React Keys and Lists - Complete Tutorial!
Summary
TLDRIn this React keys and lists tutorial, the presenter demonstrates the importance of using unique keys when rendering lists in React. By comparing the performance impact of using indices versus unique identifiers, the video illustrates how keys help React efficiently update the DOM. The tutorial emphasizes the potential for bugs if non-unique keys are used and encourages viewers to understand and implement best practices for keys in their React applications.
Takeaways
- 🔑 **Unique Identification**: Keys in React are used to uniquely identify each element in a list to ensure efficient updates.
- 🚫 **No Keys Issue**: Omitting keys can cause React to re-render all list items unnecessarily, leading to performance issues.
- 🔍 **Highlighting Effect**: When keys are not provided, React highlights every element in the list upon changes, indicating inefficiency.
- 🛠️ **Efficiency with Keys**: With proper keys, only the changed element is highlighted, demonstrating React's efficient update process.
- ⚠️ **Key Prop Warning**: React warns when keys are missing from list elements, emphasizing the importance of providing them.
- 🔄 **Index as Key**: Using the index as a key is not recommended because it can lead to incorrect updates when list items are reordered or deleted.
- 🆔 **Unique Identifiers**: It's crucial to use a unique identifier, like an ID from a database, as a key to avoid bugs and ensure correct updates.
- 🔍 **React's Perspective**: React treats elements with the same key as the same item, which can lead to bugs if not managed correctly.
- 💡 **Choosing a Good Key**: A good key should be unique and stable, not dependent on the item's position in the array or list.
- 📚 **Learning and Experimentation**: The tutorial encourages learning through experimentation with keys to understand their impact on React's rendering performance.
Q & A
What is the main topic of the video script?
-The main topic of the video script is an in-depth tutorial on React keys and lists, explaining the importance and functionality of keys in rendering lists efficiently in React applications.
Why are keys important in React when rendering lists?
-Keys are important in React for rendering lists because they help React identify which items have changed, are added, or are removed. This allows React to make efficient updates to the DOM, improving performance.
What happens if keys are not provided when rendering a list in React?
-If keys are not provided, React will use the index of the array as the key by default. This can lead to performance issues because React will not be able to efficiently update the DOM, as it will consider each item as a new element whenever the list changes.
What is the recommended way to assign keys in a list in React?
-The recommended way to assign keys in a list in React is to use a unique identifier for each element, such as an ID from a database or any other unique property that persists across renders.
Why shouldn't the index of an array be used as a key in React?
-Using the index of an array as a key in React is not recommended because when an item is added or removed from the array, the indices of the remaining items change. This causes React to re-render all items in the list, even if they haven't changed, leading to unnecessary performance overhead.
What does the video script demonstrate when a key is properly assigned in a list?
-When a key is properly assigned, React can efficiently update the DOM by only making changes to the elements that are affected by the list change, such as deleting an item, without unnecessarily re-rendering other elements.
How does React handle updates when a key is missing or improperly assigned?
-When a key is missing or improperly assigned, React will re-render all elements in the list, treating them as new elements, which can lead to performance issues and unexpected behavior.
What is the significance of the warning message 'each child in a list should have a unique key prop' mentioned in the script?
-The warning message 'each child in a list should have a unique key prop' signifies that React requires each child in a list to be uniquely identifiable by a key prop to optimize rendering and updates. This warning is shown when no unique key is provided, indicating a potential performance issue.
What are some potential issues that can arise if unique keys are not used in a React list?
-If unique keys are not used, potential issues include performance degradation due to unnecessary re-renders, and bugs where React might delete or update the wrong items because it cannot correctly identify individual elements.
What is the role of the 'handle remove' function mentioned in the script?
-The 'handle remove' function in the script is responsible for the deletion of a user from the list. It demonstrates how React manages state updates and how keys help React efficiently update the DOM when items are removed from a list.
What is the advice given in the script for choosing a good key in React?
-The advice given in the script for choosing a good key is to select a property that is unique across the array and does not change based on the order of the array. This could be an ID from a database or any other unique identifier that remains consistent even if the order of items changes.
Outlines
🔑 Understanding React Keys and List Rendering
This paragraph introduces the concept of keys in React, emphasizing their importance for efficient rendering of lists. The tutorial begins with a simple application displaying a list of users. The instructor demonstrates the consequences of not using keys properly by showing how deleting a user causes all list elements to be highlighted, indicating unnecessary re-renders. The script then explains the purpose of keys, which are unique identifiers that help React efficiently update the DOM. By adding a unique key to each list item, React can identify and update only the changed elements, leading to better performance. The paragraph concludes with an explanation of why keys should be unique and stable, preferably not derived from the array index.
