React lifecycle methods

How to use componentDidMount() in React

React lets us create either functional components or class components.
Functional components are just functions, while class components are instances of a class.

Since class components extend React.Component they automatically inherit methods defined in React for us.
Some of these methods are called component lifecycle methods because they are invoked by React at certain points in time in the life of a component.

In the class components that we define we can override these default methods with our own version so we can add functionalities that come into play at a specific point in the life of our component.

One of such methods is componentDidMount() which is called automatically after the component has been rendered on the page.
componentDidMount() can't be placed inside a functional component because a functional component only renders JSX and does little else.

We add componentDidMount() to a class component when we need to perform some operation immediately after the component was rendered on the page.
Here's how we define componentDidMount() into a class component:

class Pokemons extends React.Component {

  componentDidMount {
    // do something here
  }

}

Now that we have the method in our class, what are we going to do with it?

componentDidMount() is usually the place where we fetch data from an external data source, like an API or a database and update the state with that data.
Since we update the state with setState(), React will re-render the component with the new data displayed.

Here's an example.
Suppose the data comes into our application via an external API call and is placed into an array called pokemonData.
In the Pokemons constructor we initialize the state to an empty array, because we know pokemonData comes in as an array.

class Pokemons extends React.Component {
  constructor() {
    super();

    this.state = {
      pokemons: [] 
    }
  }
}

Next we define componentDidMount() as the place where we fetch the data and update the component state to include this new data that has just come in.

  componentDidMount() {

    // Fetch data from some external API and put it into pokemonData
    // then set the state to this new data:

    this.setState({pokemons: pokemonData})
  }

After componentDidMount() runs the pokemons property in our state is assigned the value of pokemonData.
The component is rendered and the new state is displayed in our component.

In a future article we'll see how to fetch data from an external API.

componentDidMount() is only one of several lifecycle methods available to us. We will look at other lifecycle methods in the coming days.