I have a functional component that has one function within it, renderMessages.
const MessageContainer = (props) => {
const renderMessages = () => {
return props.messages.map((message, index) => {
return(
<Message
key={index}
username={message.username}
message={message.message}
fromCurrentUser={message.fromCurrentUser}
/>);
})
}
return(
<div className='messages'>
{renderMessages()}
</div>
)
}
However, I realized that instead of wrapping renderMessages function on the map, I can just have:
const renderMessages = props.messages.map((message, index) => {
return(
<Message
key={index}
username={message.username}
message={message.message}
fromCurrentUser={message.fromCurrentUser}
/>);
})
}
And as a result, my final return would just contain
return(
<div className='messages'>
{renderMessages}
</div>
)
In a class-based component and within a render function, I'd use the last of the two. Which of the two is considered the best practice when using functional components, and why?
EDIT:
Which of the two is considered the best practice when using functional components, and why?
Best practices change with context - e.g. the team you're working on - so this is an opinion-based question out of the gate.
That being said, in my opinion, I wouldn't do either. I'd do (and I do) this:
const MessageContainer = (props) => {
return (
<div className='messages'>
{props.messages.map((message, index) => (
<Message
key={index}
username={message.username}
message={message.message}
fromCurrentUser={message.fromCurrentUser}
/>
))}
</div>
)
}
What's the purpose of the extra variable anyway?
While you're at it, don't use indexes for keys
The dirty secret about all those extra methods you stuck on your class components that encapsulated rendering logic is that they were an anti-pattern - those methods were, in fact, components.
EDIT #2
As pointed out in the other answer, the most performant solution for this specific use case is specifying the map function outside the functional component:
const renderMessage = (message,index) => (
<Message
key={index}
{...message}
/>
)
const MessageContainer = (props) => {
return (
<div classname='messages'>
{props.messages.map(renderMessage)}
</div>
);
}
But, you shouldn't prematurely optimize and I would advocate for the original solution I posted purely for simplicity/readability (but, to each their own).
Good job separating the mapping outside the component's return, because this way you'll be only calling the same function over and over again untill the .map is done iterating, but if you wrote it in the component's return, every time the .map iterate over the next item you'll be creating a new function.
Regarding the question, I'd recommend the second way, clean/readable code is always preferable.
P.S. try to use the unique message id instead of the index.
Related
I'm following the react js tutorial, and I keep running into this issue
import React from "react";
import NewsCard from "../NewsCard/NewsCard";
const NewsCards = ({ articles }) => {
return (
<div>
{articles.map((article, i) => {
<NewsCard />;
})}
</div>
);
};
export default NewsCards;
Seems like your articles does not have default value as [].
You can change as follow. And you should give key attribute when using map function.
const NewsCards = ({ articles }) => {
const data = articles ? articles : []
return (
<div>
{data.map((article, i) => {
<NewsCard key={article.id}/>;
})}
</div>
);
};
Probably articles is not initialized when you try to map throught it. Try this:
{articles?.map((article, i) => {
<NewsCard />;
})}
OR
{articles && articles.map((article, i) => {
<NewsCard />;
})}
</div>
That way you will first make sure if articles exist
This means that the articles prop is undefined.
There are several ways to solve this. The first and easiest way is by implementing the following logic:
{articles?.length ? articles.map((article, i) => <NewsCard />) : "There are no articles here."}
Another way to solve this is by implementing React proptypes - you can read about this here.
Third and "hardest" (but probably best) way to solve this is by using a static type checking tool. Flow comes to mind, but you can use TypeScript too.
If you still need help, just like what the previous answers said, make sure that articles is initialized/defined by using the && operator to make that check. Also, based upon what you wrote, the map method is returning undefined since you specified a function body (using the function body bracket notation {} ) without a return statement. So instead write the map method like this:
<div>
{articles && articles.map((article, i) => <NewsCard />)}
</div>
or like this:
<div>
{articles && articles.map((article, i) => {
return <NewsCard />
})}
</div>
The first example implies an implicit return since an arrow function is being used and a function body is not present (there are no function body brackets { }).
While looking through our code base, I found code that looks a bit like this:
const Carousel = ({ items }) => {
return (
<CarouselOuter>
{items.map((item) => (
<CarouselItemWrapper>
<CarouselItem key={item.key}>
...
</CarouselItem>
</CarouselItemWrapper>
)}
</CarouselOuter>
);
}
Notice that the key prop is on CarouselItem, not CarouselItemWrapper, the component that's directly returned from items.map. This seems to work fine, and there are no warnings in the console, but it runs counter to every example I've seen using map in React.
I want know if there's a good argument (specifically in regards to performance) for rearranging the code with the key as shown below, or if this is just a stylistic choice:
const Carousel = ({ items }) => {
return (
<CarouselOuter>
{items.map((item) => (
<CarouselItemWrapper key={item.key}>
<CarouselItem>
...
</CarouselItem>
</CarouselItemWrapper>
)}
</CarouselOuter>
);
}
Side note: CarouselOuter, CarouselItem, and CarouselItemWrapper are all styled-components, but I doubt that's relevant.
I wonder if there is a way to replicate same usage of js callback's in JSX like:
[1,2,3].map(console.log) // Here map iterator takes care of passing values to iteratee.
