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Using const Variables declared with var or let can be changed later on in the program, and reassigned. Once a const is initialized, its value can never be changed again, and it can't be reassigned to a different value.
Because we should always use the simplest construct available to avoid making errors down the road.
In my opinion this change was so welcoming that you now rarely see the usage of the function keyword in modern codebases. Implicit return Arrow functions allow you to have an implicit return: values are returned without having to use the return keyword.
The this scope with arrow functions is inherited from the execution context. An arrow function does not bind this at all, so its value will be looked up in the call stack, so in this code car. Arrow functions cannot be used as constructors either, when instantiating an object will raise a TypeError. This is where regular functions should be used instead, when dynamic context is not needed. This is also a problem when handling events.
A class definition This is how a class looks. When the object is initialized, the constructor method is called, with any parameters passed. A class also has as many methods as it needs. I am a programmer. Inside a class, you can reference the parent class calling super. Static methods Normally methods are defined on the instance, not on the class.
Getters and setters You can add methods prefixed with get or set to create a getter and setter, which are two different pieces of code that are executed based on what you are doing: accessing the variable, or modifying its value. Asynchronous means that things can happen independently of the main program flow.
In the current consumer computers, every program runs for a specific time slot, and then it stops its execution to let another program continue its execution. When a program is waiting for a response from the network, it cannot halt the processor until the request finishes. Normally, programming languages are synchronous, and some provide a way to manage asynchronicity, in the language or through libraries. Some of them handle async by using threads, spawning a new process.
If you prefer to learn by doing, start with our practical tutorial. If you prefer to learn concepts step by step, start with our guide to main concepts.
So how should you deal with this problem? Enter Babel. Babel is a compiler: This must happen at build time, so you must setup a workflow that handles this for you. Webpack is a common solution. Babel is easily installed using npm , locally in a project:. Since npm now comes with npx , locally installed CLI packages can run by typing the command in the project folder:. To solve the problem we talked about in the introduction using arrow functions in every browser , we can run.
By default, webpack starting from version 4 does not require any config if you respect these conventions:. You can customize every little bit of webpack of course, when you need.
In this example, css-loader interprets the import 'style. What kind of loaders are there? You can find the full list here. See the babel-loader options here. Plugins are like loaders, but on steroids. There are lots of plugins available. This mode introduced in webpack 4 sets the environment on which webpack works. It can be set to development or production defaults to production, so you only set it when moving to development. Production mode is slower to build, since it needs to generate a more optimized bundle.
Webpack can be run from the command line manually if installed globally, but generally you write a script inside the package. Webpack can automatically rebuild the bundle when a change in your app happens, and keep listening for the next change. One nice feature of the watch mode is that the bundle is only changed if the build has no errors.
If there are errors, watch will keep listening for changes, and try to rebuild the bundle, but the current, working bundle is not affected by those problematic builds. Webpack allows us to use images in a very convenient way, using the file-loader loader. Check the Image docs. Since webpack bundles the code, Source Maps are mandatory to get a reference to the original file that raised an error, for example.
You tell webpack to generate source maps using the devtool property of the configuration:. Its strengths are:. Jest can be installed in any other project using Yarn:. Projects created with create-react-app have Jest installed and preconfigured out of the box, but adding Jest to any project is as easy as typing. Open a math. Now create a math. Running yarn test results in Jest being run on all the test files it finds, and returning us the end result:.
Mocking is useful when you want to avoid side effects e. Even more important, if you are writing a Unit Test , you should test the functionality of a function in isolation, not with all its baggage of things it touches. Using mocks, you can inspect if a module function has been called and which parameters were used, with:.
Jest provides a convenient way to mock an entire package. Say you import mathjs. You can also use jest. You can find pre-made mocks for popular libraries. For example this package https: Snapshot testing is a pretty cool feature offered by Jest. This is a simple test on the App component of a simple create-react-app application make sure you install react-test-renderer:. If App changes, you get an error:. When using yarn test in create-react-app you are in watch mode , and from there you can press w and show more options:.
If your change is intended, pressing u will update the failing snapshots, and make the test pass.
