Rules for Render Logic: Pure Components | Lecture 131 | React.JS 🔥
Summary
TLDRThis script delves into the fundamental concepts of render logic and event handler functions in React components. It emphasizes the importance of maintaining purity in render logic, adhering to functional programming principles, and avoiding side effects. The video explains how render logic describes the component's view and should always return the same output for the same input (props). In contrast, event handlers are responsible for handling user interactions, updating state, making HTTP requests, and manipulating the application. The script underscores the significance of separating these two types of logic to ensure the application's proper functioning and maintainability. It also touches upon the useEffect hook, which allows side effects to be registered and executed when the component mounts or updates.
Takeaways
- 😃 Render logic refers to the code that participates in describing how a component's view should look like, executed when the component is rendered.
- 😎 Event handler functions are executed as a consequence of an event they are listening to, containing code that makes things happen in the application like state updates and HTTP requests.
- 🤔 Pure functions do not have side effects, meaning they don't depend on or modify data outside their scope, and return the same output for the same input.
- 🚫 Side effects are interactions with the outside world, like modifying external data, making HTTP requests, or writing to the DOM.
- ✨ React requires components to be pure functions when it comes to render logic, meaning render logic must not produce any side effects.
- 🔒 Render logic is not allowed to mutate objects or variables outside its scope, including props and state, as it would create side effects.
- 🔄 Updating state in render logic would create an infinite loop, which is why it's not allowed.
- 👍 Side effects like console.log and random number generation are technically not allowed in render logic but are generally safe to use.
- 🎯 Side effects are allowed and encouraged in event handler functions, which are not part of render logic.
- 🔍 The useEffect hook can be used to register side effects that need to be executed when the component is first rendered.
Q & A
What is render logic in React?
-Render logic is all the code that participates in describing how the view of a certain component instance should look like. It includes the code at the top level of the component function and the return block where the component returns its JSX.
What are event handler functions in React?
-Event handler functions are pieces of code that are executed as a consequence of the event that the handler is listening to. They contain code that makes things happen in the application, such as state updates, HTTP requests, reading input fields, and page navigation.
Why is it important to differentiate between render logic and event handler functions in React?
-It's important to differentiate between these two types of logic because they do fundamentally different things. While render logic is code that renders the component, event handlers contain code that actually changes and manipulates the application in some way.
What does it mean for a component to be pure when it comes to render logic in React?
-For a component to be pure when it comes to render logic, it means that if we give a certain component instance the same props (input), then the component should always return the exact same output in the form of JSX. In other words, render logic is not allowed to produce any side effects.
What are side effects in functional programming?
-A side effect happens whenever a function depends on or modifies data that is outside its scope. It can be thought of as a function's interaction with the outside world, such as mutating an outside object, making HTTP requests, writing to the DOM, or setting timers.
What are pure functions in functional programming?
-Pure functions are functions without side effects. They do not change any variable outside their scope, and when given the same input, they'll always return the same output, making them predictable.
What are the rules for render logic in React components?
-The main rule is that components must be pure functions when it comes to render logic. This means that render logic is not allowed to perform network requests, create timers, directly work with the DOM API, mutate objects or variables outside the component scope, or update state or refs.
Why can't we mutate props in React?
-We cannot mutate props in React because doing so would be a side effect, and side effects are not allowed in render logic. This is one of the hard rules of React, as mutating props would violate the principle of components being pure functions when rendering.
Where are side effects allowed in React components?
-Side effects are allowed and encouraged to be used inside event handler functions, as these are not considered render logic. Additionally, if we need to create a side effect as soon as the component function is first executed, we can register that side effect using the useEffect hook.
What is the next topic mentioned in the script after discussing render logic and side effects?
-The next topic mentioned in the script after discussing render logic and side effects is state update batching.
