Reactivity
38 bites tagged Reactivity: interview questions with model answers, and 60-second explainers.
Test yourself: Top 30 Reactivity interview questions →Multiple choice, with the correct answer and why it is correct on every question. Free, no sign-in.How do Vue, Angular, and Svelte stores trigger component re-renders?
Tests deep knowledge of reactivity primitives. Contrast Vue's proxy subscriptions, Angular's RxJS push through async pipe or markForCheck, and Svelte's compiler-generated store invalidation. Red flag: claiming all three use virtual DOM diffing or polling.
How would you implement a memoized selector for expensive derived state?
It tests cache invalidation and pure functions for reactive derived state. A strong answer covers reference tracking on inputs, returning cached results when unchanged, and requiring immutable or stable arguments.
How do Vue, Angular, and Svelte handle deep prop mutation re-rendering?
Vue and Svelte detect nested mutations via Proxies; Angular default re-renders via Zone.js, but ngOnChanges misses it since the reference is unchanged.
How does Angular's Zone.js change detection differ from Vue and Svelte reactivity?
Tests granular reactivity architecture. Angular Zone.js monkey-patches async to scan the full component tree; Vue uses runtime Proxy tracking; Svelte compiles runes into targeted DOM bindings.
How does Svelte compile count += 1 into DOM updates?
Tests if you know Svelte reactivity is compile-time via runes, not runtime proxies. Good answer: $state marks reactive vars; runes are compiler hooks; compiler emits DOM-sync code. Red flag: describing useState or VDOM.
Compare Angular Zone.js and Vue 3 Proxy reactivity
This tests update granularity. Contrast Vue 3 Proxy tracking, targeting only affected components, with Angular Zone.js patching async APIs to trigger top-down dirty checking across tree. Vue binds deps at render; Angular compares values each cycle.
What is the Virtual DOM's role during a Vue state update?
Tests your grasp of Vue's reactive render pipeline. A strong answer: state change triggers a new virtual DOM tree; runtime diffs it against the old tree to apply only necessary real DOM updates. Red flag: saying virtual DOM updates browser DOM directly.
How do you architect Svelte stores to minimize WebSocket re-renders?
Tests Svelte store granularity and subscription semantics. A great answer splits the monolith into domain stores, uses derived selectors to isolate slices, and leverages readable stores to encapsulate the WebSocket source.
How does Svelte surgically update the DOM without VDOM?
Tests VDOM overhead vs compile-time optimization. A strong answer explains that Svelte compiles reactive dependencies into direct DOM API calls, skipping tree creation and diffing. Red flag: claiming VDOM is slow because the DOM itself is slow.
Vue computed vs method: performance difference and when to choose each?
This tests Vue reactivity caching. Answer: computed caches and reruns only when reactive deps change, while methods run each render. Choose computed for derived data; methods for events/args. Red flag: saying computed is always faster or passing arguments.
How would you design useCounter to support shared or independent state?
Tests your grasp of scope in Vue composables. Great answers contrast module-level refs for global singleton state against function-level refs for per-instance isolation, and explain tradeoffs.
How do you implement a data-fetching Composable in Vue 3?
Write useFetch accepting a URL, exposing data, loading, and error refs, then return them.
Build a reactive data-fetching utility with Svelte stores
Tests store composition for async data fetching with caching and auto re-fetch. Strong answer: readable for lifecycle, derived's set callback for race-safe fetches, writable for private inputs. Red flag: exposed internals or missing cleanup.
How do you structure components and state for a dynamic Vue form?
This tests your design of normalized reactive state for dynamic Vue forms. A great answer uses a flat item array with stable keys, row components or scoped slots, and schema-driven validation. Red flags include mutating by index or using array indices as keys.
Validate matching Vue passwords: computed property or watcher?
Use a computed boolean for caching and dependency tracking; reserve watchers for side effects only.
Bind an input to a variable and validate with reactivity
Tests Svelte reactivity: connecting DOM state to validation logic. Good answer: $state for the variable, $effect to run validation when it changes, cleanup if needed. Red flag: using effects where event handlers suffice or creating infinite loops.
Implement auto-cleanup reactive logic in Vue's Composition API and Svelte
Vue pairs onMounted with onUnmounted or watchEffect cleanup; Svelte uses $store auto-subscription or onDestroy.
How do Svelte Stores work and how do you create custom ones?
Tests Svelte store contract mastery. A strong answer defines the subscribe/unsubscribe interface, wraps writable() with custom set/update logic, subscribes via $ prefix or manually, and uses $store in templates.
Fundamental difference between Vue computed and watcher with examples
Tests derived state vs side effects in Vue reactivity. Computed returns cached derived value; watch executes callback on mutation. Example: computed filters list; watch fetches data when ID changes. Red flag: watch for template state or computed for async.
How do you update Svelte arrays and objects to trigger reactivity?
This tests knowledge of Svelte 5 deep reactivity: $state wraps arrays in proxies so push and property mutations trigger updates. Mention $state first, then note legacy syntax needs reassignment. A red flag is claiming push never works in Svelte.
Vue 3 ref() vs reactive(): differences and when to use each
Tests Vue 3 Composition API reactivity primitives. Strong answer: ref() wraps any value in a .value wrapper and is the recommended default; reactive() returns a proxy accessed directly. Red flag: saying ref() is only for primitives or omitting .value entirely.
Why does todos.push miss Svelte updates but spread works?
Tests compile-time versus runtime reactivity. Answer: Svelte's compiler instruments assignments, not method calls, so push is invisible; Vue Proxies and Angular Zone.js both intercept push to trigger updates.
Explain two-way data binding and provide input binding syntax
Tests whether you see two-way binding as sugar for value binding plus an input event. Strong answer: show one syntax, then explain the prop-out, event-in pattern. Red flag: syntax only with no mention of change detection or unidirectional flow.
How do Vue and Angular detect nested object mutations?
Tests deep reactivity vs reference-based detection. Vue 3 Proxies catch nested mutations; Angular Default checks on zone ticks and OnPush misses same-reference changes. Fix via immutability, deep watchers, or ngDoCheck.
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