You can also update the snapshot by running jest -u or jest --updateSnapshot outside of watch mode. The easiest way to start with testing React components is doing snapshot testing, a testing technique that lets you test components in isolation.
I assume you created a React app with create-react-app , which already comes with Jest installed, the testing package we'll need. CodeSandbox is a great environment to try this out. Start with a React sandbox, and create an App. When CodeSandbox detects test files, it automatically runs them for you, and you can click the Tests button in the bottom of the view to show your test results:.
If you skipped it, you can go back and read how we built it, but for easier reference I add it here again. We are going to use the react-testing-library , which is a great help as it allows us to inspect the output of every component and to apply events on them. You can read more about it on https: We start by importing render and fireEvent from react-testing-library , two helpers.
The first lets us render JSX. The second lets us emit events on a component. Buttons are used in the app to accept a click event and then they call a function passed to the onClickFunction prop. We add a count variable and we create a function that increments it:. Now off to the actual tests. It shows 4 buttons and the result in the page.
We can inspect each button and see if the result increases when we click them, clicking multiple times as well:. Check the code working on this CodeSandbox: The ecosystem around React is huge. Here I introduce you to 4 of the most popular projects based upon React: React Router, Redux, Next. Routing in a Single Page Application is the way to introduce some features to navigating the app through links, which are expected in normal web applications:.
Routing links together your application navigation with the navigation features offered by the browser: React Router offers a way to write your code so that it will show certain components of your app only if the route matches what you define. Which one to use is mainly dictated by the browsers you need to support.
If you don't have to worry about older browsers, it's the recommended choice.
Route components are responsible for showing - or hiding - the components they contain. You import it from react-router-dom, and you use it to wrap all your app:. The Link component is used to trigger new routes. You import it from react-router-dom , and you can add the Link components to point at different routes, with the to attribute:. Check this example on Glitch: Notice the exact attribute. You can have a route respond to multiple paths simply using a regex, because path can be a regular expressions string:.
Instead of specifying a component property on Route , you can set a render prop:. Redux is a way to manage an application state, and move it to an external global store. There are a few concepts to grasp, but once you do, Redux is a very simple approach to the problem. Redux is ideal for medium to big apps, and you should only use it when you have trouble managing the state with the default state management of React, or the other library you use.
The only requirement of an action object is having a type property, whose value is usually a string. In a simple app an action type can be defined as a string, as I did in the example in the previous lesson. A reducer is a pure function that calculates the next State Tree based on the previous State Tree, and the action dispatched. A pure function takes an input and returns an output without changing the input or anything else.
Second, if you are building a publicly available website, you have a content SEO issue. The solution to both of those problems is server rendering , also called static pre-rendering. It provides a common structure that allows you to easily build a frontend React application, and transparently handle server-side rendering for you. This is the only thing that Next. Create an empty pages folder, and run the command again, and Next. In the pages folder create an index. Simply put, pages are inside a pages folder, and the page URL is determined by the page file name.
The filesystem is the pages API. As you can see, the HTML generated by the component is sent directly in the page source. The Next. If you point your browser to localhost: As you can see, also this page is server rendered. Note how you did not have to restart the npm process to load the second page. Server rendering is very convenient in your first page load, for all the reasons we saw above, but when it comes to navigating inside the website, client-side rendering is key to speeding up the page load and improving the user experience.
Try linking the two pages above. Now go back to the browser and try this link. As you can see, the Contact page loads immediately, without a page refresh. If you now cmd-click the link, the same Contact page will open in a new tab, now server rendered. A good use case for Next. A dynamic page is a page that has no fixed content, but instead display some data based on some parameters. You can use clean URLs without query parameters.
Styles are scoped to the component, but you can also edit global styles adding global to the style element:. A Next. At the beginning of this tutorial you created a package. The company behind Next. Of course they integrate both their products so you can deploy Next. You can set up multiple Next. Check out my Next.
It is one of the tools that allow you to build on a set of technologies and practices collectively known as JAMstack. Gatsby is one of the cool kids in the Frontend Development space right now. I think the reasons are:. All you need is a place that serves plain HTTP pages and your assets to the client. I mentioned Progressive Web Apps in the list. Gatsby automatically generates your site as a PWA, with a service worker that speeds up page loading and resource caching.