Outlines
📘 Understanding Render Logic in React
Render logic in React components is crucial for defining how a component's view should appear. It consists of all the code at the component's top level, including JSX return blocks, and is responsible for rendering the component's appearance on the screen. Additionally, render logic can also exist within functions, such as when calling a function that contributes to the component's view, exemplified by the `createList` function in the return block. This concept underscores the distinction between render logic, which is involved in rendering components, and event handler functions, which are executed in response to user events. Event handler functions are identified by their ability to perform actions such as state updates, HTTP requests, and more, thereby fundamentally differing from render logic. The emphasis on pure components in React, influenced by functional programming principles, demands that render logic must not produce side effects, ensuring components are predictable and efficient.
🔒 Rules of Render Logic and Side Effects in React
React enforces a principle that components must act as pure functions with respect to their render logic, meaning that given the same props, a component must always render the same output without side effects. Side effects, while not inherently bad, can lead to unpredictable behavior and are restricted in the context of render logic. This includes making network requests, mutating external variables, and direct DOM manipulations, which are not allowed during rendering. However, React provides avenues for managing necessary side effects through event handlers and the `useEffect` hook for effects outside of the render logic. This structure helps maintain component purity and prevents issues like infinite loops during state updates, balancing the application's interactive capabilities with reliable rendering behavior.
Mindmap
Keywords
💡Render Logic
💡Event Handler Functions
💡Side Effects
💡Pure Functions
💡State Updates
💡Functional Programming Principles
💡Props
💡JSX
💡useEffect Hook
💡State Update Batching
Highlights
Render logic is basically all the code that lifts at the top level of your component functions, and that participates in describing how the view of a certain component instance should look like.
Render logic is all the code that is executed as soon as the component is rendered.
Event handler functions are pieces of code that are executed as a consequence of the event that the handler is listening to.
While render logic is code that renders the component, event handlers contain code that actually does things, like state updates, HTTP requests, reading input fields, page navigation, and more.
React requires that components are pure when it comes to render logic, in order for everything to work as expected.
A side effect happens whenever a function depends on any data that is outside the function scope, or whenever a function modifies data that is outside its scope.
Pure functions are functions without side effects, they do not change any variable outside their scope, and when given the same input, they'll always return the same output.
Side effects are not bad in themselves, any useful program must have some interaction with the outside world.
Components must be pure functions when it comes to render logic, meaning if given the same props (input), they should always return the exact same output in the form of JSX.
Render logic is not allowed to perform network requests, create timers, directly work with the DOM API, mutate objects or variables outside its scope, mutate props, or update state or refs.
Side effects like console.log or creating random numbers are technically not allowed in render logic but can be safely used.
Side effects are only forbidden inside render logic, they are allowed and encouraged inside event handler functions.
To create a side effect as soon as the component function is first executed, we can register that side effect using the useEffect hook.
State update batching is another super important topic related to render logic and side effects in React.
Transcripts
In order for the rendering process to work
in the way that we just learned before,
our render logic needs to follow some simple rules.
So, let's look at these rules in this lecture
but first of all, what actually is render logic?
Well, in order to understand that,
let's actually take a look at the two types
of logic that we can write in React components.
So, that's render logic and event handler functions.
So, render logic is basically all the code that lifts
at the top level of your component functions
and that participates in describing how the view
of a certain component instance should look like.
So in this code example, there is a lot of render logic.
So we have these two lines of code at a top level
and then also the return block where our component returns
it's JSX.
So these describe exactly how the component
will be displayed on the screen.
However, if we look closely, we can identify
yet another piece of render logic here, even though
this code is actually inside a function.
So as you can see in the return block, the code there
is actually calling this createList function.
And therefore, that logic also participates
in describing the component view.
And so, it's also render logic.
So basically, render logic is all the code
that is executed as soon as the component is rendered.
So each time that the function is called.
Now moving on to event handler functions,
those are very easy to identify.
So, they're basically pieces of code that are executed
as a consequence of the event that the handler
is listening to.
So in our example, we have this line of code
that essentially registered handle new answer
for the change event and therefore handle new answer
is our event handle function.