🚀 Best Practices for Using Keys in React
The second paragraph delves into the best practices for using keys in React. It contrasts the use of array indices as keys, which leads to performance issues, with the use of unique identifiers like user IDs. The instructor clarifies that React will throw an error if keys are not provided, as it cannot efficiently update the list without them. The paragraph also discusses the potential for bugs when non-unique keys are used, as React may incorrectly identify elements as the same. The tutorial encourages viewers to experiment with different key values to understand their impact on React's rendering performance. It concludes with a call to action for viewers to engage with the content, subscribe for more tutorials, and join the Discord community for further learning and support.
Mindmap
Keywords
💡React Keys
💡List Tutorial
💡Efficient Updates
💡Unique Identifier
💡Re-rendering
💡State
💡Handle Remove Function
💡Console Warning
💡Performance Optimization
💡Index as a Key
💡Unique ID
Highlights
Introduction to React keys and lists tutorial, promising comprehensive understanding.
Demonstration of the issue without proper keys leading to inefficient updates.
Explanation of the necessity for keys in React for efficient rendering.
The importance of unique keys in lists for React to manage updates correctly.
How React uses keys to identify elements for efficient DOM updates.
The consequence of not providing a key, leading to React using the index as a key.
Visual demonstration of React's re-rendering behavior without unique keys.
The impact of using the index as a key on React's update efficiency.
Why using the index as a key can lead to bugs and performance issues.
The difference between static and dynamic data when considering keys.
Guidance on choosing a good key for lists in React applications.
The potential for bugs when keys are not unique in a list.
Practical advice on using unique identifiers like IDs from databases as keys.
Discussion on the importance of understanding keys for React developers.
Encouragement for viewers to experiment with keys to grasp the concept.
Conclusion and call to action for viewers to engage with the content.
Invitation to join the Discord community for more learning resources.
Closing remarks and sign-off from the presenter.
Transcripts
what's up guys there's cousin here
welcome back to the last and the best
react keys and list tutorial you're ever
gonna have to watch I promise you if you
watch this video and you watch it until
the end you're not gonna have to watch
another video on Keys ever again because
you will completely understand what they
are and how they are used cool let's
begin I have here as usual a very very
simple application we have here a list
of users from zero to nine and what I
want to do first is I want to show you
what happens if you don't put your keys
properly and then talk about keys and
why you should actually even do them so
the first thing that I'm going to do is
I'm going to open up this console here
and I'm going to go and find the HTML
elements that represents our users here
and here they are they are a bunch of
buttons because I have an on click event
listener to them when I click them this
user is going to get deleted I want you
to focus your attention here at the
bottom and see what happens to these
elements whenever I delete the user what
Chrome is going to do is whenever
there's a change to any element it's
going to high highlight that element
indicating to us that something has
changed so watch what happens when I
delete user 0. I delete and every single
element has highlighted I delete user 1
and every single element has highlighted
again if I now go and look at the code
for this you're going to see that it's
really simple we have here our state
with our users it's getting initialized
by this default users here if I open
that it's just a list of static users
with an ID and a name property so I
wouldn't worry too much about that then
we have this handle remove function
which handles our deletion of a user and
then we have the code here that renders
each individual user one by one
obviously you're going to see that we
have an error here it's saying that
we're missing a key prop for the element
in an iterator and actually if we go
back to our code and go to our console
we're going to see a similar error in
our console warning each child in a list
should have a unique key prop so now
let's see what happens if I add a proper
key to this button so I'll do key and
then I'll do user.id go back to our
application refresh go back to our
elements again and open up the buttons
and now pay attention to what is
highlighted when I delete every user so
I delete user 0 nothing is highlighted
only only this first div here right I
delete user one nothing again is
highlighted only this div because
something is changing in a div right a
user is getting removed so what's really
going on here why is it that whenever
we're providing a key nothing is being
highlighted but when we don't provide a
key everything is highlighted well for
this you have to understand how Keys
work in react and what they actually are
so from reacts perspective keys are
identifiers they are a way to uniquely
identify every single element in your
code to then be able to make efficient
updates whenever react has to so
whenever we make a change such as
deleting a user having a unique key will
allow react to identify each element and
make that change as efficiently as
possible now in our case what does it
mean to be efficient right well we have
to think about what is the work required
for react to do to efficiently delete
with a user so if we're deleting a user
we just deleted user 0 really the only
thing that react would have to do is
delete this user 0 from this list and
then leave everything else untouched
because user one two three and so on
they haven't changed they're the same
they have no new props they have no new
text no new state they are exactly the
same so react doesn't need to do
anything to them now when we have a key
we delete this and you see that nothing
is being done to those Elements which is
great it's exactly what we want but then
if I come to the code and I delete the
key that is no longer happening I'll
refresh I'll open up the elements again
I delete user 0 and everything is being
