SolidJS Interview Questions & Answers for 2026 Interviews
Key Takeaways
- ✓SolidJS uses fine-grained reactivity built on signals, so only the exact DOM nodes that depend on a changed value update — there is no virtual DOM and no component re-render.
- ✓createSignal returns a getter and setter; reading the getter inside an effect or JSX subscribes that computation to the signal.
- ✓Components run once. The function body is setup code, not a render function that re-runs on every state change like React.
- ✓Derived state uses createMemo, side effects use createEffect, and nested reactive objects use createStore with fine-grained updates.
- ✓Control flow uses <Show>, <For>, and <Switch> instead of ternaries and array.map, which keeps updates fine-grained and keyed.
Why SolidJS Comes Up in Frontend Interviews
Signals and Fine-Grained Reactivity
import { createSignal, createEffect } from 'solid-js'
function Counter() {
const [count, setCount] = createSignal(0)
// runs whenever count() changes — the component body does NOT re-run
createEffect(() => console.log('count is', count()))
return (
<button onClick={() => setCount((c) => c + 1)}>
{/* only this text node updates, not the whole component */}
Count: {count()}
</button>
)
}
export default CounterWhy SolidJS Has No Virtual DOM
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import { createStore } from 'solid-js/store'
import { createMemo } from 'solid-js'
function Cart() {
const [cart, setCart] = createStore({ items: [{ name: 'Pen', qty: 2 }] })
// derived, cached value — recomputes only when quantities change
const total = createMemo(() => cart.items.reduce((n, i) => n + i.qty, 0))
// fine-grained update: only subscribers of items[0].qty re-run
const add = () => setCart('items', 0, 'qty', (q) => q + 1)
return <button onClick={add}>Total items: {total()}</button>
}
export default CartControl Flow and Rendering
- •<Show when={cond()} fallback={...}>: conditional rendering without re-running siblings
- •<For each={items()}>: keyed list rendering that only moves or updates changed rows
- •<Index each={items()}>: index-keyed lists for primitive arrays
- •<Switch>/<Match>: multi-branch conditional rendering
- •Avoid raw items().map() in JSX — it loses fine-grained, keyed updates
- •Props are reactive getters: do not destructure them early or you break reactivity
Common SolidJS Interview Mistakes
- •Destructuring props (const { value } = props) — this reads the value once and loses reactivity
- •Calling a signal without parentheses — count is the getter, count() is the value
- •Expecting the component body to re-run on state change like React
- •Using array.map in JSX instead of <For>, losing keyed fine-grained updates
- •Putting side effects in the component body instead of createEffect
- •Overusing signals where a derived createMemo would be clearer and cached
How to Prepare for a SolidJS Interview
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See PlansFrequently Asked Questions
What makes SolidJS different from React?
SolidJS uses fine-grained reactivity with signals instead of a virtual DOM. Components run once and updates flow directly to the exact DOM nodes that depend on a changed value, so there is no re-render-and-diff cycle and no need for memoization helpers.
What is a signal in SolidJS?
A signal is the core reactive primitive. createSignal(initial) returns a getter and a setter. Reading the getter inside a tracking scope subscribes that scope to the signal, so calling the setter re-runs only the subscribed computations, not the component.
Why does SolidJS not use a virtual DOM?
Because reactivity is fine-grained, Solid knows at compile time which DOM nodes depend on which signals and updates them directly. A virtual DOM exists to diff what changed on each render, but Solid never re-renders, so there is nothing to diff.
Why shouldn't you destructure props in SolidJS?
Props are reactive getters. Destructuring them (const { value } = props) reads the value once at setup time and breaks reactivity, so the UI stops updating. Access props as props.value, or use splitProps/mergeProps when you need to separate them.
What is the difference between createMemo and createEffect?
createMemo returns a cached derived value that recomputes only when its dependencies change and is meant for computing data. createEffect runs side effects (logging, DOM, subscriptions) whenever its dependencies change and does not return a value.
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