Building reactive UIs with SvelteKit's reactive statements


Building reactive UIs with SvelteKit's reactive statements


- 14 min to read

Building reactive UIs with SvelteKit's reactive statements

Explore SvelteKit's approach to creating UI elements using reactive statements and JSONPlaceholder API. Dive into SvelteKit's features and build an engaging project in this guide.

By Chidera Chukwuka

Building reactive UIs with SvelteKit's reactive statements

In contemporary web development, creating reactive user interfaces (UIs) is essential to offering a smooth and dynamic user experience. An interactive and captivating application is produced by reactive user interfaces, which react in real-time to user actions and modifications in the underlying data. However, employing traditional methods to create and manage reactive user interfaces can be difficult.

In this article, you will explore how to build reactive UIs with SvelteKit, a framework that offers a creative and sophisticated method of creating reactive UI elements from reactive statements using JSONPlaceholder API. You will explore SvelteKit's salient features, analyze the notion of reactive statements, and receive guidance in building a project that showcases its capabilities. To view the project's source code, head over to Bejamas' GitHub profile.

Understanding reactive UIs

What is reactive UI, and why is it important

Reactive UI describes user interfaces that spontaneously update in response to modifications in the fundamental data or user behaviors. It is necessary because it improves the user experience by lowering the need for manual updates, giving real-time feedback, and building a more dynamic and seamless interface. Reactive user interfaces (UIs) offer advantages to developers as well. They can streamline the logic and code of user interface components, minimize complexity and defects, and enhance the applications' scalability and performance.

Challenges of conventional approaches

It's not simple to create and maintain reactive user interfaces (UIs), especially when using traditional approaches that depend on event-driven techniques like callbacks, promises, observables, etc. These techniques frequently include multiple boilerplate code, tedious state management, and manual DOM manipulation, which make the user interface components challenging to comprehend, troubleshoot, and test.

SvelteKit offers a declarative and reactive method for creating user interfaces in order to overcome these obstacles.

Overview of SvelteKit

Overview of SvelteKit as a framework

SvelteKit is a cutting-edge web framework that utilizes reactive programming to streamline the user interface development process. Svelte, a component-based framework, serves as its foundation and converts components into highly efficient JavaScript during build time. SvelteKit aims to offer a more versatile and robust framework that can support a range of use cases and scenarios, such as server-side rendering (SSR), static site generation (SSG), single-page applications (SPAs), hybrid applications, etc. It has excellent performance optimization, reactivity, and simplicity.

Key features of SvelteKit for reactive UIs

SvelteKit is a fantastic option for creating reactive user interfaces because of a number of important features, including:

  • Reactivity: SvelteKit's emphasis on reactivity is one of its most notable aspects. The idea of automatic updates forms the foundation of the framework's reactivity model. Effective updates based on data or state changes are made possible via SvelteKit's build-time component compilation, which eliminates the need for a virtual DOM. Reactive statements can be written using SvelteKit, and they can update the UI components immediately as data or state changes—all without the need for subscriptions, callbacks, or event listeners.
  • File-based routing: The file structure of the src/routes directory is the basis for the file-based routing system that SvelteKit utilizes to automatically construct routes. Routes that are nested, catch-all, redirect, and dynamic with parameters can all be constructed. Moreover, a standard layout for every route in a directory can be defined using the $layout.svelte file.
  • Server-side rendering: Server-side rendering is supported by SvelteKit, enabling developers to pre-render pages on the server before delivering them to the client. Search engine optimization (SEO) is enhanced, and the first page loads more quickly as a result.
  • Hydration: Thanks to SvelteKit's support for hydration, client-side interactivity and responsiveness are added to HTML that is rendered by the server. SvelteKit treats the remaining portions of the page as static HTML and employs a method known as "islands architecture" to hydrate only the sections of the page that require it. As a result, the web app functions better and is more user-friendly.
  • Hooks and endpoints: SvelteKit offers endpoints and hooks, which are functions that can operate client-side or server-side and carry out several actions like data retrieval, request validation, page rendering, and so on. The functionality and security of web applications can be improved with the aid of hooks and endpoints.

