This blog post continues the React Hooks series and gives an introduction to Custom Hooks in React.
Let's learn what it takes to create a custom React Hook as well as all the rules we must keep in mind when using Hooks.

What is Custom Hook?

Custom Hook is a JavaScript function which we create by ourselves, when we want to share logic between other JavaScript functions. It allows you to reuse some piece of code in several parts of your app.

When and How to Use

When we want to share the logic between other components, we can extract it to a separate function. According to official documents, the custom hook has to:

• start with the key word use
• call other hooks

Because custom hook is a JS function, the Rules of Hooks apply to it as well. Those are:

1. Never call Hooks from inside a loop, condition or nested function
2. Hooks should sit at the top-level of your component
3. Only call Hooks from React functional components
4. Never call a Hook from a regular function
5. Hooks can call other Hooks

How to Create Custom Hook?

You create the hook same way as you create any JS function. Look at it as a refactoring of code into another function to make it reusable. It will speed up your productivity and save your time.

Let's consider the following example, where we have useEffect()hook which updates the document title:

import {useState, useEffect } from 'react';

    export const Counter=()=> {
      const [count, setCount] = useState(0);
      const incrementCount = () => setCount(count + 1);
      useEffect(() => {
        document.title = `You clicked ${count} times`
      });

      return (
        <div>
          <p>You clicked {count} times</p>
          <button onClick={incrementCount}>ClickMe</button>
        </div>
      )
    }

What we want to do is to create a custom hook, which accepts a piece of text and updates the document title for us. Here is how we do that:

const useDocumentTitle = (title) => {
        useEffect(() => {
          document.title = title;
        }, [title])
      }

Our useDocumentHook() now accepts the string and calls useEffect() hook inside, which updates the document title with a given title, when the title was changed (we pass title as dependency here).

So our final code would look like this:

import { useState, useEffect } from "react";

const useDocumentTitle = (title) => {
  useEffect(() => {
    document.title = title;
  }, [title]);
};

export const Counter = () => {
  const [count, setCount] = useState(0);
  const incrementCount = () => setCount(count + 1);
  useDocumentTitle(`You clicked ${count} times`);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={incrementCount}>Click me</button>
    </div>
  );
};

And that is all, as simple as that :) In my next blog post I'll share 5 the most commonly used custom hooks, which make your development faster and easier.

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