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React native foreach loop

Note: I am not an experienced React developer, it is my project to teach myself React and to blog as I do, so that I learn exactly what is happening and why see this post for why.

See additional reading notes at the end of the article is you feel you need more information on these topics. We can then pass this state down to our child component using props.

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See notes at the end for more information. If we take a look at the React panel in Chrome DevToolswe can see that our data is now being passed down to our child component as Props. When we look at what is being passed down though, it becomes clear pretty quickly that in order to display this data in a tabular way, we are going to have to loop through and output each array item as a table row. At this point you would be forgiven for thinking that you could loop through the data and output each row using either a for or foreach loop.

But before we can do that we first have to assign keys to the data so that we could access it within our loop. To start with I thought this would be a simple case of using the. This is because, although Chrome Dev Tools lists the data in Props as an array, at the moment it is still an object containing JSON data this took me quite a while to realise and therefore the map function will not work yet. What this does is, return an array of data where the elements keys are all strings that match the data within the JSON object.

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For example:. Looking at the data in githubDatathe Object. So now we have access to the data, all that is left is to loop through it so that we can create a table of github issues. And to do that we would combine Object. It looks like there is a lot going on here but if we break it down, we are first taking the Props githubData and creating keys that are strings matching the data within.

react native foreach loop

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Notify me of new comments via email. Notify me of new posts via email.Given the code below, we use the map function to take an array of numbers and double their values.

We assign the new array returned by map to the variable doubled and log it:. In React, transforming arrays into lists of elements is nearly identical. Below, we loop through the numbers array using the JavaScript map function. Finally, we assign the resulting array of elements to listItems :.

Try it on CodePen. We can refactor the previous example into a component that accepts an array of numbers and outputs a list of elements. Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity:. The best way to pick a key is to use a string that uniquely identifies a list item among its siblings.

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Most often you would use IDs from your data as keys:. This can negatively impact performance and may cause issues with component state. If you choose not to assign an explicit key to list items then React will default to using indexes as keys.

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A good rule of thumb is that elements inside the map call need keys. Keys used within arrays should be unique among their siblings. We can use the same keys when we produce two different arrays:. If you need the same value in your component, pass it explicitly as a prop with a different name:.

react native foreach loop

With the example above, the Post component can read props. In the examples above we declared a separate listItems variable and included it in JSX:. JSX allows embedding any expression in curly braces so we could inline the map result:. Sometimes this results in clearer code, but this style can also be abused.

Like in JavaScript, it is up to you to decide whether it is worth extracting a variable for readability. Keep in mind that if the map body is too nested, it might be a good time to extract a component. Key should be specified inside the array.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. New to React and trying to loop Object attributes but React complains about Objects not being valid React children, can someone please give me some advice on how to resolve this problem?

I've added createFragment but not completely sure where this needs to go or what approach I should take? The problem is the way you're using forEachas it will always return undefined. You're probably looking for the map method, which returns a new array:. If you're writing ES6, you can accomplish the same thing a bit neater using an arrow function :. I highly suggest you to use an array instead of an object if you're doing react itteration, this is a syntax I use it ofen. Read more here.

If your looking for itteration, this is the way to do it. Learn more. How to loop an object in React? Ask Question. Asked 3 years, 6 months ago. Active 6 months ago. Viewed k times. I got a similar error like this before.

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This was interesting also: stackoverflow. Active Oldest Votes. Banzy Banzy 5 5 bronze badges. When using an object, the keys are often IDs rather than indexes. If, however, the keys are all indexes, I agree with you. I appreciate your comment, but I'm not talking about an objects. Working with array seems the best for me and it's also good provided by the docs.

Keys are not neccessary at all, they are meant to keep the order of your elements. Arrays are not always the best choice, even though they often are. When caching stuff e. And while React will add keys, based on indexes, when you don't provide them explicitly, there're definitely use cases for doing so.

Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. The Overflow How many jobs can be done at home? Socializing with co-workers while social distancing.Some components that make it easier to construct dynamic and interactive scenes using React Native. If you are looking for the React Web version of this library, go to react-game-engine.

Rendering an Array of Data with map() and JSX

Take a look at Studious Beara super-polished puzzle game with great visuals and music. One of the first published games to use RNGE. If you've used react-native-game-engine before and understand the core concepts, take a look at react-native-game-engine-template.

It's a sort of game kickstarter project that allows you to prototype ideas quickly and comes preloaded with a bunch of stuff like:. Let's code a scene that incorporates some multi-touch logic. To start with, let's create some components that can be rendered by React. Create a file called renderers.

Finally let's bring it all together in our index.

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Build and run. Each entity is a "finger" and is assigned to a particular touch id. The touch ids increase as you place more fingers on the screen. Move your fingers around the screen to move the entities. As an exercise, try add a system that will insert another finger entity into the game state when a "start" touch event is encountered. What about adding a system that removes the closest entity from the game state when a "long-press" is encountered?

