Fundamentals of Reactivity in Vue.js 3

Anton Ioffe - December 25th 2023 - 10 minutes read

Welcome to the inner sanctum of Vue.js 3, where the pulse of reactivity not only shapes app behavior but represents a paradigm shift in JavaScript UI development. In the following deep dive, we'll unravel the mechanisms that define Vue's cutting-edge reactivity—from the cunning use of JavaScript Proxies to the abstract dance of reactive APIs. We'll dissect composables and their orchestration of reusable reactivity, navigate through the treacherous yet revealing edge cases, and finally pull back the curtain on performance tuning and debugging in the reactive landscape. This expedition is for those with seasoned keystrokes and architect minds, ready to master and harness the full potential of reactivity in Vue.js 3.

Proxies and the Heart of Vue 3 Reactivity

JavaScript Proxy objects are foundational to the reactivity system in Vue.js 3, marking a departure from the framework’s earlier version that relied on the Object.defineProperty method. Proxies wrap around objects to provide custom behavior when properties are accessed or modified, serving as a sophisticated interception mechanism. When a component's template or a computed property uses a certain data property, the Proxy's "get" handler registers the consuming entity as a dependent. Upon subsequent changes to that property, captured through the Proxy's "set" handler, Vue swiftly notifies dependents, triggering a re-render to reflect the new state.

The use of Proxy objects allows Vue 3 to perform fine-grained reactivity tracking. Unlike the previous system, which could potentially become verbose when defining every property’s getter and setter, Proxies are less invasive and more dynamic. They allow Vue to capture all operations on an object’s properties, including property addition and deletion, array index setting, and even direct property access, which goes well beyond the scope of what Object.defineProperty can intercept. This plays a significant role in allowing Vue to automatically update the DOM in response to the myriad ways in which data might change.

The reactivity system’s reliance on Proxies confers several advantages in terms of performance and memory use. Proxies enable Vue to track property access during rendering automatically and avoid tracking unused data properties that do not contribute to the view. This tailored tracking prevents unnecessary memory bloat and ensures that only the components that rely on specific reactive data are updated, contributing to Vue’s reputation for performant applications.

However, the shift to Proxies also introduces certain complexities. It requires developers to understand the nuances of how Proxies work, particularly since the Proxy is not identical to the original object it wraps. This can lead to subtle bugs if developers assume object identity or attempt to use non-reactive APIs that bypass Proxy handlers. Moreover, Proxy-based reactivity is ECMAScript 6 specific, and therefore, it lacks support in older browsers, such as Internet Explorer.

Finally, it's noteworthy that the Proxy-based system, while powerful, must be used with consideration for reactive updates. Vue will react to any and all changes made to observed data, meaning developers must be mindful of the side effects their mutations may cause. Starkly, this design requires a disciplined approach to state management to avoid unnecessary re-renders and maintain optimal application performance. The Proxy-based reactivity presents a versatile yet intricate model, challenging developers to embrace its possibilities while respecting its boundaries.

The Reactivity APIs: reactive, ref, computed, and watch

Vue 3’s reactivity system hinges on a set of APIs—reactive, ref, computed, and watch—that collectively enable developers to craft reactive data structures and react to changes in state. The reactive function allows the creation of a reactive object, deeply converting each of its properties. Practically, it is often used when dealing with objects with nested properties or arrays. Here’s a brief glimpse into its implementation:

const state = reactive({ count: 0 });

The count property is automatically tracked, and any mutations will prompt the dependent views to update. Despite its deep reactivity strength, reactive can introduce overhead, especially with large nested objects, and may lead to overutilization if not managed judiciously.

Ref is generally preferred for primitive values, like strings or numbers, which are not reactive when used directly in JavaScript. The ref function wraps these values in an object with a .value property to make them reactive:

const count = ref(0);

Its simplicity is appealing for single reactive values within a component's state and provides a streamlined interface for reactivity but necessitates accessing the .value to interact with the underlying primitive.

Computed properties are integral in deriving reactive state and optimizing performance through lazily evaluated, cached dependencies. Created with a computed function, these properties only recalculate when dependencies change, preventing unnecessary computations and safeguarding against performance bottlenecks:

const firstName = ref('John');
const lastName = ref('Doe');
const fullName = computed(() => `${firstName.value} ${lastName.value}`);

This approach yields a readable, declarative pattern in coding style, and in execution smartly minimizes the workload during updates.

