is to move the state initialization to the constructor or to a property initializer, like so:

// After
class AppComponent extends React.Component {
  state = {
    selected: this.props.selected,
    color: 'red',
  };
}

The solution is to move data-fetching into componentDidMount:

// After
class AppComponent extends React.Component {
  state = {
    data: null,
  };

  componentDidMount() {
    fetch('https://sentry.io/data').then(res => {
      this.setState({ data: res.json() });
    });
  }
}