Pratica di Shadowing: React Interview Questions Senior Level (React Fiber, Reconciliation, Virtual DOM) - Impara a parlare inglese con i 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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Vocabolario e note di pronuncia per questa lezione
Questa lezione di conversazione si basa sul video “React Interview Questions Senior Level”. Le parole che ritornano più spesso: react, fiber, tree, Dom, render. Questo video contiene 253 frasi e 3294 parole da ripetere con lo shadowing. Il parlato dura 17:15. Chi parla va veloce, circa 191 parole al minuto: aspettati suoni legati e ridotti. Solo il 80% delle parole rientra nelle 3.000 più comuni dell’inglese, quindi il lessico è impegnativo.
Vocaboli chiave di questo video
15 parole del video che vale la pena imparare, con pronuncia e significato:
| Parola | Pronuncia | Significato |
|---|---|---|
| react verbo | /ɹiːˈækt/ | reagire |
| render verbo | /ˈɹɛn.dɚ/ | rendere |
| virtual aggettivo | /ˈvɜɹt͡ʃuəl/ | virtuale |
| priority sostantivo | /pɹaɪˈɒɹɪti/ | priorità |
| component sostantivo | /kəmˈpoʊ.nənt/ | componente |
| developer sostantivo | /dɪˈvɛləpɚ/ | sviluppatore, sviluppatrice |
| element sostantivo | /ˈɛləmənt/ | elemento |
| stack sostantivo | /stæk/ | pila, bica |
| pause sostantivo | /pɔːz/ | pausa |
| reconciliation sostantivo | /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/ | ravvicinamento, riconciliazione |
| commit verbo | /kəˈmɪt/ | affidare, impegnarsi |
| input sostantivo | /ˈɪnpʊt/ | immissione |
| queue sostantivo | /kju/ | fila, coda |
| compare verbo | /kəmˈpɛɚ/ | paragonare |
| framework sostantivo | /ˈfɹeɪm.wɜːk/ | struttura portante |
I phrasal verb che sentirai
| Parola | Pronuncia | Significato |
|---|---|---|
| figure out verbo | scoprire, rendersi conto | |
| go ahead verbo | /ˌɡoʊ əˈhɛd/ | procedere, andare avanti |
| go through verbo | attraversare |
Frasi da ripetere
Frasi brevi e complete del video che puoi riutilizzare nella conversazione di tutti i giorni:
- 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?
La grammatica di questo video
Le strutture che chi parla usa di più, con le parole esatte del video:
| Struttura | Nel video |
|---|---|
| Frasi condizionali if + frase, will/would + verbo — una condizione e il suo risultato | if they're not unique, react won't be |
| “Used to” used to + verbo — un’abitudine o uno stato passato che non è più vero | used to work · used to be |
| Forma passiva be + participio passato — conta ciò che accade, non chi lo fa | will be translated · is done · can be paused |
| Frasi relative who / which + frase — un’informazione in più su una persona o una cosa | node, which is · today, which is · function, which is |
Pronuncia a cui fare attenzione
Chi parla usa 41 contrazioni e forme ridotte, come you're, I'm, don't. Pronunciale nella forma breve, così come le senti.
- I suoni “th”: algorithm /ˈælɡəɹɪðm̩/, thread /ˈθɹɛd/, strengthen /ˈstɹɛŋ(k)θən/
- I suoni “sh” e “zh”: reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, explanation /ˌɛkspləˈneɪʃən/, motivation /ˈmoʊ.tə.veɪ.ʃən/, efficient /ɪˈfɪʃənt/, animation /ˌæn.ɪˈmeɪ.ʃən/
- Parole lunghe — attenzione all’accento: priority /pɹaɪˈɒɹɪti/, developer /dɪˈvɛləpɚ/, reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, complexity /kəmˈplɛk.sɪ.ti/, fundamental /ˌfʌn.dəˈmɛn.təl/
I suoni difficili per chi parla italiano:
- /h/ — va soffiata, non è muta: whereas /(h)wɛɹˈæz/, hook /hʊk/, heuristic /hjuˈɹɪstɪk/
- Consonante finale — senza aggiungere una vocale dopo: react /ɹiːˈækt/, component /kəmˈpoʊ.nənt/, node /noʊd/, element /ˈɛləmənt/, stack /stæk/
- /æ/ — più aperta della “e”: react /ɹiːˈækt/, stack /stæk/, algorithm /ˈælɡəɹɪðm̩/, gap /ɡæp/, whereas /(h)wɛɹˈæz/
Come esercitarsi con questo video
- Ascolta tutto il video una volta senza parlare e annota le parole che non conosci.
- Inizia a velocità 0,75×, fai shadowing frase per frase e torna alla velocità normale quando diventa facile.
- Registrati e confronta con l’originale, facendo attenzione a parole come react, render, virtual.
Cos'è la tecnica dello Shadowing?
Shadowing è una tecnica di apprendimento delle lingue supportata da studi scientifici, originariamente sviluppata per la formazione dei traduttori professionisti e resa popolare dal poliglotta Dr. Alexander Arguelles. Il metodo è semplice ma potente: ascolti un audio in inglese di madrelingua e lo ripeti immediatamente ad alta voce — come un'ombra che segue il parlante con un ritardo di solo 1–2 secondi. A differenza dell'ascolto passivo o degli esercizi di grammatica, lo shadowing costringe il tuo cervello e i muscoli della bocca a elaborare e riprodurre simultaneamente i modelli di discorso reale. La ricerca dimostra che migliora significativamente la precisione della pronuncia, l'intonazione, il ritmo, il discorso connesso, la comprensione dell'ascolto e la fluidità del parlato — rendendolo uno dei metodi più efficaci per la preparazione alla prova di speaking dell'IELTS e per la comunicazione reale in inglese.