For side effects and deeper reactive interactions, Vue 3 offers both watch and watchEffect. While watch requires explicitly specifying the reactive sources to observe, watchEffect operates on any reactive dependencies it encounters during its execution. Here is an example using watch within the Composition API context, involving the requisite imports and a reactive state interaction:

import { reactive, watch } from 'vue';

const state = reactive({ firstName: 'John', lastName: 'Doe' });

watch(() => state.firstName, (newValue, oldValue) => {
    console.log(`Name changed from ${oldValue} to ${newValue}.`);

Alternatively, watchEffect allows you to perform side effects without the need to manually track reactive dependencies:

import { ref, watchEffect } from 'vue';

const firstName = ref('John');

watchEffect(() => {
    console.log(`Name is now: ${firstName.value}.`);

However, developers must be prudent with watch and watchEffect, as mismanagement can lead to frequently called expensive operations—a common performance pitfall.

Given these tools, thoughtful architecture decisions become imperative. Reactive and ref provide flexible ways to establish reactivity, computed leverages performance by limiting needless computations, and watch and watchEffect enable response to changes reactively. Each requires a nuanced approach to be utilized effectively. Striking the right balance involves understanding the costs and benefits, aiming for clarity and efficiency while avoiding overuse that can degrade performance and readability.

Deep Dive into Composables and Standalone Reactivity

Composable functions in Vue 3 serve a pivotal role in encapsulating and managing reactive logic, fostering a maintainable and modular codebase. When constructing composables, we isolate reactive states and associated functions into discrete, reusable units. These units can be utilized across various parts of an application, thereby maximizing code reusability. To ensure reactivity remains intact, avoid destructuring the reactive object directly, as this can disrupt its reactive capabilities—a common error where developers lose reactivity by destructuring properties directly from the reactive system.

To circumvent such pitfalls, it's essential to structure composables so they return their reactive state unaltered. A robust pattern for this is by returning the full reactive object to maintain its reactivity, as shown in the following example where a composable provides the user session state as a whole rather than piecemeal:

import { reactive } from 'vue';

function useSessionState() {
    const state = reactive({ user: null, isLoggedIn: false });
    // Encapsulated reactive logic and functionalities related to the session state here

    // Returning the entire reactive object ensures reactivity conservation
    return state;

Documenting the composable’s API clearly and consistently is crucial for delineating the state and functions provided, thus reducing incorrect usage while enhancing collaboration. Furthermore, by adhering to the Single Responsibility Principle—developing narrowly focused composables for specific functionalities—we can better maintain modularity and scale seamlessly. This approach simplifies the composition of these discrete logic pieces within components:

function useUserProfile() {
    const userProfile = reactive({ name: '', bio: '' });
    // Logic to handle user profile changes...

    return {
        updateProfile(name, bio) {
            // Encapsulated logic to update userProfile
   = name;
   = bio;

function useUserPosts() {
    const userPosts = reactive({ posts: [] });
    // Logic to manage user posts...

    return {
        addPost(post) {
            // Encapsulated logic to add a new post

The distinction between Single Responsibility Principle adherence and side-effect management should be seamless. Notably, the encapsulation of side effects within composables is intrinsic to this principle—ensuring effects pertinent to a composable's purpose are self-contained, thereby promoting testability and reliability:

import { reactive, onMounted, onUnmounted } from 'vue';

function useDataFetcher(apiUrl) {
    const data = reactive({ items: [] });
    let stopPolling = null;

    function startFetching() {
        stopPolling = setInterval(() => {
            // Encapsulated logic for data fetching...
        }, 1000);

    function stopFetching() {
        stopPolling = null;

    // Lifecycle hooks are incorporated to manage side effects

    return {

export default {
    setup() {
        const { data, startFetching, stopFetching } = useDataFetcher('some-api-url');


        return { data };

By meticulously applying these principles and practices to your composables, you craft reactive constructs that are robust, maintainable, and holistic within the Vue 3 ecosystem.

Reactivity Edge Cases and Common Mistakes

Vue 3's reactivity system greatly simplifies building dynamic user interfaces, but it isn't without its edge cases. One common oversight occurs with arrays. While Vue can react to array mutation methods like push, pop, and splice, direct assignment using an index does not trigger reactivity.

const array = reactive(['Vue 3', 'Reactivity']);
// This won't be reactive
array[1] = 'Edge Case';
// Use array methods for reactivity
array.splice(1, 1, 'Edge Case Corrected');

Similarly, when working with collections like Map or Set, developers should remember that Vue can't detect value changes using methods like .set() or .add() unless they result in size changes. The reactive wrapper by itself doesn't upgrade these collections for deep reactive changes.

