Pre-mounting with componentWillMount()

Now that the props and state are set, we finally enter the realm of Life Cycle methods. The first true life cycle method called is componentWillMount(). This method is only called one time, which is before the initial render. Since this method is called before render() our Component will not have access to the Native UI (DOM, etc.). We also will not have access to the children refs, because they are not created yet.

The componentWillMount() is a chance for us to handle configuration, update our state, and in general prepare for the first render. At this point, props and initial state are defined. We can safely query this.props and this.state, knowing with certainty they are the current values. This means we can start performing calculations or processes based on the prop values.

Person.js [^1]

import React from 'react';
import classNames from 'classnames';
class Person extends React.Component {
  constructor(props) {
    super(props);
    this.state = { mode: undefined } ;
  }
  componentWillMount() {
    let mode;
    if (this.props.age > 70) {
      mode = 'old';
    } else if (this.props.age < 18) {
      mode = 'young';
    } else {
      mode = 'middle';
    }
    this.setState({ mode });
  }
  render() {
    return (
      <div className={ classNames('person', this.state.mode) }>
        { this.props.name } (age: { this.props.age })
      </div>
    );
  }
}
Person.defaultProps = { age: 'unknown' };
export default Person;

In the example above we call this.setState() and update our current state before render. If we need state values on calculations passed in props, this is where we should do the logic.

Other uses for componentWillMount() includes registering to global events, such as a Flux store. If your Component needs to respond to global Native UI events, such as window resizing or focus changes, this is a good place to do it[^2].

Next Up: Component render()

[^1] In our example above, we are using the classNames() library, which was originally included as a React Addon. However, the feature has been removed from React and moved to its own library for use with or without React.

[^2] It’s important to remember that many Native UI elements do not exist at this point in the life cycle. That means we need to stick to very high-level/global events such as window or document.