Shadowing Practice: React Interview Questions Senior Level (React Fiber, Reconciliation, Virtual DOM) - Learn English Speaking with Video
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So today we will answer some of the hardest senior
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React interview questions our M &T's got in real front -end interviews.
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We are going to deep dive into concepts such as concurrent
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React, React Fiber, the reconciliation process, and the virtual DOM.
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Bogdan is going to answer the interview questions, but also give you the mental models and the reasons why behind his explanations.
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Bogdan, are you ready?
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Yes, let's go.
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And let's go with the first question.
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What is React Fiber and how is it different from the virtual DOM?
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Perfect.
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So I'm going to go ahead and share my screen for this one.
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What is React Fiber and how is it different from the virtual DOM?
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Well, to put this simply, back in the days we had our React elements
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that would create a virtual DOM node and then that will be translated into an actual DOM node with reconciliation.
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But the problem with this is that it's a very synchronous process.
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That means imagine we have a huge tree of elements.
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We need to render all of those.
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I compute the state
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and with JSX compute the virtual DOM node all done synchrously in one shot and then render the DOM.
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And what React introduced in version 16 with React asynchronous is the fiber node, which is an intermediate structure that gets created.
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And basically all the work of rendering component is done by fiber, by this fiber node.
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So just like you had a tree of virtual DOM nodes, now you have a fiber tree.
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Whenever we need to re -render, we work on this tree, we compute the new state, we run certain lifecycle methods.
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And once we are done, we can then compute the virtual DOM from the fiber tree.
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And the big advantage of having this structure
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and the why behind it is that differently from the virtual node and traditional rendering, the fiber reconciliation can be paused.
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So we can actually pause, you know, when we go into the fiber and we have a higher priority update, we could actually pause this,
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do something else, and then come back to this.
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And that was not possible before fiber.
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So that would be the big difference where the virtual DOM, it's only used now to actually compute the differences to the DOM.
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And the fiber, it's a new intermediate structure that allows us to pause
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and prioritize the render process to make it asynchronous or what they call concurrent React.
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Cool, Bogdan.
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This leads me to our second question today, which is, can you explain the reconciliation process in React?
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And also, if you can tell me what does the introduction of React Fiber means for the reconciliation process as well?
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So in the reconciliation, in traditional reconciliation, we had our component.
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It will trigger a state change, and that will trigger a re -render of the component re.
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And re -rendering means we would go from the root node, the root component, all the way to the children,
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all the way to what we call the leaf nodes, the very bottom components, and run the render function recursively.
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And we would put that in the call stack.
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So all these things will be into the call stack, all the way to the bottom.
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Then we would execute them.
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And finally, from that, we would extract the new virtual DOM.
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That's what they call the stack reconciler.
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And this one, it's kind of synchronous in the sense of, imagine we have a huge component tree,
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we would need to put recursively all those render functions in here, finish them, and then finally render.
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When we have a huge component tree, you might have what we call a blocking UI.
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Imagine we're fetching some data and rendering things on the screen.
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If you are completing a form at the same time as a user,
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you would perceive that form as non -responsive because the call stack is busy by rendering these components that are being fetched.
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And so we can't really re -render, for example, the UI.
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The browser cannot repaint, so maybe you're typing in an input field, but you don't see anything happening.
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So that was kind of the old reconciliation process.
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And after that, we had our VDome.
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We would make our diffing algorithm and finally commit to the DOM, the changes needed.
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That was how
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React used to work.
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With Fiber, things are a bit more complex.
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So what React does is from the component tree, it will build what we call a fiber tree.
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And a fiber, it's nothing but, let me zoom in, I think I have a fiber around here.
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It's a JavaScript object.
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And inside there's many properties, but probably the most important are the state and the new props, like the props the component has,
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the memorized props, everything you need about the component, the function that runs it, the siblings and the child are all here.
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And so React will create a fiber tree with this internally.
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And it will run the render function for that fiber tree.
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And from it, it will deduct the virtual DOM.
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And then the process is just like before.
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The virtual DOM gets compared with our diffing algorithm and we commit changes to the DOM.
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So far, nothing changed, but we have this intermediate step.
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Nice.
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Now, what I still don't understand by the minute, right now we have the fiber tree, right?
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As a, in between the virtual DOM and the state changes the plate.
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And I'm wondering...
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Why the fiber tree?
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Why do I need fiber in the first place?
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Because it adds extra complexity.
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And I'm wondering, what does fiber allows me to do that the virtual DOM couldn't?
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You mentioned this asynchronous rendering.
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Could you expand a bit on that?
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Because it feels to me like it's adding a lot of complexity.
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React was already complex.
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The diffing algorithm was already complex.
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And now we have a stage in between that's kind of definitely adding another layer of abstraction, of complexity.
