useEffect - React Hooks Series

Welcome back to the series of React Hooks. In part one of the series, I wrote about the useEffect react hook.

Series path

useState
useEffect

What is `useEffect` hook?

The useEffect hook lets you perform side effects in functional components.

Few examples of side effects

Data fetching
Setting up a subscription
Manually changing the DOM in React

If you are familiar with React class lifecycle methods, you can think of useEffect hook as componentDidMount, componentDidUpdate and componentWillUnmount combined.

What does `useEffect` hook do?

By using this hook, you let React know that your component needs to perform something after rendering of the component. React will remember the function which you passed and react call it after performing the DOM updates.

In this effect we can do many things like setting document title, fetch some data from an API, setting event listeners.

Does useEffect run after every re-render?

Yes! By default, the effect runs both after the first render and after every update of the component. Rather than thinking in terms of mounting and updating, you might find it simpler to think that effects happen after render. React guarantees the DOM has been updated by the time it runs the effects.

Example

Let's change the document title for better understanding.

Using class component

import React from "react";

class ExampleComponent {
  //After rendering
  componentDidMount() {
    document.title = "Updating the title in componentDidMount";
  }

  //After updating
  componentDidUpdate() {
    document.title = "Updating the title in componentDidUpdate";
  }

  render() {
    return <div>Setting the title</div>;
  }
}

export default ExampleComponent;

Using functional component

import React, { useEffect } from "react";

const ExampleComponent = () => {
  useEffect(() => {
    document.title = "Setting title using useEffect";
  });

  return <div>Setting the title using useEffect hook</div>;
};

In function component, you have to define one useEffect function instead of componentDidMount and componentDidUpdate.

Getting deeper

Now we know what useEffect is. Let us try to understand it deeper. useEffect function accepts two-parameter. i) A function which gets called on every update/re-rendering. ii) An array of dependencies value on which the function has to get called.

1. The useEffect below will always get called on rendering and updating of the component.

useEffect(() => {
  console.log(
    "I will be called each time the component renders and re-renders"
  );
});

2. The useEffect below will get called only once. i.e. the first time it renders. It is equivalent to componentDidMount. The second parameter [] is called the dependencies array. An empty array means no dependency.

useEffect(() => {
  console.log("I will be called only once when the component is mounted");
}, []);

3. The useEffect below will get called each time the value of name is changed. It is like componentDidUpdate.

useEffect(() => {
  console.log("I will be called first when the component is mounted and each time the name is changing");
}, [name]);

4. If we want to do any clean-ups before the component is unmounted.

useEffect(() => {
  // some tasks

  return () => {
    console.log("I do cleanups");
    console.log(
      "will first run on component mount then, will run before useEffect and lastly before unmounting"
    );
  };
});