Improvements to Recently Viewed Component in React - reactjs

Question

I have this small app I created using a REST Countries API.
https://rest-country-react.netlify.app/
If you click on one country card then it is displayed under the "Recently Viewed" header. So far it works fine, but I wanna tune it a little bit. What I thought I'd do:
#1 Add a limit of three recently viewed countries, so basically if the user clicks on 4,5,6 countries, only the three most recent clicked countries are displayed.
#2 Visited countries are currently sorted in an "oldest" to "newest" order. I wanna reverse that so the newest gets the first spot, then the second newest, then the third and so on.
I am stuck because I am not sure how to implement these tweaks. For the first one I thought I'd filter the state array before mapping it in the component, saying something like... if index > 2, filter it out the element.
But for the second, I haven't found a solution yet. Maybe instead of using concat() method, I should use unshift()? From what I read in the React documentation, it's not advised to directly edit the state or its array, so I don't know what to do.
onCountryClick(country) {
const uniqueRecent = [
...new Set(this.state.recentlyViewed.concat(country)),
];
this.setState({
// ... other state updates here
recentlyViewed: uniqueRecent
});
}

Answer 1

There are actually multiple solutions, but let's take the one which is very clean and understandable:
onCountryClick(country) {
const { recentlyViewed } = this.state;
const newState = {}; // add your other updates here
if (!recentlyViewed.includes(country)) {
// firstly take first two items as a new array
newState.recentlyViewed = recentlyViewed.slice(1);
// add country into beginning of new array
newState.recentlyViewed.unshift(country);
}
this.setState(newState);
}
Firstly we check if country already exists in the recentlyViewed array and if no - then continue.
We must use new array when updating the state, so simply calling unshift() method will not work for us as it modifies original array and doesn't return new one. Instead, we firstly call .slice() method which solves two main things for us: it takes a portion of original array (in our case items with index 0 and 1) and return new array with them. Great! Now we can easily use unshift to add country into beginning of new array. Then simply update the state.
With this solution you always get a new array with max of 3 countries, where the first item is the newest.