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Is it worth it?
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Do we really need why? and why do we really need this step, this fiber tree?
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Yeah, very good question.
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So what the fiber tree allows us to do is that it works together with a priority queue and a scheduler.
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And rather than having this process synchronous, like we really, as I explained before about the stack reconcilator,
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we would go like have to run the whole render in one shot.
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We can actually pause.
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We can actually stop at a certain node, pause and do something else and then come back to this render.
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The way that's done, and I'm going to try to explain it really quickly, is that whenever an update, let's say, imagine a set state happens,
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an update gets attached to this fiber tree.
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So let's imagine there's a state change here for this form component.
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There is an update queue for this form component and that change that will be queued there.
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And what React will do after that is to copy the current fiber tree in the background and do work on it.
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That means just go through each node and see
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if there's any updates and compute the new memoized state and run the render method.
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But it does all this in the background.
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And so that means while React does all this, right?
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Imagine we have a huge component tree.
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It can actually, it does it asynchronously.
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It doesn't take all the main thread to do this.
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Basically, what React does, it goes to our main thread, to the web browser, and calls this function, which is, I do not have it here,
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but it's called the request idle callback, which basically, it tells the browser, hey, do you have some free time?
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Are you idle?
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And the browser says, yes, I am idle, and I will let you do some work for around 50 milliseconds.
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That's the default in the browser.
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And then React goes and starts doing this work, right?
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Ticker, ticker, ticker it doesn't need to finish.
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It will just do some work.
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When React finished the frame budget, we call it that way, like the browser says, okay,
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you've done enough, but now I need to free the main thread to repaint or re -render the UI, then React will pause everything.
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And this was impossible before.
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You really needed to finish the render before doing anything else.
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One follow -up question that comes to my mind is, how can we manage all this in memory?
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I mean, how can we prevent memory leaks? the whole fiber tree will basically stay somewhere in memory
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and we're kind of adding changes to those
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and how can we make sure nothing nothing goes wrong
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so what react does really well is to reuse a lot of the fiber tree
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so you see when we create this new work in progress
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that we're rendering a lot of these nodes are copied especially
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if react see that they didn't change so
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if imagine one of those fiber uses the use memo hook or the react .memo and the props didn't change, React will not re -render.
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It will just maintain a reference to the previous one.
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So it's very optimized and it tries to use as much as possible of the previous fiber tree.
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Understood.
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So it's kind of prepared for that to be as efficient as possible and not use too many resources.
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You mentioned a high priority update.
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How does that work?
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What do you mean by that?
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What I mean is that imagine we are fetching some data
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and rendering a huge list of elements that would be a low priority update
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because it's nothing that we need to instantaneously show to the user.
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Whereas whenever the user is typing something on an input field
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and we want to show a focus or an outline on that input field or even an error message, we want to react to user input,
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then yes, that would be a much higher priority update.
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So React will pause this low priority render, it will create a new work in progress tree for the high priority update,
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finish that one, commit, like compute the virtual DOM and commit the changes to the DOM and then go back to the low priority update.
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That's why it's called concurrent react or synchronous react
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because you can actually use a priority queue to under the hood prioritize those updates.
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So for the end user, even if we're doing a lot of background work, the UI feels a lot more fluid.
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Here's basically the list of internal priorities that the React developers added to the framework.
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The top priority would be a discrete event, like if they click or they type on an input.
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The second one would be they scroll, and we need to show something on screen because the user scrolled.
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A normal priority would be something like data fetch, and a very low priority would be a route change, for example.
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And all this is internal to the React code base.
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We don't need to worry about it.
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But it's important for us to know that this is how
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React handles now.
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Again, re -renders in updates.
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And so that's quite a bit of complexity, I would say, for the average React developer.
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And I have a follow -up question prepared.
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But first, let's answer the third question that I have prepared here,
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which is, what can you tell me about the diffing algorithm and the virtual DOM?
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So as I mentioned, this render process, it can be paused and asynchronous.
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But whenever we finish this, we have a synchronous phase, which is the commit phase, which is I know what changes I need to do.
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And now I'm going to commit them to push them to the actual DOM.
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So what we do is once we have each fiber tree finished and we have the memory state,
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we can compute the VDOM, the new virtual DOM.
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This didn't change much with previous React we will compare the VDome,
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the new one, with the current feature DOM and try to figure out the differences.
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Now, traditionally, comparing two trees like that would take, I think the big O for this would be N to the third.
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So N by N by N, N being the number of elements.
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So imagine we have a thousand elements.
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It would take a billion operations to actually compare those trees.
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Wow. So, yeah.
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So React has two heuristics, which is it basically assumes that if the type of an element change,
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so if this form is not a form anymore, then it will discard all the children and try to recompute that.