updated well that's because react when
you don't provide a key it's actually
not going to not provide a key it's
going to apply one behind the scenes
without you knowing it's going to apply
the index of the element as a key which
means that that is exactly equivalent if
I were to do something like user and
then index here and then manually
provide key Index this would be the same
in reacts perspective as not providing a
key and we can prove this by going back
to our application reloading again
opening up our elements again and then
deleting a user and you're going to see
that even with the key of index
everything is being highlighted as I
delete I delete and everything is
highlighted so why is that what's really
going on well we said that react uses
these keys to uniquely identify every
single item when making an update and we
also said that the most efficient update
for this when deleting a user is to just
remove that user and leave everything
else untouched but if we have the index
as a key which we have now in our code
when we delete the user actually before
we delete a user user 0 has the key of
zero because that is index 0 it is the
first element in the array user 1 has a
key of one user 2 has a key of two and
so on but now when I click delete user 0
react is going to make that deletion and
when it comes time to make those updates
what you're gonna see is that user 1 is
now in the first position of the array
which if it has the index as a key that
means that it's now receiving the key of
0 which is a different key than it had
on the previous render user 2 has now a
key of 1 because it's the first element
well the second element in the array
user 3 has a key of two and so on from
reacts perspective every single one of
these users now has a different key
which means that react is going to
consider that a whole new element so
that's what you're seeing here I delete
user 1 every user is offset by one every
user has a different key react sees
those as a different element and so it
will recreate and remount the entire
element which is why everything is
highlighting as I delete a user but now
if I come back to my code and instead of
index I remove this and I put user
dot ID like we had before and then I
refresh and do the same things again the
reason why nothing is highlighting is
because even though we've deleted user 0
user 1 still has the same ID that it has
before user 2 has the same ID every
single user has the same ID because that
ID is not dependent on the order that
they are in the array so that's what
keys are and that's why they are always
required and really important whenever
you're mapping over things in your code
and that is why if you don't provide a
key react is going to be very obvious
about it and it's going to scream at you
that hey provide a key because otherwise
you're throwing every single performance
optimization that I try to do out the
window and even though that you can do
the index here right and you can do this
and then index right react is no longer
going to complain it's no longer going
to throw an error even though this is
equivalent to not having a key which
might be confusing why is react allowing
this it's because react doesn't really
care what key you're actually giving it
it actually has no way to really know if
it's good key or not all that it cares
about is that you explicitly provide a
key as long as you provide any key it
could really be a string of like one or
something or even just one
react is going to be happy because from
its perspective you've provided a key
and the responsibility to find a good
key is solely on you so now the question
is what makes a good key we've seen that
if you provide the index that's not good
so then what should you do what you
should really think about finding
something that is unique across your
array something that no two elements are
going to have that is going to be the
same for example here in our default
users right if I go to this definition
we have an ID property and although in
this case it is the index this user's
array is never going to change so this
ID here this index is always going to be
the same we have it here in the state
and it's actually the state that is
going to change so this users here is
going to change so that's why we can't
use the index of this array but this
users here is static it'll never change
and so this ID is always going to be the
same we could have also used the name
property here in our case because here
we can guarantee that no two users are
going to have the same name but this
will differ in your case so be sure to
pick a prop property that is unique but
we could have just as easily come here
and instead of ID we could have done
name and this would have been exactly
the same the point is for you to pick
something that you can guarantee will be
unique in that array if you're dealing
with entities that come from a database
usually they will have an ID that is
already unique so it's as easy as using
that ID if you're dealing with something
that doesn't really have something you
have to get creative and make sure that
you give some sort of unique identifier
as your key now if you don't do this if
you don't provide a unique key you're
going to have some very strange bugs
because again react uses those keys to
identify unique items and if two items
have the same key in reacts perspective
they are the same item so if you delete
a user that has the same key it's going
to delete both users because they both
have the same key right so this is
really important so make sure that you
spend all of the time that you need to
really understand this take this quote
play around with it on your own really
understand and try to put different
values for the keys and see what kind of
results you get this is a really really
important Concept in react and it's
actually one of the fundamental concepts
of react and how it's used cool so there
you go that was a tutorial on react keys
and lists if you've enjoyed this video
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honestly what are you doing you are
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below with that being said my name has
been nurse cousin this is cousin
Solutions thank you so much for watching
and I will see you all in the next video
ciao
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