And this is of course nothing new at this point.
So we have done this many times in the course, right?
But it's still important to differentiate
between these two types of logic
because they do actually do fundamentally different things.
So while render logic is code that renders the component,
event handlers contain code that actually does things.
So basically code that makes things happen
in the application.
So, event handlers contain things like state updates,
HTTP requests, reading input fields, page navigation,
and many more.
So all things that basically change
and manipulate the application in some way.
Now why is this all so important?
Well, it's important because React requires
that components are pure when it comes to render logic
in order for everything to work as expected.
But what does pure actually mean?
To answer that, let's have a quick refresher
on functional programming principles,
which are quite important in React in general.
And let's start with side effects.
So, a side effect happens whenever a function depends
on any data that is
outside the function scope, or even more importantly
whenever a function modifies data that is outside its scope.
And I like to think as a side effect
as a functions interaction with the outside world.
For example, this function is mutating an outside object.
And so this is clearly a side effect.
Other examples of side effects are HTTP requests,
riding to the DOM, setting timers and more.
The other important functional concept is pure functions,
which are basically functions without side effects.
So basically, these are functions that do not change
any variable outside their scope.
Also, when we give a pure function the same input,
it'll always return the same output.
For example, this function is clearly a pure function
because if we give it the same argument r,
it'll always give us the area
of the circle based on that r value.
So the output only depends on the inputs,
which makes this function predictable.
This other function right here on the other hand
is completely unpredictable
because it returns a string that contains a date
and that date changes every second.
So in this case, even
if we give the function the same input,
the output will always be different
because the date will always be different
and therefore, this is an impure function.
And the same is true for the second function.
So notice how this function mutates an outside variable,
which of course makes this function impure as well.
Now, calling functions impure makes it sound
as if side effects are somehow bad
but that's actually not true.
So, side effects are not bad in themselves.
In fact, if we think about it,
any program can only be useful if it has some interaction
with the outside world at some point, right?
Like a web application that never affects any data
or never writes to the DOM is just completely useless.
However, in order to make useful and bug-free applications,
we need to know when and how to create side effects,
which brings us back to React
and its rules for render logic.
And essentially, there's just one big rule,
which is that components must be pure functions
when it comes to render logic.
This means that if we give a certain component instance
the same props, so the same input,
then the component should always return
the exact same output in the form of JSX.
In practice, this means that render logic
is not allowed to produce any side effects.
So in other words, the logic that runs
at the top level and is responsible
for rendering the component should have no interaction
with the outside world.
This means that render logic is not allowed
to perform network requests to create timers
or to directly work with the DOM API.
For example, listening to events using at event listener.
Now, according to what we learned previously,
render logic must also not mutate objects
or variables that are outside the scope
of the component function.
And this is actually the reason why we cannot mutate props,
which remember is one of the hard rules of React.
And so now you know why that rule exists.
It's because doing so would be a side effect
and side effects are not allowed.
Finally, we really cannot update state
or refs in render logic.
And updating state in render logic would actually create
an infinite loop, which is why we can never do that.
State updates are technically not side effects
but it's still important for them to be on this list.
Now, there are other side effects that are technically
not allowed as well, but that we create all the time
like using console.log or creating random numbers.
So these are clearly interactions with the outside world
but they don't seem to do any harm.
And so we can safely keep doing them.
Alright, but now you might be wondering if all this stuff
is not allowed, then how will I ever be able
to make an API call to fetch some data?
Well, keep in mind that these side effects
are only forbidden inside render logic.
This means that you have other options
for running your side effects.
First of all, we saw earlier that event handler functions
are not render logic and therefore, side effects
are allowed and actually encouraged
to be used inside these functions.
And second, if we need to create a side effect as soon
as the component function is first executed,
we can register that side effect
using a special hook called useEffect.
But we will learn all about that in the next section.
For now, let's move on to another super important topic,
which is state update batching.
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