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So because that's a lot cheaper, like re -rending that from scratch, it's a lot cheaper than comparing everything.
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So that's the first one, which is, you know, if different type, then redo.
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And the second one is assume keys are stable.
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I'll get in a second into keys.
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They're stable and unique.
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So whenever we have a list of elements, we react will ask for that key prop.
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And it will use that to actually really fast compare those elements.
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If our keys are not stable, that means they change values all the time.
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We're pushing a lot of new work.
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And if they're not unique, react won't be really able to use them
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because it really tries to identify a leaf tree with the key
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so imagine here we'd have um 15 inputs it would use
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that key to differentiate which ones do i need to render
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which ones i don't have this is why for example
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you should never use array indexes as keys in a real component because they're not stable right exactly because if you
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You would delete an element in position five of the array.
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All the others will shift.
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And all of a sudden, maybe React doesn't render element number six, even if the actual content or the underlying state has changed.
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So this is a trail of heuristic.
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And you need to know this as a developer in order to really avoid having weird issues.
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So, of course, I have a follow -up question prepared for that.
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But I just wanted to take a break.
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And for everyone watching last year, probably watching this video because you are either interviewing right now, and I guess you are a JavaScript engineer,
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whether it's front -end with React or front -end general, full -stack, even back -end developers interviewing right now, then we have a present for you.
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So we prepared a free technical assessment for you to understand what your technical gaps are
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and to also adjust your interviewing process
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and try to fix those gaps for you to be formed in your next technical interview.
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It's available for free in the comments.
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The link is going to be there.
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And you can go to the CNETF website and take it and I wish you the best of luck.
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With that being said, my big question right now, after seeing all this stuff, I mean, React was already,
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you know, the learning curve was quite steep and the framework itself is becoming pretty complex.
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I mean, you spent a few hours to have this kind of thing on that.
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This knowledge is also not readily available online.
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So you have to kind of dig deeper into the source code, the documentation.
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So the million -dollar question is, how much, how deep should the average
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React developer go?
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How much should a mid -level developer know?
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How much should a React engineer know?
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I guess to answer that, it very much depends of what kind of developer you are.
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If you're a senior full -stack developer, a bit more spread out, you cannot really spend so much time in React.
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And so I would just learn maybe the motivation of the React team behind it and a top -level API.
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But if you are a senior
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React developer, what we have seen in the market is that it did became a bit more overcrowded than it used to be.
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And so this level of technical depth, although daunting, it will really help you stand out in an already overcrowded market.
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And the good news I have for everybody else is that the data structures that are being used, like queues and priority heaps and trees,
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they will develop your thinking model and your language.
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And so you don't really lose by going this deep.
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If you're a senior
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React developer, I think you will have to go pretty deep into it because this is your thing.
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You are a senior React developer.
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If you're a senior front -end developer also, but if you're a senior full stack, I would just look at the top -level ideas behind it
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and try to understand the motivation of the React team behind
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it you will not have to interact with the Fiber directly at any point as a developer.
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So there's no need to go too deep into it.
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And if you're a React developer trying to deepen your knowledge about React internal, trying to learn more about Fiber 3, where would you start?
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I would start with the browser APIs.
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There are two main functions that they built concurrent
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React around, which is request idle callback, which is, hey, the browser is free and says, you can give me some work to do.
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I will do it in the background.
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And that's when React starts rendering.
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And I think the other one was a request animation frame, which is the browser saying, I'm going to repaint and re -render the UI.
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Is there anything really quickly that you want me to do before?
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And that's when React commits changes to the DOM, high priority changes.
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So you see them in the next refresh, in the next repaint.
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If you learn those, first of all, you can use them in other frameworks.
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And then you'll realize that everything they built into Fiber was to be able to use these APIs in a concurrent mode.
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And that's why they did a priority queue to it and the whole fiber thing.
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So I would start there.
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Which is why, folks, and for the people watching us, you will see this across the whole Synodave channel and across everything we do,
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but fundamentals are the way to go, right?
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Because as you can see, whatever frameworks are trying to build upon, it's always upon basic browser functionality and JavaScript fundamentals, right?
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We talked about basic data structures.
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We'll talk about the call stack.
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And as you can see, when Barton was actually explaining and going to the Fibre explanations, he did a complexity analysis.
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He was always referring to data structures that are agnostic and to JavaScript characteristics that are built into the language.
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So if you really, funny enough, if you really want to master the advanced features of a framework like
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React, you actually need to master and strengthen your fundamentals first.
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If you're interested into strengthening your fundamentals, then you're in the right place.
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Make sure you subscribe to the channel.
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Let us know in the comments if there's any other advanced react question, senior level question that you want Bogdan or I to cover,
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or if there's anything in this video that you want us to go deeper into.
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And we expect a lot more videos like this coming your way.
