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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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Từ vựng và ghi chú luyện nói cho bài học này
Bài luyện nói này dựa trên video “React Interview Questions Senior Level”. Những từ được nhắc lại nhiều nhất trong bài: react, fiber, tree, Dom, render. Video này có 253 câu và 3294 từ để luyện shadowing. Phần lời nói dài 17:15. Người nói nói nhanh, khoảng 191 từ mỗi phút, nên sẽ có nhiều chỗ nối âm và nuốt âm. Chỉ 80% số từ nằm trong 3.000 từ tiếng Anh thông dụng nhất, nên từ vựng khá khó.
Từ vựng quan trọng trong video
15 từ đáng học trong video, kèm phiên âm và nghĩa:
| Từ | Phiên âm | Nghĩa |
|---|---|---|
| priority danh từ | /pɹaɪˈɒɹɪti/ | sự ưu tiên |
| component danh từ | /kəmˈpoʊ.nənt/ | bộ phận |
| developer danh từ | /dɪˈvɛləpɚ/ | nhà phát triển |
| node danh từ | /noʊd/ | gióng |
| element danh từ | /ˈɛləmənt/ | yếu tố |
| stack danh từ | /stæk/ | chồng |
| reconciliation danh từ | /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/ | hoà giải |
| queue danh từ | /kju/ | xếp nối đuôi |
| compare động từ | /kəmˈpɛɚ/ | so sánh, so |
| stable danh từ | /ˈsteɪ.bəl/ | chuồng, chuồng ngựa |
| algorithm danh từ | /ˈælɡəɹɪðm̩/ | thuật toán |
| fetch động từ | /fɛt͡ʃ/ | tìm |
| thread danh từ | /ˈθɹɛd/ | chỉ |
| trigger danh từ | /ˈtɹɪɡɚ/ | cò |
| leaf danh từ | /lif/ | lá |
Những câu đáng luyện lại
Các câu ngắn, trọn ý trong video mà bạn có thể dùng lại khi giao tiếp hằng ngày:
- 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?
Ngữ pháp trong video
Những cấu trúc người nói dùng nhiều nhất, kèm đúng cụm từ trong video:
| Cấu trúc | Trong video |
|---|---|
| Câu điều kiện if + mệnh đề, will/would + động từ — điều kiện và kết quả | if they're not unique, react won't be |
| “Used to” used to + động từ — thói quen hoặc tình trạng trong quá khứ nay không còn | used to work · used to be |
| Câu bị động be + quá khứ phân từ — nhấn vào việc xảy ra, không phải người làm | will be translated · is done · can be paused |
| Mệnh đề quan hệ who / which + mệnh đề — thêm thông tin về người hoặc vật | node, which is · today, which is · function, which is |
Phát âm cần chú ý
Người nói dùng 41 dạng rút gọn, ví dụ you're, I'm, don't. Hãy nói theo dạng ngắn đúng như bạn nghe.
- Âm “th”: algorithm /ˈælɡəɹɪðm̩/, thread /ˈθɹɛd/, strengthen /ˈstɹɛŋ(k)θən/
- Âm “sh” và “zh”: reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, explanation /ˌɛkspləˈneɪʃən/, motivation /ˈmoʊ.tə.veɪ.ʃən/, efficient /ɪˈfɪʃənt/, animation /ˌæn.ɪˈmeɪ.ʃən/
- Từ dài — đặt trọng âm cho đúng: priority /pɹaɪˈɒɹɪti/, developer /dɪˈvɛləpɚ/, reconciliation /ˌɹɛkənˌsɪliˈeɪʃ(ə)n/, complexity /kəmˈplɛk.sɪ.ti/, fundamental /ˌfʌn.dəˈmɛn.təl/
Những âm người Việt thường gặp khó:
- Cụm phụ âm cuối — đọc đủ từng âm, đừng nuốt âm cuối: react /ɹiːˈækt/, component /kəmˈpoʊ.nənt/, element /ˈɛləmənt/, assessment /əˈsɛsmənt/, efficient /ɪˈfɪʃənt/
- Âm /tʃ/, /dʒ/ và /ʒ/ — tiếng Việt không có, đừng đọc thành “ch” hay “gi”: virtual /ˈvɜɹt͡ʃuəl/, fetch /fɛt͡ʃ/, scratch /skɹæt͡ʃ/, adjust /əˈd͡ʒʌst/, attach /əˈtæt͡ʃ/
- Âm /æ/ — mở miệng rộng hơn âm “e”: react /ɹiːˈækt/, stack /stæk/, algorithm /ˈælɡəɹɪðm̩/, gap /ɡæp/, whereas /(h)wɛɹˈæz/
Cách luyện với video này
- Nghe hết video một lần, chưa cần nói, và ghi lại những từ bạn chưa biết.
- Bắt đầu ở tốc độ 0,75×, nói đuổi từng câu, rồi quay lại tốc độ bình thường khi đã quen.
- Ghi âm giọng mình rồi so với bản gốc, chú ý các từ như priority, component, developer.
Phương Pháp Shadowing Là Gì?
Shadowing là kỹ thuật học ngôn ngữ có cơ sở khoa học, ban đầu được phát triển cho chương trình đào tạo phiên dịch viên chuyên nghiệp và được phổ biến rộng rãi bởi nhà đa ngôn ngữ học Dr. Alexander Arguelles. Nguyên lý cốt lõi đơn giản nhưng cực kỳ hiệu quả: bạn nghe tiếng Anh của người bản xứ và lặp lại to ngay lập tức — như một "cái bóng" (shadow) đuổi theo người nói với độ trễ chỉ 1–2 giây. Khác với luyện ngữ pháp hay học từ vựng bị động, Shadowing buộc não bộ và cơ miệng phải đồng thời xử lý và tái tạo ngôn ngữ thực tế. Các nghiên cứu khoa học xác nhận phương pháp này cải thiện đáng kể phát âm, ngữ điệu, nhịp điệu, nối âm, kỹ năng nghe và độ lưu loát khi nói — đặc biệt hiệu quả cho người luyện IELTS Speaking và muốn giao tiếp tiếng Anh tự nhiên như người bản ngữ.