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Cover image for Wrapping arrays in ReactJS
Murat Can YΓΌksel
Murat Can YΓΌksel

Posted on

Wrapping arrays in ReactJS

I want to display some sort of text, a reviews kind of a thing maybe, or just pictures I can see the next and the previous image by clicking the respective buttons. To do so, I'll need to know how to wrap around arrays. In this post, I'll be showing that.

You can find the source code here => https://github.com/muratcan-yuksel/react-wrap-array

To start with, I create a data.js file and put the following inside =>

const data = [
  {
    id: 1,
    text: "1st Text",
  },
  {
    id: 2,
    text: "2nd Text",
  },
  {
    id: 3,
    text: "3rd Text",
  },
  {
    id: 4,
    text: "4th Text",
  },
  {
    id: 5,
    text: "5th Text",
  },
];
export default data;
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In my application, I don't want to map through this data array of objects. No, I want to display only one of them each time. Check my App.js file=>

import React, { useState } from "react";
import data from "./data";
import "./style.css";

const App = () => {
  const [index, setIndex] = useState(0);
  const { id, text } = data[index];

  const nextItem = () => {
    setIndex((index) => {
      let newIndex = index + 1;
      return wrapItems(newIndex);
    });
  };

  const previousItem = () => {
    setIndex((index) => {
      let newIndex = index - 1;
      return wrapItems(newIndex);
    });
  };

  const wrapItems = (itemIndex) => {
    if (itemIndex > data.length - 1) {
      return 0;
    } else if (itemIndex < 0) {
      return data.length - 1;
    } else {
      return itemIndex;
    }
  };

  return (
    <div id={id} className="container">
      <div className="title">{text} </div>
      <div className="buttons">
        <button onClick={previousItem} className="btn">
          Previous
        </button>
        <button onClick={nextItem} className="btn">
          Next
        </button>
      </div>
    </div>
  );
};

export default App;
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I start by importing the useState hook from React, the data.js file I've just created, and a style.css file you shouldn't be worried about. If you want to check it out, I've provided the source code above anyway.

I set my state to 0, this will be the index of the element I want to display on the page from the data array. With const { id, text } = data[index]; I destructure the data coming so that whenever I want to use the text or id variables, they'll point to the item in the data array at the index of state. That means that if the state were to change, the id and the text would point to a different item in the data array from data.js.

Now, in my return statement I display the β‚Ίext, and also have two buttons: One of them pointing to the previous item while the other one pointing to the next item. I also have some functions attached to them.

The function nextItem sets the state to plus one of whatever it is. Now, since I can't modify the state directly, I'll have to work around and write a function to manipulate index. The function previousItem is the same except that it sets the state to minus one of whatever it is at that moment. But you'll also notice that the returned newIndex is wrapped inside the wrapItems'function.

The reason wrapItems function exist is that if it weren't, whenever we reach to the last item in the data array, which is the 5th item because I only put 5 items in the array, which means the 4th index as indexes start at 0, the nextItem would continue to update the state to plus one, which would make the index 5, and our application would crash. It would crash because we don't have any item at the index 5 of our data array. Hell, we don't have the index 5 at all. The same would happen with the previousItem function obviously, the app would crash at index -1. wrapItems function fixes that problem.

Now, wrapItems function takes a number and probably is interpreted like so=>

  • If the given number, which is the index coming from the state, is bigger than the last index of the data array (which is 4, as there are 5 items and indexes start at 0), return 0 so that we'd start from the beginning.
  • If the index is smaller than 0, which means we're at the first index, which also means we might fall into the world of negative numbers that'll crash our app, let's update the state to the last index of the data array, so that when we fire the previousItem function by clicking the appropriate button when we're at the first item, we'd be presented with the last item and not a crashed app.
  • else, things are fine, do not do anything.

That's all. Cheers!

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