The Key Prop | Lecture 128 | React.JS 🔥
Summary
TLDRThis video script offers an in-depth explanation of the key prop in React, a crucial concept for optimizing performance and managing state. It begins by introducing the key prop and its role in the diffing algorithm, enabling React to uniquely identify elements. The script then delves into the two main use cases: using stable keys in lists to preserve DOM elements across renders, and changing keys to reset component state when necessary. With clear examples and a logical progression, the script aims to provide viewers with a comprehensive understanding of when and why to use the key prop, reinforcing best practices for efficient React development.
Takeaways
- 🔑 The `key` prop is a special prop in React that allows the diffing algorithm to uniquely identify elements, both DOM elements and React elements.
- ✅ Using a stable `key` (one that stays the same across renders) for an element allows React to keep that element in the DOM even if its position in the tree changes, resulting in better performance.
- ♻️ When the `key` of an element changes between renders, React will destroy the old element and create a new one in its place, even if the position in the tree is the same, allowing for state reset.
- 📝 Use the `key` prop when rendering lists of elements of the same type, otherwise React will complain and give a warning.
- ⚡ Using `key` properly can greatly improve performance, especially when dealing with large lists with thousands of elements.
- 🔄 To reset the state of a component instance, change the `key` prop of that instance, forcing React to create a new instance with a fresh state.
- 🧠 React cannot inherently determine if an element is the same across renders, so using `key` provides that information to React.
- 🚫 Without using `key` in lists, React will remove and recreate DOM elements unnecessarily, leading to wasted work and decreased performance.
- 🔑 The two main use cases for the `key` prop are: 1) Using stable keys in lists, and 2) Changing keys to reset component instance state.
- 📖 The script provides a clear explanation of the `key` prop and its importance, along with practical examples and use cases.
Q & A
What is the purpose of the key prop in React?
-The key prop is a special prop that we can use to uniquely identify elements in React. It helps the diffing algorithm distinguish between multiple instances of the same component type, allowing React to keep track of which elements have been modified, added, or removed.
Why is it important to use keys when rendering lists in React?
-When rendering lists of components, it's crucial to provide a unique key for each item. This allows React to efficiently update the list when items are added, removed, or reordered. Without keys, React would have to re-render the entire list, which can lead to performance issues, especially for large lists.
What happens when an element has a stable key across renders?
-If an element has a stable key (a key that stays the same across renders), React will keep that element in the DOM, even if its position in the tree has changed. This behavior improves performance by avoiding unnecessary re-renders or recreations of the same DOM element.
What is the purpose of changing an element's key across renders?
-Changing an element's key across renders tells React to treat it as a different instance, even if the element's position in the tree is the same. This is useful when you need to reset the state of a component instance, as React will create a new DOM element and discard the previous state.
How does the key prop affect state preservation in React components?
-If an element's key is unchanged and its position in the tree is the same, React will preserve its DOM element and state across renders. However, if the key changes, React will create a new DOM element and reset the component's state.
Can you provide an example of when changing an element's key would be useful?
-One scenario where changing an element's key would be useful is when you have a component that displays a question, and the question changes. By changing the key, you can force React to create a new component instance with a fresh state, ensuring that the old answer doesn't persist and confuse the user.
What happens if you don't provide keys when rendering a list of components in React?
-If you don't provide keys when rendering a list of components, React will issue a warning in the console. Additionally, when items are added, removed, or reordered in the list, React will have to re-render the entire list, which can lead to performance issues, especially for large lists.
Is it necessary to use keys for all React components?
-No, it's not necessary to use keys for all React components. Keys are primarily used when rendering lists of components or when you need to reset the state of a component instance based on changes in props or other conditions.
Can keys be duplicated within the same list of components?
-No, keys must be unique within the same list of components. Duplicating keys can lead to unexpected behavior and potential bugs in your React application.
What data type can be used for keys in React?
-Keys in React can be strings or numbers, but it's recommended to use strings for keys when possible, as they provide a more reliable way of uniquely identifying components.
Outlines
🔑 The Key Prop: Enhancing Performance and Uniqueness
This section explains the importance of the key prop in React and how it aids the diffing algorithm in identifying unique elements. It discusses how keys allow React to efficiently update the DOM by preserving stable elements and their state across renders, even when their positions change. Using keys in lists is crucial for performance, especially with large lists. The key prop also enables resetting component state by creating new instances when keys change across renders.
🔄 Resetting State with Dynamic Keys
This paragraph explores the use case of resetting component state by changing the key prop. It explains how, when the key changes for an element at the same position in the tree, React creates a new DOM element and resets its state. This behavior is useful when the component's props change significantly, making the existing state irrelevant. Dynamically changing the key allows for a clean slate and logical state management in such scenarios, which is essential for maintaining the application's integrity.
Mindmap
Keywords
💡Key Prop
💡Diffing Algorithm
💡Stable Key
💡Changing Key
💡Lists
💡State
💡Diffing Rules
💡Performance Optimization
💡Component Instance
💡DOM (Document Object Model)
Highlights
The key prop is a special prop that we can use to tell the diffing algorithm that a certain element is unique, and this works for both DOM elements and React elements.
Whenever an element has a stable key that stays the same across renders, the element will be kept in the DOM, even if the position in the tree has changed.
When the key of a certain element changes between renders, the element will be destroyed and a new one will be created in its place, even if the position in the tree is exactly the same as before.
Using keys in lists is a big use case of the key prop, as it allows React to efficiently update the DOM by identifying and keeping the same elements when their positions change.