With ES2015 (ES6) syntax (spread and arrow functions)

If you're curious, our GameEngine component is a loose implementation of the Compenent-Entity-System pattern - we've written up a quick intro here. This depends on your definition of production quality. You could however create some more basic games doesn't mean they can't be fun gamesor even jazz up your existing business applications with some interactive eye candy. If you're aware of any others or wouldn't mind a shameless plug here - please reach out.

RNGE does not come with an out-of-the-box physics engine. We felt that this would be an area where the game designers should be given greater liberty. There are lots of JS-based physics engines out there, each with their pros and cons. Check out Matter JS if you're stuck. How you render your entities is up to you. You can use the stand React Native components View, Image or try react-native-svg or go full exotic with gl-react-native.

I felt that this would be a nice-to-have and for most use cases it would not be required.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here.

How to loop inside React JSX

Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I'm developing a little app in React Native and I am looking for something like a foreach function.

I just can't find a foreach loop. Not on StackOverflow and not even in the docs. I've found something with 'map', but I don't know if this is what I'm looking for. With a request to my Server I get multiple objects of users.

When I log the result, it looks like this:. I'd like to output this to a list. How is it possible to loop trought this objects? I know that I could also use a simple for loop, but this would definitly not be my first choice EDIT: I saved this value in this.

State is defined in my constructor. I try to access this with this. Am I doing this right? You can use map or for-of or any other. If data is proper then above example will work properly. Learn more. React Native foreach loop Ask Question. Asked 2 years, 7 months ago. Active 8 months ago. Viewed 55k times.Download the book now! There's one more thing we're going to cover before you know enough React basics to be able to move on to a real project, and that's how to loop over an array to render its contents.

Right now we have a single person with a single country, but wouldn't it be neat if we could have 10 people with 10 countries, and have them all rendered? Sure it would. Luckily for us, this is easy to do in JSX thanks to an array method called map. Note that I'm using short-hand syntax to set the people value in this.

Now that our state has an array of data to work with, we can loop over it using map by modifying our render method like this:. There are quite a few parentheses in there thanks to the way map works, but that code block does the following:. If you save the file and look in your browser, you'll probably see ten greeting messages in there so it looks like everything is working. But if you open your browser's error console you'll see a large warning: Each child in an array or iterator should have a unique "key" prop.

react native foreach loop

That error is pretty clear, but just in case you're not sure what it means here goes: if you use a loop like we're doing here with map you need to give every top-level item printed by that loop a key attribute that identifies it uniquely.

The reason for this is called reconciliation and it becomes very important when you make more advanced apps — and can cause some really weird bugs if you don't understand it fully! Now imagine your component's state changes, and now it prints the following:.

What happened? Well you and I can both see that "Dave from Malaysia! React asks for a key attribute so that it knows which item is which.

If we re-wrote the previous examples it would look like this:. Back to our Detail component with its random names and places: we can provide a key by using the index value we are receiving from maplike this:. That works fine for now, but if you ever want to add, remove or move items, you'll need to use a key attribute that doesn't change when items are moved or rearranged. Trust me, I've been there: if you use the position of an item as its key when move items around, you'll get some marvellously weird behavior!

Get the complete, unabridged Hacking with React e-book and take your learning to the next level - includes a day no questions asked money back guarantee! If this was helpful, please take a moment to tell others about Hacking with React by tweeting about it!

Follow me: twostraws. Hacking with React.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here.

Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. However, coming from template land and being new to JSXI am unsure how I would achieve the above adding a component multiple times. Think of it like you're just calling JavaScript functions. You can't use a for loop where the arguments to a function call would go:.

See how the function tbody is being passed a for loop as an argument, and of course that's a syntax error. Not sure if this will work for your situation, but often map is a good answer.

Iterating & Rendering with Loops in React components

You could write it like this with map :. If you don't already have an array to map like FakeRainBrigand's answer, and want to inline this so the source layout corresponds to the output closer than SophieAlpert's answer:. Re: transpiling with Babel, its caveats page says that Array.

I have a documentation issue open to clarify that. But use at your own risk or polyfill. With Array. Thanks to FakeRainBrigand for correcting my mistake in an earlier version of the fill solution see revisions.

In all cases the key attr alleviates a warning with the development build, but isn't accessible in the child. You can pass an extra attr if you want the index available in the child. See Lists and Keys for discussion. Using Array map function is very common way to loop through an Array of elements and create components according them in Reactthis is a great way to do a loop which is pretty efficient and tidy way to do your loops in JSXIt's not the only way to do it, but the preferred way.

Also, don't forget having a unique Key for each iteration as required. Map function creates a unique index from 0 but it's not recommended using the produced index but if your value is unique or if there is a unique key, you can use them:.