In the previous lesson, you learned how to handle lifecycle effects in Svelte using onMount and onDestroy. These functions allow you to perform actions when a component is first rendered and clean up resources when it is removed from the DOM. Now, we’ll explore another important concept in Svelte: waiting for DOM updates using the tick() function.

When you update the state of a component in Svelte, the DOM is updated asynchronously. This means that the changes you make to the state might not immediately reflect in the DOM. In some cases, you need to wait for the DOM to update before performing certain actions, such as scrolling to a newly added element or measuring its dimensions. This is where tick() comes in.

The tick() function is a utility provided by Svelte that allows you to wait for the DOM to update after a state change. Think of it as a way to ensure that the DOM is in sync with your component’s state before proceeding with the next steps. For example, if you add a new message to a list and want to scroll to it, you’ll need to wait for the DOM to update before scrolling. tick() makes this possible.

In this lesson, you’ll learn how to use tick() to wait for DOM updates and perform actions like scrolling to elements or measuring their dimensions. By the end of this lesson, you’ll be able to build components that ensure the DOM is updated before performing critical actions.

Let’s dive into an example to see how tick() works in practice. We’ll build a simple component that allows you to add messages to a list and automatically scroll to the last message after it’s added. Here’s the code:

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1<script>
2    import { tick } from 'svelte';
3
4    let messages = $state(["Message 1", "Message 2", "Message 3"]);
5    let messageContainer;
6
7    async function addMessage() {
8        messages = [...messages, `Message ${messages.length + 1}`];
9
10        // If you comment out the tick(), you will notice that the site behaves incorrectly when a message is added.
11        await tick(); // Wait for the DOM to update
12
13        // Scroll the last message into view after DOM updates
14        const lastMessage = messageContainer?.lastElementChild;
15        if (lastMessage) {
16            lastMessage.scrollIntoView({ behavior: "smooth" });
17        }
18    }
19</script>
20
21<button onclick={addMessage}>Add Message</button>
22
23<div bind:this={messageContainer} style="max-height: 150px; overflow-y: auto; border: 1px solid #ccc; padding: 10px;">
24    {#each messages as message}
25        <p>{message}</p>
26    {/each}
27</div>

In this example, we start by importing the tick() function from Svelte. We then define a messages array using the $state rune to manage the list of messages. The messageContainer variable is used to reference the container element that holds the messages.

The addMessage function adds a new message to the messages array. After updating the array, we use await tick() to wait for the DOM to update. Once the DOM is updated, we use scrollIntoView to scroll the last message into view smoothly. The bind:this directive is used to bind the messageContainer variable to the container element, allowing us to reference it in our code.

When you run this code and click the "Add Message" button, a new message is added to the list, and the container automatically scrolls to the last message. This demonstrates how tick() ensures the DOM is updated before performing actions like scrolling.

Before we move on, let’s recap some key concepts in Svelte 5 that are relevant to this lesson. First, reactivity in Svelte 5 is handled using the $state rune. This allows you to define reactive variables that automatically update the DOM when their value changes. In our example, the messages array is a reactive variable that updates the list of messages when a new message is added.

Second, event handling in Svelte 5 is done using attributes like onclick. In our example, the onclick attribute is used to call the addMessage function when the button is clicked. This is a straightforward way to handle user interactions in Svelte.

Finally, Svelte 5 does not use stores or the $: syntax for reactivity. Instead, it relies on runes like $state to manage reactive data. This makes the code more explicit and easier to understand.

The tick() function is useful in several scenarios where you need to wait for the DOM to update before performing an action. Here are some common use cases:

1. Scrolling to Elements: As shown in our example, tick() can be used to scroll to a newly added element after the DOM updates. This ensures that the element is rendered before scrolling to it.
2. Measuring Elements: If you need to measure the dimensions of an element after a state change, you can use tick() to wait for the DOM to update before taking measurements.
3. Synchronizing Animations: When updating the state of a component that triggers an animation, you can use tick() to ensure the animation starts only after the DOM is updated.

These use cases highlight the importance of tick() in ensuring that the DOM is in sync with your component’s state before performing critical actions.

In this lesson, you’ve learned how to use the tick() function in Svelte to wait for DOM updates before performing actions like scrolling to elements or measuring their dimensions. We walked through an example where you added messages to a list and scrolled to the last message after it was added. You also learned about key concepts in Svelte 5, such as reactivity with $state and event handling with onclick.

In the upcoming practice exercises, you’ll have the opportunity to apply these concepts by building components that use tick() to wait for DOM updates. You’ll experiment with adding features to the message list and explore other use cases for tick(). Remember that the CodeSignal environment has Svelte 5 pre-installed, so you can focus on coding without worrying about setup.

Great job completing this lesson! You’re now ready to take on the challenges of managing DOM updates in your Svelte applications. Keep up the good work!