Without keys, React will remove and recreate DOM elements when their positions change, even if they are the same elements, which is inefficient.
Keys allow developers to uniquely identify elements, providing React with information it doesn't have on its own to optimize updates.
Always use the key prop when you have multiple child elements of the same type, as React will otherwise complain and give a warning.
The second use case for the key prop is to reset state in component instances by changing the key, which forces React to create a new DOM element and reset the state.
If the same element is at the same position in the tree, and only the props change, React will preserve the state, which may not be desired in some cases.
Changing the key tells React that it should be a different component instance, creating a new DOM element and resetting the state.
Resetting state by changing the key is necessary when you need to reset the state of a component instance to a logical state based on new props or data.
Small lists may not show a noticeable performance difference with keys, but large lists with thousands of elements can benefit greatly from using keys.
React has no way of knowing that DOM elements are actually the same when their positions change, so keys provide this information to optimize updates.
Removing and rebuilding the same DOM element is wasted work, which keys help avoid by identifying stable elements across renders.
The transcript provides a detailed explanation of the key prop, its two main use cases, and the importance of using it correctly in React applications.
Transcripts
Remember how I mentioned the key prop
when we talked about how the diffing algorithm works.
And so, let's not take some time
to look at the key prop in detail
in order to learn what it does and when we need to use it.
So, the key prop is a special prop
that we can use to tell the differing algorithm
that a certain element is unique.
And this works for both DOM elements and React elements.
So in practice, this means that we can give
each component instance a unique identification,
which will allow React to distinguish
between multiple instances of the same component type.
Now that's all great, but why do we actually need this?
Well, remember that the second assumption
of the diffing algorithm
is that whenever an element has a stable key,
so a key that stays the same across renders,
the element will be kept in the DOM,
even if the position in the tree has changed.
And this is the whole reason why we should use the key prop
in lists as we have already done
so many times throughout the course.
And so, in the next slide,
you will finally learn why we need to do that.
On the other hand, when the key
of a certain element changes between renders,
the element will be destroyed and a new one will be created
in its place, even if the elements positioned
in the tree is exactly the same as before.
And so this is great to reset state,
which is the second big use case of the key prop.
But let's go back to the first big use case of the key prop,
which is to use keys in lists.
And let's start by considering this example without keys.
So here, we have a list with two question items,
which clearly have no key prop
but let's see what happens when we add a new item
to the top of the list.
Well, the two list items that we already had are clearly
still the same, but they will now appear
at different positions in the React Elementary.
They're no longer the first and second children
but now they are the second and the third children.
So, we basically have the same elements
but at different positions in the tree.
And so according to the differing rules
that we learned earlier, these two DOM elements
will be removed from the DOM and then immediately recreated
at their new positions.
And this is obviously bet for performance because removing
and rebuilding the same dumb element is just wasted work,
right? But the thing is that React
doesn't know that this is wasted work.
Of course, we developers intuitively know
that these two elements are actually the same as before
but React has no way of knowing that.
But what if we could actually change that?
Well, that's where keys come into play because remember,
a key allows us developers to uniquely identify an element
so we can give React that information
that it doesn't have on its own.
And so now when we add a new item to the top of the list,
the two original elements are of course,
still in different positions of the tree
but they do have a stable key.
So, a key that stays the same across renders.
So that's q1 and q2 in this case.
And so according to the differing rules,
these two elements will now be kept in the dump
even though their position in the tree is different.
So, they will not be destroyed.
Entry result will be a bit more of a performant UI.
Now of course, you won't really notice this difference
on small lists, but it will make a huge difference
when you have a really big list with thousands of elements,
which can actually happen in some applications.
So in summary, always use the key prop
when you have multiple child elements of the same type.
So just like the question elements in this example
and you already knew that you should do that because well,
otherwise, React will complain and give us a warning
but now, you hopefully understand exactly
why you need to do it.
Alright, so we looked at the use case for a stable key.
And so now let's look at the use case for a changing key,
which is used to reset state in component instances.
Now here, we don't need a big code example
because we will do this in practice in the next lecture.
But let me just show you what I mean by resetting state.
So, let's say we have this question, inside question box
and we pass in this object as a prop.
Now the question component instance has an answer state,
which right now is set to React allows us
to build apps faster.
But now, let's imagine that the question changes
to this one.
So, we still have the same element at the same position
in the tree.
All that changed was the question prop.
So, what do you think will happen to the state in this case?
Well, let's remember one of the diffing rules, which says
that if we have the same element at the same position
in the tree, the DOM element and its state will be kept.
Therefore, what's gonna happen is that the state of question
will be preserved.
So, it will still show the answer
that was in the component state before.
But that answer is
of course completely irrelevant to this new question, right?
So, it doesn't make any sense to keep this state
around here.
So basically, what we need is a way to reset this state.
And as you can guess,
this is where the key prop comes into play once again.
So now, we have a key of q23
in this first question, which allows React
to uniquely identify this component instance.
Then when a new question appears,
we can give it a different key.
And so by doing this, we tell React
that this should be a different component instance
and therefore, it should create a brand new DOM element.
And the result of doing this
is that the state will be reset,
which is exactly what we need in the situation
in order to make this small app work in a logical way.
So, whenever you find yourself
in a position where you need to reset state,
just make sure that you give the element a key
and that the key changes across renderers.
Now, this actually is necessary very often
but you will sometimes find yourself in this situation.
And so when this happens,
it's very important to know that this is the solution.
Okay.
And to make this even more clear now,
let's go back to our small project.
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