React

Component Lifecycle Methods

React Course

Introduction

In the previous section, you’ve learned about a component’s lifecycle. In functional components, we’ve primarily used the useEffect hook to perform tasks throughout the lifecycle, but in class components we have to use specific lifecycle methods instead.

We’ll start with a short recap of what a components lifecycle is. There are three stages to a component’s life: mounting, updating, and unmounting. Each of these have a method assigned to them within class components, which is what we are going to cover in this lesson.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

  • How to use lifecycle methods in a class component

render()

The render function is the most used lifecycle method, and one that you’ve come across in the last class components lesson. It is the only required lifecycle method in a class component. It runs on mount and update of a component. Render should be pure, meaning it doesn’t modify component state, returns the same thing each time it’s called (given the same inputs), and doesn’t directly interact with the browser.

componentDidMount()

This method is run after the component is mounted (inserted in the DOM tree). You should make any calls to fetch data that is needed for the component here. It is also a good place to do anything that is reliant on the component, such as drawing on a canvas element that you just rendered.

componentDidUpdate()

This method is run after a component re-renders. Because of this, you have to be careful about what you update in this method, as if you’re updating state indiscriminately, a re-render is caused, and you’ll end up in an endless loop. You can avoid this issue by using conditional statements about the equality of previous and current props when updating state.

In this method you should be updating anything that needs to be changed in response to either the DOM changing, or any states that you might want to act on upon change. For example, you’d want to refetch user data if the user changes.

componentWillUnmount()

This is the last lifecycle method, which is called before a component is unmounted and destroyed. In this method you should be performing cleanup actions, so that would be cancelling network requests, clearing timers, etc.

How useEffect() combines the lifecycle methods

Now that we’ve learnt about class lifecycle methods, it’s useful to understand that the useEffect hook used in functional components is essentially a combination of componentDidMount, componentDidUpdate, and componentWillUnmount. Which method/methods it relates to varies based on it’s dependency array, and if it returns anything.

  • An empty dependency array would be equivalent to componentDidMount.
  • A dependency array with a value/values in it would be a combination of componentDidMount and componentDidUpdate, but only updating when dependencies change.
  • No dependency array would be equivalent to componentDidMount and componentDidUpdate combined.
  • A return function inside of a useEffect() hook would be equivalent to componentWillUnmount.

For example:

  useEffect(() => {
    placeholderFunction();
    return () => cleanupFunction();
  }, [])

In this snippet, the useEffect contains the functionality of componentDidMount, and componentWillUnmount via the return function. This example doesn’t have the componentDidUpdate functionality because of an empty dependency array.

Assignment

  1. Check out this component lifecycle diagram to see a good visual representation of a components lifecycle methods.

  2. Read through component documentation from ‘constructor(props)’ up to ‘componentWillUnmount()’ in the Reference section, being mindful of deprecated APIs. Consider other APIs as supplementary information.

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

Additional resources

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