Add Hooks

2 minutes

You can use the Effect Hook to keep an eye on the state of your application and run code in response to updates. We register the function we want to run in response to changes by using useEffect.

Scenario

Our application allows the user to tap individual items to mark them as prepared. If some items aren't prepared yet, we want to display the message Just keep chopping. When all items are finished, we want to display Prep work done!

We'll set up this behavior by adding a new state object. We'll update the object from our Effect Hook.

Important

In our example, we modify state inside useEffect. By default, useEffect runs whenever any stateful object is modified. This behavior can create an endless loop. In this loop, we modify state and the Hook is run, which modifies state, which runs the Hook, and so on.

To avoid the endless loop, we can use the dependency parameter on useEffect to look at only one object. We'll do this when we create our code.

Add the new state property

Open the App.jsx file.
To add the new state property, insert the following code below the line that reads, TODO: Add new state property.
```
// TODO: Add new state property
const [ prepared, setPrepared ] = useState(false);
```

Add the Effect Hook listener

To add the Effect Hook listener, insert the following code below the line that reads, TODO: Add the effect hook.

// TODO: Add the effect hook
useEffect(() => {
    setPrepared(recipe.ingredients.every(i => i.prepared));
}, [recipe]);

The code uses setPrepared to update prepared. It uses the every method, which returns a Boolean value based on every item that matches the criteria we specify. In our case, we're checking whether every item is prepared. If not, the method returns false.

The second parameter on useEffect is set to [recipe]. This setting provides the dependency to ensure our code runs only when the recipe object changes.

Add the display

Now display the message to the user if the preparation work is done. To do so, add the following code below the line that reads, TODO: Add the prep work display.

{/* TODO: Add the prep work display */}
{ prepared ? <h2>Prep work done!</h2> : <h2>Just keep chopping.</h2>}

We look at the prepared object. If it's true, we display Prep work done! Otherwise, we display Just keep chopping.

Test the page