React is the entry point to the React library. If you use ES6 with npm, you can write import React from 'react'. React components let you split the UI into independent, reusable pieces, and think about each piece in isolation. React components can be defined by subclassing React. Component or React. See Using React without ES6 for more information. Each JSX element is just syntactic sugar for calling React.
You will not typically invoke the following methods directly if you are using JSX. React also provides a component for rendering multiple elements without a wrapper. Today, Suspense only supports one use case: loading components dynamically with React.
In the future, it will support other use cases like data fetching. Hooks are a new addition in React They let you use state and other React features without writing a class.
Hooks have a dedicated docs section and a separate API reference:. Component is the base class for React components when they are defined using ES6 classes :. See the React. Component class. PureComponent is similar to React. The difference between them is that React.
PureComponent implements it with a shallow prop and state comparison. PureComponent for a performance boost in some cases.
If these contain complex data structures, it may produce false-negatives for deeper differences. Only extend PureComponent when you expect to have simple props and state, or use forceUpdate when you know deep data structures have changed. Or, consider using immutable objects to facilitate fast comparisons of nested data. Furthermore, React. PureComponent but for function components instead of classes. If your function component renders the same result given the same props, you can wrap it in a call to React.React makes it painless to create interactive UIs.
Design simple views for each state in your application, and React will efficiently update and render just the right components when your data changes. Build encapsulated components that manage their own state, then compose them to make complex UIs.
React can also render on the server using Node and power mobile apps using React Native. React components implement a render method that takes input data and returns what to display. Input data that is passed into the component can be accessed by render via this. JSX is optional and not required to use React. In addition to taking input data accessed via this. Using props and statewe can put together a small Todo application. This example uses state to track the current list of items as well as the text that the user has entered.
Although event handlers appear to be rendered inline, they will be collected and implemented using event delegation.
React allows you to interface with other libraries and frameworks. Declarative React makes it painless to create interactive UIs. Declarative views make your code more predictable and easier to debug. Component-Based Build encapsulated components that manage their own state, then compose them to make complex UIs. A Simple Component React components implement a render method that takes input data and returns what to display. A Stateful Component In addition to taking input data accessed via this.
An Application Using props and statewe can put together a small Todo application. A Component Using External Plugins React allows you to interface with other libraries and frameworks. Get Started. Take the Tutorial.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:.
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.React For Everyone #12 - Iterating Over Data With JSX
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. 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:. 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.
State and Lifecycle
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. Edit this page. Main Concepts. Advanced Guides.
Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. You need to pass an array of element to jsx. The problem is that forEach does not return anything i. So better use map because it returns an array like this.
Learn more. Asked 2 years, 4 months ago. Active 1 year, 6 months ago. Viewed k times. Active Oldest Votes. Prakash Sharma Prakash Sharma 8, 3 3 gold badges 15 15 silver badges 26 26 bronze badges. Use var i as key is not a good choice in some situation for virtual dom.
Here's a good post on why medium. FWIW you can also pass in other kinds of collections. You just need to unroll them so they'll work with. 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.
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Out of the box, ReactJS already gives us some context-binding magic. When an Element Component is instantiated in ReactJS, the constructor function auto-binds the class specification methods to the instance of the element. This way, whenever you invoke those methods, they are invoked in the context of the element, even if they're invoked as naked function references. As such, we never need to use. Since iteration methods like.
However, if you look at the signatures for these methods, they all accept an optional second argument, which is the context ie, the [this] binding in which to execute the iterator. These methods already provide the functionality that some people are trying to recreate with. If you are not sure what I am talking about, there are a number of demos including demos in the ReactJS documentation that are performing iteration like this:. Notice the. This is unnecessary.
To see the native context-binding functionality for iteration methods, I've put together a tiny ReactJS demo that has to map a collection of friends onto a collection of Friend elements.
To do this, I'm using the. This allows me to use the "natural" context for the Element. As you can see, my call to. Doing this allows me to reference this. NOTE : Function. So, if you're going to buy into support for. Since I've never really taken advantage of native iteration methods - I usually use lodash. To do so, I put together a quick test case:. NOTE : lodash. This just makes sure that the iterator methods always executes as if bound to the "context" object.
And, when I run this is in all the browsers, including IE9, I get the following console output:. As you can see, all is good.Although we can write a for loop in React, ES6 provides the more appropriate map function for us to use.
Out of the three iterators above, our best option to iterate over an array in React inside of JSX is the Map function. Introduced in ES6, the Map array function is by far my most used method of iterating over an array in React. Why is it my most commonly used method of looping through arrays in React? We begin by initializing an array called names and populate it with five strings. You might recognize some of the names in this array.
Remember, since the release React We can add state to functional components, therefore I like to always use functional React components in my tutorials. Our functional React component returns a single div element with some JSX inside. JSX is absolutely brilliant. Next, we simply loop through the names array using the brilliantly simple Map Array function.
You do this by providing a key attribute on the HTML element that you are rendering inside of the Map function. In our example above, we rendered a set of li tags in our Map function. Therefore, we have to add an additional attribute to the li tag, like so:. We use the index of the array element for the unique key.
What exactly does this mean? It simply calls a provided function on each element in your array. This callback is allowed to mutate the calling array. Meanwhile, the map method will also call a provided function on every element in the array.
The difference is that map utilizes return values and actually returns a new Array of the same size. Consider the below array. If we wanted to double each element in the array, we could use either map or forEach. Note that you would never return from a forEach function as the return values are simply discarded:.
Rendering List of Elements in React With JSX
Here are the results of my forEach vs map test:. Your browser is probably different. You can check out the full test results here:.
This is because forEach affects and changes our original Array, whereas map returns an entirely new Array — thus leaving the original array unchanged. And map might be preferable when changing or altering data. Not only is it faster but it returns a new Array. This means we can do cool things like chaining on other methods mapfilterreduceetc. After this, we filter through the array and only save the elements that are greater than 5. This leaves us with a final arr2 of [6,8,10].
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