Benefits of SvelteKit

SvelteKit has a lot to offer web app developers and users. Among the advantages are:

  • Quick and lightweight: SvelteKit creates web applications that are quick to load, light in weight, and have a small footprint. SvelteKit uses Svelte capabilities to compile the UI elements into standard JavaScript, doing away with the requirement for a runtime library. Additionally, SvelteKit optimizes online performance by utilizing modern web technologies like code-splitting, hydration, and SSR.
  • Flexible and adaptive: SvelteKit can be tailored to meet a variety of needs and use cases. Web apps of all kinds, including hybrid apps, single-page apps, progressive web apps, and static web pages, are supported by SvelteKit. Furthermore, SvelteKit supports a variety of deployment targets, including edge, serverless, and conventional servers. Also, SvelteKit supports different kinds of data fetching, including client, server, and static data.
  • SEO-friendly applications: SvelteKit makes it easier to create applications that are search engine friendly by providing built-in support for server-side rendering. Search engines are able to index content more efficiently when pages are pre-rendered on the server.
  • Community support and ecosystem: SvelteKit benefits from an expanding and vibrant developer community. A multitude of plugins and extensions are available for the framework to improve functionality and optimize development processes, creating a helpful ecosystem.

Reactive statement in SvelteKit

What are reactive statements in SvelteKit

Reactive statements are expressions that immediately update when the underlying data is modified. It is a fundamental feature of Svelte and SvelteKit. Compared to conventional event-driven techniques, they offer a simpler means of managing state and updates by enabling components to automatically update in response to changes in their dependencies. SvelteKit uses automatic dependency tracking to achieve reactivity. SvelteKit monitors the dependency between a component and a variable or store used by reactive statements, updating the component automatically when the variable or store changes.

Reactive statements in SvelteKit are distinct from the traditional event-driven approach—which makes use of promises, observables, event listeners, callbacks, etc.— to updating the user interface. Due to the manual management of state changes and UI updates, the event-driven approach can be verbose, complex, and prone to errors. Conversely, reactive statements in SvelteKit allow you to specify the logic for UI updates and let Svelte take care of the rest. They are straightforward, elegant, and dependable. Because they only execute when the data they depend on changes and only update the impacted areas of the user interface, reactive statements in SvelteKit are also more efficient.

Conventional event-driven method vs. reactive statements

Reactive statements and the traditional event-driven approach are two distinct approaches to managing UI updates and data changes in web applications. They can be applied to various scenarios and goals, and they each have unique benefits and drawbacks.

The foundation of reactive statements is the declarative syntax, which updates the user interface (UI) automatically when the data depends on changes. With reactive statements, the developer can write elegant and succinct code that specifies how the user interface should respond to changes in the data, and Svelte will take care of the rest. Because reactive statements only run when the data changes and only update the affected areas of the user interface, they are straightforward, dependable, and efficient. They do this by doing away with the need for event listeners, callbacks, promises, observables, and other mechanisms.

The traditional event-driven approach relies on reacting to application events—like user interactions, network requests, timers, and others—by means of event listeners, callbacks, promises, observables, and other mechanisms. The developer must manually oversee state changes, UI updates, and communication between various application components when using the event-driven approach. Due to the need to manage asynchronous operations, avoid race conditions, write a lot of boilerplate code, and deal with callback hell, the event-driven approach can be verbose, complex, and prone to errors.

The control flow of the application is the primary distinction between reactive statements and the traditional event-driven method. The developer must write code to handle each event and its effects when using the event-driven approach, which bases control flow on the events that take place in the application. The developer only needs to write code to declare the logic of the UI updates in reactive statements, where the control flow is dictated by the data that changes in the application.

Creating reactive statements in SvelteKit components

In your terminal, type the following commands to start a new SvelteKit project:

Proceed to create a component inside the src then create the user list component.

This is an elementary Svelte component that shows user information and includes a detail view button. The HTML in the main section specifies the structure, and the logic for the component will be contained in the script section.