Is there a similar way to pass component like that, ie. rather then this:
<List data={data} renderItem={item => <Post {...item} />} />
something like this:
<List data={data} renderItem={<Post/>} />
Ahh ok, just found out you can use components as plain js functions:
<List data={data} renderItem={Post} />
You can, but speaking from experience this leads to pain and bad design later on once these components grow and requirements change. The List component becomes more esoteric, and it's hard to find where things should be.
For instance, if you wanted to do something as simple as render a sponsored Post, the code becomes needlessly complex:
// Index.js
<List data={data} renderItem={Post} renderSponsoredItem={SponsoredPost} />
// List.js
function List({ data, renderItem, renderSponsoredItem }) {
<div className={styles.list}>
data.map(dataItem => {
if (dataItem.sponsored) {
return renderSponsoredItem(dataItem);
} else {
return renderItem(dataItem);
}
});
</div>
}
...and in a way, this ends up looking not like React at all, and doesn't harness the ease-of-use and readability thats possible with React.
An alternative could look something like:
// Index.js
<List>
{data.map(dataItem => {
if (dataItem.sponsored) {
return <SponsoredPost post={dataItem} />;
} else {
return <Post post={dataItem} />
}
})
</List>
// List.js
function List({ children }) {
<div className={styles.list}>
{children}
</div>
}
I've found the first form to lend itself to unreadable code and unworkable, unreadable components in the long term, and the latter to be much easier to read/work with.
I am building a website that is reliant on a json file for all of its information.
In my app.js the json info is showing properly when I console.log() it, but when I try and pass it to my functional components it is giving me undefined.
in app.js
<Route
exact
path="/puppies"
render={props => (
<Puppies {...props} propdata={this.state.propdata} />
)}
/>
This seems to be working fine, however when I try and map it inside the component it tells me that its undefined.
function Puppies(propdata) {
return <div>{propdata.puppies.map(puppies =>
<h1>{puppies.name}</h1>
)}</div>;
}
I have done this before but with a class component. So most likely I am making a mistake with the functional component.
The full code is viewable here:
https://github.com/Imstupidpleasehelp/Puppywebsite/tree/master/src
Thank you for your time.
You'll probably need to check that the data is null of undefined. You are passing a big object with data, I recommend to pass more specific props instead of a big object.
I like to prevent my data to be undefined in 2 ways:
lodash.get
Optional Chaining
Usage:
import _ from 'lodash';
function Puppies({ propdata }) {
const puppies = _.get(propdata, 'puppies', []);
return (
<div>
{puppies.map(puppies => <h1>{puppies.name}</h1>)}
</div>
);
}
or
function Puppies({ propdata }) {
const puppies = propdata?.puppies || [];
return (
<div>
{puppies.map(puppies => <h1>{puppies.name}</h1>)}
</div>
);
}
What you have as propdata is actually just an object containing all properties that you have passed in. You should use destructuring to get the actual propdata value.
Solution:
function Puppies({propdata}) {
return (
<div>
{propdata.puppies.map(puppies =>
<h1>{puppies.name}</h1>
)}
</div>
);
}
Since this is asynchronous request to get the data, your data is not readily available hence you need to handle that scenario.
function Puppies(propdata) {
return (
{
propdata.puppies.length>0 ? <div>
propdata.puppies.map((puppies)=>{
<h1>{puppies.name}</h1>
})
</div> :null
}
)
Hi I have some sort of the following code:
class First extends Component {
constructor(props){super(props)}
myfunction = () => { this.card //do stuff}
render() {
return(
<Component ref={ref => (this.card = ref)} />
)}
}
Why is it not possible for me to access the card in myfunction. Its telling me that it is undefined. I tried it with setting a this.card = React.createRef(); in the constructor but that didn't work either.
You are almost there, it is very likely that your child Component is not using a forwardRef, hence the error (from the React docs). ref (in a similar manner to key) is not directly accesible by default:
const MyComponent = React.forwardRef((props, ref) => (
<button ref={ref}>
{props.children}
</button>
));
// ☝️ now you can do <MyComponent ref={this.card} />
ref is, in the end, a DOMNode and should be treated as such, it can only reference an HTML node that will be rendered. You will see it as innerRef in some older libraries, which also works without the need for forwardRef in case it confuses you:
const MyComponent = ({ innerRef, children }) => (
<button ref={innerRef}>
{children}
</button>
));
// ☝️ now you can do <MyComponent innerRef={this.card} />
Lastly, if it's a component created by you, you will need to make sure you are passing the ref through forwardRef (or the innerRef) equivalent. If you are using a third-party component, you can test if it uses either ref or innerRef. If it doesn't, wrapping it around a div, although not ideal, may suffice (but it will not always work):
render() {
return (
<div ref={this.card}>
<MyComponent />
</div>
);
}
Now, a bit of explanation on refs and the lifecycle methods, which may help you understand the context better.
Render does not guarantee that refs have been set:
This is kind of a chicken-and-egg problem: you want the component to do something with the ref that points to a node, but React hasn't created the node itself. So what can we do?
There are two options:
1) If you need to pass the ref to render something else, check first if it's valid:
render() {
return (
<>
<MyComponent ref={this.card} />
{ this.card.current && <OtherComponent target={this.card.current} />
</>
);
}
2) If you are looking to do some sort of side-effect, componentDidMount will guarantee that the ref is set:
componentDidMount() {
if (this.card.current) {
console.log(this.card.current.classList);
}
}
Hope this makes it more clear!
Try this <Component ref={this.card} />