Understanding the onMount Function in Svelte -Core Elements, Gotchas, and Best Practices

Svelte is one of my favorite Javascript frameworks. The framework has been gaining a lot of traction due to it’s simplicity and powerful features it includes. While it’s simplicity allows basically anyone with Javascript, HTML, and CSS knowledge to jump in and build reactive web applications, some of it’s features can be overlooked. In this article, I intend to delve into one of the most overlooked features - the onMount function.

onMount is a lifecycle function that plays a crucial role in component behavior. Similar to React, this function does not need to be defined in the component for the component to be mounted. Rather, it presents an opportunity to the developer to run specific functions when the component is mounted. With it’s definition out of the way let’s dive in and explore the anatomy of the lifecycle function, it’s core elements, common pitfalls, and best practices to help you use it effectively in your projects.

Basic Anatomy

As mentioned above, the onMount function is a lifecycle function that gets called once the component is first rendered in the DOM. In this regard, if one wishes to run some code right after the component is rendered such as fetching data among other actions, this is most likely the best place to place the snippet. The function takes a callback function that is executed once the onMount function is called. In the example below, Mounted is logged once the component is mounted in the DOM.

<script>
	import { onMount } from 'svelte';

	onMount(() => {
		console.log('Mounted');
	});
</script>

Core Elements of onMount

Invocation Timing

The onMount function is invoked immediately after the component is added to the DOM. This aspect of the lifecycle function is important as it ensures any further DOM manipulations one might want to perform can only be done once the component has been rendered. This aspect of the function also allows developers to handle any code that could have otherwise blocked the rendering of the component. This also implies that the callback passed to the onMount function does not need to be synchronous as shown below;

<script>
	import { onMount } from 'svelte';

	onMount(async () => {
		// Perform some api / long running task that could have blocked the component render
	});
</script>

onMount Callback Return Value

The return value of the onMount function is equally important as the function that is executed initially. If the function returns a function, this function is executed once the component is destroyed. As such, it becomes a particularly useful tool when one wants to perform any level of cleanup related to the rendered components. In the wild, this is often used to remove event listeners and unsubscribing from services/events.

Common Gotchas and Issues

• Handling Race Conditions: When dealing with asynchronous operations, race conditions can arise. This is especially true if multiple onMount functions or different components are interacting with shared state or resources. In this regard, it is important to take precautions to prevent this eventuality.
• Memory Leaks with Cleanup Functions: While this cannot be considered a Gotcha per se, it is still an important issue that is tied to the cleanup usage of the onMount function. If a component tends to be frequently mounted and unmounted, and it keeps initializing memory or event listeners, one might end up with memory leaks. This is especially true if you forget to return a cleanup function. As such, returning a function that releases the assigned memory alongside dropping any lingering event listeners is paramount.
• Multiple onMount Calls: Fun fact, Svelte allows multiple onMount calls within the same component. While some swear by this, often touting modularity among other reasons, I think having different functions that are encapsulated in a single one that is then invoked by the onMount function is a better approach. However, if you would still like to use multiple onMount functions within the same component, it is important to note that it’ makes it quite easy to lose track of what each onMount is doing. This increases the likelihood of potential bugs or conflicts. Documentation can help in these situations, but I would recommend avoiding this path altogether to prevent any issues this might bring about.

Best Practices for Using onMount

• Keep it Simple: The onMount function should ideally be kept simple and focused. Avoid cramming too much logic into it. If necessary, break down the logic into smaller functions and call them from within onMount. Alternatively, you could encapsulate these functions in a separate function to ensure the onMount function has only one function call. I tend to think this approach is cleaner and makes it easy to follow for anyone reading your code.
• Handle Edge Cases: Consider edge cases such as what happens if an API call fails or if the component is unmounted before an asynchronous operation completes. Handling these scenarios gracefully will make your component more robust and reduce the likelihood of a single component being the single point of failure in your app. In some cases, these, among other, svelte errors could block the execution, hence resulting in the UI being blocked.
• Avoid Side Effects Outside onMount: As much as possible, keep side effects (e.g., DOM manipulations, API calls) inside onMount. This ensures that your side effects only occur when the component is fully mounted and ready. Another approach to handling this would be to ensure such code is ran after the component has been mounted.
• Use Cleanup Functions: Always return a cleanup function when necessary. This prevents memory leaks and ensures that your component cleans up after itself properly. Alternatively, the cleanup function can be ran inside the onDestroy lifecycle function - which is also provided by svelte.

Conclusion

The onMount function is a powerful tool in Svelte that allows you to perform operations when a component is first rendered. Harnessing its full potential requires having a proper understanding of it’s core elements, common pitfalls and following best practices is recommended. Regardless of the task, whether you’re fetching data, manipulating the DOM, or setting up or removing event listeners, onMount will help you manage your component’s lifecycle effectively. Ultimately, it is important to test, test, and test again. After all, discovery requires experimentation 😉.