Working with variables and states

Importance of variables and state in building reactive UIs

In SvelteKit, variables and states are essential for creating responsive user interfaces (UIs). They are the cornerstone for handling dynamic data, reacting to user input, and making sure that the user experience is smooth and quick.

  • Dynamic data representation: The data that powers a reactive user interface's dynamic nature is stored in variables and states. They depict the data that is updated in response to user input, outside circumstances, or data retrieval.
  • Controlling user interactions: States and variables are essential to user interaction management. By enabling the tracking of user inputs, clicks, and other events, they allow the user interface to be updated.
  • Conditional rendering: The circumstances in which particular UI elements are rendered or hidden are determined by variables and states. They allow the UI to be dynamically adjusted by conditional logic in response to changing conditions.
  • Enhancing efficiency: Using variables and states effectively can help enhance performance. Unnecessary re-renders can be prevented by selectively updating only the UI elements that are required.

Runes in SvelteKit's reactive statements

Explanation of Runes

A recent addition to Svelte is Runes, which gives you a succinct and expressive syntax for generating reactive variables and statements. Inspired by Solid, Svelte sister project, Runes uses a signal-based reactivity model.

Data fetching and display

With the help of Svelte load functions and reactive statements, SvelteKit offers a straightforward and user-friendly method for retrieving and displaying data in a reactive manner.

Load functions are unique functions that can be created in the src/routes folder's +page.js or +page.server.js files that go with the +page.svelte files. When a load function returns an object that the page component can use to access the data, it means that the function has successfully fetched the data the page requires. Additionally, load functions can return other properties to control the page's response, like status, error, headers, etc.

Inside the userlist folder create a +page.js where you would like to fetch the data to.

So src/routes/userlist/+page.js looks as so:

This SvelteKit code above defines a load function, which is a unique function used to fetch data prior to rendering the page. This enables you to retrieve information from the server side, making it accessible for search engine optimization or enhancing the speed of the initial load.

A GET request is sent to the designated API endpoint—in this case,—using the fetch function. Since it is an async function, after submitting the request it awaits the response before parsing the JSON data. The data variable is then used to hold the parsed data. The retrieved data is returned by the function as an object with a user's property. The Svelte component that makes use of this object will have it available as a prop.

A try-catch block encloses the code to handle any errors that might arise during the fetch or data parsing procedure. An error is reported to the console if it happens.

This code component displays a list of users by using the data that was fetched in the previous load function. The script section makes variable data accessible to the parent component by exporting it using Svelte's export keyword. Next, the data.users is assigned to a newly created local variable called users.

The component's functionality involves presenting a list of users in a card format, enabling users to click on the "See Details" button and be taken to a page that is unique to each individual. Iterating over the array of users is done using the #each block, which receives the data as a prop (data) from a parent component.

Handling user interactions reactively

Reactive user interactions can be handled easily and naturally with SvelteKit by utilizing the reactive statements and event directives offered by Svelte. To listen for and handle particular events, like on:click, on:input, on:hover, etc., event directives are special attributes that can be added to any HTML element. Regular JavaScript expressions that update data or call functions when the event takes place can be passed through to event directives.

Inside the users folder, create the [userId] folder along with the +page.js and +page.svelte to handle the user details.

In the src/routes/users/[userId]/+page.js add the following code:

An asynchronous function called load is exported by the code. SvelteKit calls this function automatically when rendering occurs on the server. An object with the properties fetch and params is given to the function. HTTP requests are made with the fetch function, and the parameters from the route are contained in params.

Using the userId parameter, the load function retrieves user information from the JSONPlaceholder API. Following the fetching process, the user details are returned as an object, which the Sveltekit component can use as rendering props.

Proceed to add the code for the src/routes/users/[userId]/+page.svelte as follows:

The user object is extracted from a data prop by the SvelteKit component, which then shows the user's details in a Bootstrap card. It's a neat and tidy method of displaying user data in an ordered manner.

Conditional rendering

In SvelteKit, conditional rendering allows you to manage an element or component's visibility according to predetermined criteria. This gives you an adaptable method to show or hide content based on changes in the application's state. SvelteKit's syntax simplifies the implementation and maintenance of conditional rendering in your components, whether you're dealing with straightforward visibility toggles or intricate multi-condition scenarios.