Complex data structures sometimes include nested properties. A common mistake is destructuring such properties, which breaks reactivity. Vue provides toRef and toRefs to maintain reactivity when we need references to nested properties.

const state = reactive({ nested: { count: 0 } });
// Incorrect: Destructures and loses reactivity
const { count } = state.nested;

// Correct way: `count` maintains reactivity
const count = toRef(state.nested, 'count');

Moreover, understanding when to use toRef over toRefs is vital. While toRef creates a ref for a single property, toRefs converts an object into a flat structure of refs which is handy when you need to maintain reactivity over multiple properties after spreading or passing them around.

const state = reactive({ one: 1, two: 2 });
// Incorrect: `one` and `two` lose reactivity
const { one, two } = state;

// Correct: Both `one` and `two` maintain reactivity
const { one, two } = toRefs(state);

Lastly, another pitfall is forgetting that reactive makes a deep copy, so if you try to compare the original and reactive object or any of their nested references, the equality check will fail.

const original = { nested: { count: 0 } };
const state = reactive(original);
// Incorrect: This will always return false
console.log(state === original);

// Correct: Use refs or Vue's built-in comparison methods
console.log(toRef(state.nested) === original.nested);

Vue's reactivity system is an elegant solution with nuances that require attention. Developers must strategically use the appropriate helper functions and maintain a solid understanding of the subtleties involved to avoid common pitfalls and ensure that they are making the most of Vue's reactivity capabilities. Have you encountered any reactivity edge cases in your experience, and how did you tackle them?

Reactivity Under the Hood: Debugging and Performance Optimization

Delving into the internals of Vue's reactivity system, one can appreciate the elegance of how Vue updates the DOM in response to data changes. By leveraging the reactive system's track and trigger functionalities, developers have the means to not only observe changes but also implement strategies that minimize performance impact.

When using onRenderTracked and onRenderTriggered, you're essentially hooking into Vue's reactivity events. The following advanced example shows how to capture these events and analyze which dependencies are affecting performance:

import { onRenderTracked, onRenderTriggered } from 'vue';

export default {
    setup() {
        onRenderTracked((debugInfo) => {
            console.log('Tracking operation:', debugInfo);
            // Is this dependency being tracked more than necessary?

        onRenderTriggered((debugInfo) => {
            console.log('Update triggered:', debugInfo);
            // What caused this update? Could it have been batched with other updates?

        // Further reactive state and logic setup

The above hooks can provide valuable feedback on what part of the state is being used in rendering and what triggered a given update – information crucial in optimizing your reactivity patterns. Have you ever encountered a situation wherein your component updated unexpectedly or more often than you would anticipate? Exploring these hooks' outputs can shed light on such scenarios.

Regarding batched updates, Vue's intelligent reactivity queue ensures that synchronous code only leads to one update, but when dealing with asynchronous operations, the nextTick function becomes vital. Let's consider how we might optimize an operation that touches multiple reactive sources:

import { ref, nextTick } from 'vue';

export default {
    setup() {
        const firstName = ref('John');
        const lastName = ref('Doe');

        const updateFullName = async () => {
            firstName.value = 'Jane';
            lastName.value = 'Smith';

            await nextTick();
            // Any other operations here are guaranteed to occur after the DOM has been updated
            console.log('Full name updated in DOM:', `${firstName.value} ${lastName.value}`);

        return {

In this example, nextTick is used after the main reactive updates, ensuring we're working with the most current DOM – essential for chained updates or operations depending on the post-update state.

Resource cleanup in components is as critical as ever in single-page applications where component instances can be mounted and unmounted frequently. Consider how you manage reactive listeners or subscriptions. Are there resources or connections that can be overlooked, potentially leading to memory leaks?

import { reactive, onUnmounted } from 'vue';

export default {
    setup() {
        const state = reactive({ itemId: 1, itemDetails: null });

        // Assume fetchData is a method that fetches data based on the itemId
        const fetchData = () => {
            // Logic to fetch and update itemDetails based on itemId...

        onUnmounted(() => {
            // Here, you ensure that all the reactive details or fetches are properly cancelled or disregarded.
            // How might memory leaks manifest if such cleanups are omitted?

        return { state, fetchData };

Expansion of one's understanding of Vue's reactivity mechanics promotes more performant applications and better user experiences. As a Vue developer, how often do you consider the impact of your reactivity patterns on application performance? The journey towards reactivity mastery involves continuous learning and applying incisive performance optimizations, culminating in the zenith of seamless user interaction.


In this article, the author explores the fundamentals of reactivity in Vue.js 3, highlighting the use of Proxies in the reactivity system and the role of reactive APIs such as reactive, ref, computed, and watch. They emphasize the importance of understanding the nuances and edge cases of reactivity, including array mutations, nested properties, and working with collections like Map or Set. The article also discusses the benefits of using composables to encapsulate and manage reactive logic. The author concludes by delving into debugging and performance optimization techniques, showcasing the use of onRenderTracked and onRenderTriggered hooks. Overall, the article provides a comprehensive overview of reactivity in Vue.js 3, challenging developers to think critically about reactivity patterns and to optimize their applications for performance and user experience. A challenging technical task related to this topic would be to refactor an existing Vue.js 3 application to make use of composables and optimize its reactivity patterns, using debugging techniques to identify and resolve any performance bottlenecks.

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