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Bogdan, thank you so much, and we will see you folks in the next one.
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Vocabulary and speaking notes for this lesson
This speaking lesson is built on the video “React Interview Questions Senior Level”. The speaker keeps coming back to these words: react, fiber, tree, Dom, render. This video has 253 sentences and 3294 words to shadow. The speech runs for 17:15. The speaker talks fast, about 191 words per minute, so expect linked and reduced sounds. Only 80% of the words are among the 3,000 most common in English, so the vocabulary is demanding.
Key vocabulary in this video
15 words from the video worth learning, with pronunciation and meaning:
| Word | Pronunciation | Meaning |
|---|---|---|
| react verb | /ɹiːˈækt/ | To act in response. |
| fiber noun | /ˈfaɪ.bɚ/ | A single elongated piece of a given material, roughly round in cross-section, often twisted with other fibers to form thread. |
| render verb | /ˈɹɛn.dɚ/ | To cause to become. |
| virtual adjective | /ˈvɜɹt͡ʃuəl/ | In effect or essence, rather than in fact or reality; also, imitated, simulated. |
| priority noun | /pɹaɪˈɒɹɪti/ | An item's relative importance. |
| component noun | /kəmˈpoʊ.nənt/ | A smaller, self-contained part of a larger entity. Often refers to a manufactured object that is part of a larger device. |
| developer noun | /dɪˈvɛləpɚ/ | A person or entity engaged in the creation or improvement of certain classes of products. |
| node noun | /noʊd/ | A knot, knob, protuberance or swelling. |
| element noun | /ˈɛləmənt/ | One of the simplest or essential parts or principles of which anything consists, or upon which the constitution or fundamental powers of anything are based. |
| browser noun | /ˈbɹaʊ.zəɹ/ | One who or which browses. |
| stack noun | /stæk/ | A large pile of hay, grain, straw, or the like, larger at the bottom than the top, sometimes covered with thatch. |
| pause noun | /pɔːz/ | A temporary stop or rest; an intermission of action; interruption; suspension; cessation. |
| reconciliation noun | /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/ | The re-establishment of friendly relations; conciliation, rapprochement. |
| commit verb | /kəˈmɪt/ | To give in trust; to put into charge or keeping; to entrust; to consign; used with to or formerly unto. |
| input noun | /ˈɪnpʊt/ | The act or process of putting in; infusion. |
Phrasal verbs you will hear
| Word | Pronunciation | Meaning |
|---|---|---|
| follow up verb | To take further actions remaining after an event; to continue, revisit, or persist; especially, to maintain communication or verify. | |
| figure out verb | To come to understand; to discover or find a solution; to deduce. | |
| go ahead verb | /ˌɡoʊ əˈhɛd/ | To proceed with; to begin. |
| go back to verb | To stem from; to originate in. | |
| go through verb | To travel from one end of something to the other. | |
| spread out verb | Become further apart. |
Sentences worth repeating
Short, complete lines from the video that you can reuse in everyday conversation:
- Why do I need fiber in the first place?
- Could you expand a bit on that?
- Do we really need why?
- I mean, how can we prevent memory leaks?
- What do you mean by that?
Grammar in this video
The structures the speaker uses most, with the exact words from the video:
| Structure | In the video |
|---|---|
| Conditional sentences if + clause, will/would + verb — a condition and its result | if they're not unique, react won't be |
| “Used to” used to + verb — a past habit or state that is no longer true | used to work · used to be |
| Passive voice be + past participle — the focus is on what happens, not who does it | will be translated · is done · can be paused |
| Relative clauses who / which + clause — extra information about a person or thing | node, which is · today, which is · function, which is |
Pronunciation to watch
The speaker uses 41 contractions and reduced forms, such as you're, I'm, don't. Say them the short way, as you hear them.
- The “th” sounds: algorithm /ˈælɡəɹɪðm̩/, thread /ˈθɹɛd/, strengthen /ˈstɹɛŋ(k)θən/
- The “sh” and “zh” sounds: reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, explanation /ˌɛkspləˈneɪʃən/, motivation /ˈmoʊ.tə.veɪ.ʃən/, efficient /ɪˈfɪʃənt/, animation /ˌæn.ɪˈmeɪ.ʃən/
- Long words — get the stress right: priority /pɹaɪˈɒɹɪti/, developer /dɪˈvɛləpɚ/, reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, complexity /kəmˈplɛk.sɪ.ti/, fundamental /ˌfʌn.dəˈmɛn.təl/
How to practise with this video
- Listen to the whole video once without speaking and note the words you do not know.
- Start at 0.75× speed, shadow it sentence by sentence, then go back to normal speed once it feels easy.
- Record yourself and compare with the original, paying attention to words like react, fiber, render.
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.