React: Creating a Custom Hook for Pagination
Tags: react, tutorial
Published: 2020-03-01 11:07:49

In this article, we will implement a generic, reusable pagination mechanism. We will achieve this by implementing our own custom hook.

## What is a Custom Hook?

To quote the [official React documentation](https://reactjs.org/docs/hooks-custom.html):

> A custom Hook is a JavaScript function whose name starts with "use" and that may call other Hooks.

In essence, a custom hook is a React functional component which exports props where at least one prop is prefixed with the keyword `use`. The `use` prop acts like a constructor: It defines the initial value that the functional component needs. Other props can be functions or state values of your hook - you decide what you want to expose.

## Requirements

Pagination is about dividing a list of items into different "pages". It allows users to browse between the pages, either by going to the next, the previous or to jump to any page inside this collection. Therefore, we identify the following requirements:

1. should allow to jump to the previous or the next page
1. should allow to jump to any page in the collection
1. should return the data that corresponds to the current page

For the pagination custom hook, this means these requirements:

1. should be configurable about the number of items per page
1. should be stateful to keep the current page

## Step 1: Configure Number of Items & Current Page

Let’s start the implementation of the custom hook. First, we define the `usePagination` constructor. It receives the data and the number of items per page. We also implement a stateful variable `currentPage`.

```jsx
1 import React, {useState} from 'react';
2
3 function usePagination(data, itemsPerPage) {
4   const [currentPage, setCurrentPage] = useState(1);
5   const maxPage = Math.ceil(data.length / itemsPerPage);
6 }
7
8 export default usePagination;
```

In more detail:

* In line 3, we implement the `usePagination` function, passing the parameters `data` and `itemsPerPage`
* In line 4, we define the `currentPage` state variable by using the `useState` built-in hook [^1]
* In line 5, we set the `maxPage` variable, which defines the upper limit of the number of pages that we can show
* In line 8, we export the `usePagination` function

## Step 2: Increase, Decrease and Go to any Page

These requirements provide the core features of our hook. We will implement them as functions and export them. Thereby, we need to ensure that the value of `currentPage` always lies within the range of 1 and the `maxPage` value.

Let’s code:

```jsx
1  function next() {
2    setCurrentPage((currentPage) => Math.min(currentPage + 1, maxPage));
3  }
4
5  function prev() {
6    setCurrentPage((currentPage) => Math.max(currentPage - 1, 1));
7  }
8
9  function jump(page) {
10   const pageNumber = Math.max(1, page)
11   setCurrentPage((currentPage) => Math.min(pageNumber, maxPage));
12 }
```

* Line 1: The `next` function increases the `currentPage`by 1, but does not exceed `maxPage`
* Line 5: The `prev` function decreases the `currentPage`by 1, but does not go below 1
* Line 9: The `jump` function takes care that `currentPage` stays within both limits

## Step 3: Returning Data of the Current Page

The last step is to implement that only data of the current page is shown.  

```jsx
1 function currentData() {
2   const begin = (currentPage - 1) * itemsPerPage;
3   const end = begin + itemsPerPage;
4   return data.slice(begin, end);
5 }
```

The variable `data` hold all items of the pagination component. From this, we select exactly `itemsPerPage`, beginning with the value `currentPage - 1` because array indexes start with `0`.

## Pagination Hook: Complete Component

Here is the complete component:

```jsx
1  import React, { useState } from "react";
2
3  function usePagination(data, itemsPerPage) {
4    const [currentPage, setCurrentPage] = useState(1);
5    const maxPage = Math.ceil(data.length / itemsPerPage);
6
7  function currentData() {
8    const begin = (currentPage - 1) * itemsPerPage;
9    const end = begin + itemsPerPage;
10   return data.slice(begin, end);
11  }
12
13  function next() {
14    setCurrentPage((currentPage) => Math.min(currentPage + 1, maxPage));
15  }
16
17  function prev() {
18    setCurrentPage((currentPage) => Math.max(currentPage - 1, 1));
19  }
20
21  function jump(page) {
22    const pageNumber = Math.max(1, page);
23    setCurrentPage((currentPage) => Math.min(pageNumber, maxPage));
24  }
25
26  return { next, prev, jump, currentData, currentPage, maxPage };
27 }
28
29 export default usePagination;
```

## Conclusion

In this article, I showed how to implement a custom react hook. The hook exposes its functions and parts of its state to its caller. The caller invokes functions on the hook, and decides how to render its result and the pagination. Custom hooks are powerful and help you to define reusable functionality.

 [^1]: Yes, you can reuse built-in/custom hooks inside other hooks.