Keeping track of on/off states of React components

July 28, 2018
πŸ’« Originally posted here. Broken? Let me know ~

Adokiye Iruene asked a question on StackOverflow regarding applying a style on a clicked component not all the sibling components.

Change style for only clicked on view not all views

The problem is that the Asokiye generated components with a list of records in a state.

When a user clicked on a component, not only clicked component had a style applied, but also sibling components.

How can we apply a style only on a clicked component from a list of components?
If you want to know the answer you can cheat 😈  by going directly to the answer.

πŸ—ž Preface

I will use a simple webpage that displays a list of texts wrapped in a component, Child.

Sample Code Output
How it works

When you click on an item, the page will highlight only the clicked line. by applying the following class, highlight.

View Gist on GitHub

Let’s see how to apply that style per component on click.

Edit so.answer.51516825
You can follow along on CodeSandBox

πŸ‘©β€πŸ’» Relevant Codes

Child component returns texts and applies a style depending whether it’s clicked or not (using isClicked prop).

View Gist on GitHub

App.js renders Child components.

View Gist on GitHub

πŸ“Š Analysis

What needs to happen is that we need to keep a track of all on/off states of each component so that we can turn the state of each component on/off.
So let’s track on/off states.

View Gist on GitHub

I’ve declared it as an object, instead of as an array, I will get to it later.
(I promise 🀞)

Let’s look at what happens when a user clicks on a Child component

View Gist on GitHub

OK, it’s kind of hard to digest it so let’s go through it line by line.

On line#4, I am getting all previously clicked states.
const clicked = { ...prevState.clicked };
using an object spread syntax.

, toggle previous state.
clicked[i] = !clicked[i];

Lastly, set the clicked state to the updated one.
return { clicked }; // same as return { clicked: clicked };
Note that if the property name is same as the object key, you can shorten it.

Now the question is, there is no property in the first place to set to in clicked[i] = !clicked[i]???

βš’Β A bit of Hack

OK, I’ve used a bit of JavaScript weirdness to set the clicked state of currently selected item.
I won’t go into too much details as JavaScript’s truthiness gets very hairy πŸ˜ πŸ’’.

So I refer you to this article, Mastering JavaScript’s && and || logical operators by Nicolas Marcora if you want more details.

But what you need to know is that !undefined returns true.

So what happens in clicked[i] = !clicked[i] is that, clicked is an empty object initially {}.
And !clicked[i] will be undefined and negating it with ! operator will turn it into true.


Negating undefined

clicked object will have an item with value of 1 as the key and the on/off state as the value.

Now let’s get back to the previous question, why use an object instead of an array to keep a track of clicked states?

πŸ€” Why use an object?

This is to save previous memory as setting an empty array by index greater than 0 results in filling rest of space with undefined.

Suppose that we declared state = { clicked: []}, then setting a value above first item would populate the array with undefined as shown below.

Wasted memory

I’ve set a value for the 4th item, clicked[3] = !clicked[3]; and the array ended up adding the !clicked[3] value with undefined (<3 empty slots>) for first 3 items.

Array vs. Object

You can πŸ‘€ see ☝️ that the object version stores states of clicked items only.

πŸš€Β Full Source Code

As mentioned above, you can see the working demo on CodeSandBox.
Here is the full source code (for completeness).

View Gist on GitHub

πŸ‘‹ Partying Words

The gist is that, keep a track of each component state in an object and toggle it.