Logic blocks from Svelte, like if, else if, else, each, and await, can be used with SvelteKit to enable conditional rendering. The rendering of the elements or components can be managed by using logic blocks, which are unique syntax that can be used inside component markup. Normal JavaScript expressions that return true or false, as well as other values that can be forced to be boolean, can be passed into logic blocks.

The Svelte component checks to see if the users array contains any elements before dynamically rendering a message stating that "no users are available" or a list of users. When working with data that may or may not be present, this is a great way to make the user experience more user-friendly. {#if users.length > 0} verifies whether there are any elements in the users array; If this is the case, it enters the block and shows the user list. It moves into the :else block if the condition in {#if} is not satisfied, that is, if the users array is empty.

Best practices for building reactive UIs with SvelteKit’s reactive statements

To create effective and responsive user interfaces, there are numerous best practices for utilizing SvelteKit's reactive statements.

  • Utilize declarative syntax: Adopt SvelteKit's declarative syntax to lucidly communicate the behavior of the user interface. Rather than specifying how the user interface (UI) must change in reaction to events imperatively, use a declarative syntax to articulate the intended state on data changes. Code becomes easier to read and maintain as a result.
  • Reactive statements shouldn't be used for side effects that should only occur once.: Reactive statements can run more than once during the lifecycle of a component because they are executed whenever the data they depend on changes. Reactive statements can be problematic if they are used for one-time side effects like data fetching, event listener setup, timer creation, etc.
    Use the onMount lifecycle function rather than a reactive statement for side effects that should only be executed once to prevent this. Once the component is mounted, the onMount function—which can accept an async function or return a promise—is only ever called once.
  • Employ stores for the global state: Store is offered by SvelteKit to manage shared state between components. Stores allow reactive data to be shared and encapsulated without requiring props to be passed through several tiers of components. Stores give state management a centralized approach that guarantees consistency and ease of maintenance.
  • Respond reactively to user interactions: To manage user interactions, make use of the reactive statements and built-in event modifiers in SvelteKit. When handling complex logic that is triggered by user actions, keep event handlers brief and utilize reactive statements.
  • Utilize the $: prefix sparingly: As the expression is reactive—that is, it will be executed whenever the data it depends on changes—the $: prefix indicates that. That being said, not all expressions must be reactive, and overusing the $: prefix can result in additional calculations and make the code harder to understand. For this reason, you should only include the $: prefix in expressions that require reactivity and leave it out of expressions that are static or constant.

Optimizing performance of SvelteKit

It's crucial to optimize performance when developing reactive user interfaces (UIs) using SvelteKit's reactive statements in order to develop web applications that run quickly and smoothly.

  • Reduce reactive statement complexity: Individual reactive statements should be clear and concise. Divide complicated logic into manageable chunks and, if needed, utilize helper functions. This guarantees peak performance and enhances the readability of the code.
  • Updates in batches using {#await}: Employ the {#await} directive to batch updates when handling several asynchronous processes or API calls. When the asynchronous processes are finished, this avoids needless intermediate renders and boosts the effectiveness of UI updating.
  • Apply the {#key} directive to adaptable lists: To aid SvelteKit in uniquely identifying each item when interacting with dynamic lists, use the {#key} directive. When items are added, removed, or repositioned, this improves the framework's ability to update the DOM quickly.
  • Lazy load components: Reduce the time it takes for the page to load initially by loading components slowly. Optimize the perceived performance of your application by dynamically loading components only when necessary.


In this article, you learned about building reactive UIs with SvelteKit reactive statements, its features, and benefits. You also learned how it works better than traditional methods. To get more information about Sveltekit and working with reactive statements, visit their documentation.


Written by

Chidera Chukwuka Writing Program Member

An ardent and internally propelled developer proficient in blending the art of design with the skill of programming to deliver a top-notch and immersive user experience. I have 4+ years of sophisticated experience working with organizations in the software and engineering sector as a Frontend Engineer. I am keen on the aesthetics, usability, and accessibility of applications.

Readers also enjoyed