Updating and componentWillReceiveProps()

Now that we have discussed starting an Update, let's dive into the Update life cycle methods. The first method available to us is componentWillReceiveProps(). This method is called when props are passed to the Component instance. Let's dig a little deeper into what this means.

Passing props

The most obvious example is when new props are passed to a Component. For example, we have a Form Component and a Person Component. The Form Component has a single <input /> that allows the user to change the name by typing into the input. The input is bound to the onChange event and sets the state on the Form. The state value is then passed to the Person component as a prop.

Form.js

import React from 'react';
import Person from './Person';

export default class Form extends React.Component {
  constructor(props) {
    super(props);
    this.state        = { name: '' } ;
    this.handleChange = this.handleChange.bind(this);
  }

  handleChange(event) {
    this.setState({ name: event.currentTarget.value });
  }

  render() {
    return (
      <div>
        <input type="text" onChange={ this.handleChange } />
        <Person name={ this.state.name } />
      </div>
    );
  }
}

Any time the user types into the <input /> this begins an Update for the Person component. The first method called on the Component is componentWillReceiveProps(nextProps) passing in the new prop value. This allows us to compare the incoming props against our current props and make logical decisions based on the value. We can get our current props by calling this.props and the new value is the nextProps argument passed to the method.

Updating State

So why do we need componentWillReceiveProps? This is the first hook that allows us to look into the upcoming Update. Here we could extract the new props and update our internal state. If we have a state that is a calculation of multiple props, we can safely apply the logic here and store the result using this.setState().

Use this as an opportunity to react to a prop transition before render() is called by updating the state using this.setState(). The old props can be accessed via this.props. Calling this.setState() within this function will not trigger an additional render.

-- https://facebook.github.io/react/docs/component-specs.html#updating-componentwillreceiveprops

Props may not change

A word of caution with componentWillReceiveProps(). Just because this method was called, does not mean the value of props has changed.

To understand why, we need to think about what could have happened. The data could have changed between the initial render and the two subsequent updates ... React has no way of knowing that the data didn’t change. Therefore, React needs to call componentWillReceiveProps, because the component needs to be notified of the new props (even if the new props happen to be the same as the old props).

-- See (A => B) !=> (B => A)

The core issue with props and componentWillReceiveProps() is how JavaScript provides mutable data structures. Let's say we have a prop called data and data is an Array.

// psuedo code
this.setState({ data: [1, 2, 3] });

<MyComponent data={ this.state.data } />

If somewhere in our app, a process updates the data array via push(), this changes the content of the data Array. Yet, the Array itself is the same instance. Because it is the same instance, React can not easily determine if the internal data has changed. So, to prevent a lot of issues, or having to do deep comparisons, React will push the same props down.

With this being said, componentWillReceiveProps() allows us to check and see if new props are coming in and we can make choices based on the data. We just need to make sure we never assume the props are different in this method. Be sure to read the great post (A => B) !=> (B => A) by Jim Sproch.

Skipping this method

Unlike the other methods in the Mounting phase, not all our Update phase methods are called every time. For example, we will skip componentWillReceiveProps() if the Update is triggered by just a state change. Going back to our Form.js example above:

 // ...
 handleChange(event) {
    this.setState({ name: event.currentTarget.value });
  }

  render() {
    return (
      <div>
        <input type="text" onChange={ this.handleChange } />
        <Person name={ this.state.name } />
      </div>
    );
  }
// ...

When the user types in the <input /> we trigger a setState() method. This will trigger an Update phase in our Form Component and the Person Component. For our Form Component, we did not receive new props, so componentWillReceiveProps() will be skipped.

Up Next: Using `shouldComponentUpdate()