This command creates a brand new Gatsby site in the mysite folder, using the starter available at https: A starter is a sample site that you can build upon. Another common starter is default , available at https: The browser should instantly hot reload the component which means the page does not actually refresh, but the content changes - a trick made possible by the underlying technology.
To add a second page, just create another. You can link those pages by importing a Gatsby-provided React component called Link:.
Gatsby provides lots of things out of the box, but many other functionalities are provided by plugins.
A Gatsby plugin is installed in 2 steps. First you install it using npm , then you add it to the Gatsby configuration in gatsby-config. In gatsby-config. Once you are done tweaking the site and you want to generate the production static site, you will call. Once you build the site using gatsby build , all you need to do is to deploy the result contained in the public folder.
Here are some great guides for some popular hosting platforms where you can deploy Gatsby. I hope this book has helped you get started with React, and maybe it gave you a head start in exploring some of the most advanced aspects of React programming.
In addition to npm start , create-react-app added a few other commands: If you already have a React app installed using an older version of React, first check the version by adding console.
CodeSandbox is a great way to start a React project without having to install it locally. Codepen Another great solution is Codepen. I will mention a list of things to get you up to speed quickly. Variables A variable is a literal assigned to an identifier, so you can reference and use it later in the program. Using var Until ES, var was the only construct available for defining variables.
Using let let is a new feature introduced in ES and it's essentially a block scoped version of var. Using const Variables declared with var or let can be changed later on in the program, and reassigned. Implicit return Arrow functions allow you to have an implicit return: It works when there is a one-line statement in the function body: When defined as a method of an object, in a regular function this refers to the object, so you can do: DOM Event listeners set this to be the target element, and if you rely on this in an event handler, a regular function is necessary: Clone an object with: The most important one is the ability to use an array as function argument in a very simple way: Rest properties: Tom, age: The syntax also works on arrays: The syntax at a first glance is very simple, just use backticks instead of single or double quotes: First Second an easy way to fix this problem is by having an empty first line, and appending the trim method right after the closing backtick, which will eliminate any space before the first character: A class definition This is how a class looks.
In this case hello is a method and can be called on all objects derived from this class: If the inherited class has a method with the same name as one of the classes higher in the hierarchy, the closest method takes precedence: I am a programmer. Inside a class, you can reference the parent class calling super. Static methods Normally methods are defined on the instance, not on the class.
Static methods are executed on the class instead: Getters and setters You can add methods prefixed with get or set to create a getter and setter, which are two different pieces of code that are executed based on what you are doing: Asynchronous means that things can happen independently of the main program flow. Lines of code are executed in series, one after another, for example: This event handler accepts a function, which will be called when the event is triggered: One common example is by using timers: However every callback adds a level of nesting, and when you have lots of callbacks, the code starts to be complicated very quickly: How do we solve this?
How promises work, in brief Once a promise has been called, it will start in pending state. Consuming a promise In the last section, we introduced how a promise is created. Chaining promises A promise can be returned to another promise, creating a chain of promises. Running fetch returns a response , which has many properties, and within those we reference: Handling errors In the above example, in the previous section, we had a catch that was appended to the chain of promises.
Orchestrating promises with Promise. How it works An async function returns a promise, like in this example: Here's an example: This is why this code is valid: I did something and I watched and I watched as well Easier debugging Debugging promises is hard because the debugger will not step over asynchronous code. For example, this module exports a function that returns a string uppercase: In a file however you can export more than one thing, by using this syntax: What does this mean?
Only very innovative products worked differently, and experimented with new approaches. Examples of Single Page Applications Some notable examples: Gmail Google Maps Facebook Twitter Google Drive Pros and cons of SPAs An SPA feels much faster to the user, because instead of waiting for the client-server communication to happen, and wait for the browser to re-render the page, you can now have instant feedback.
Overriding the navigation Since you get rid of the default browser navigation, URLs must be managed manually. Declarative What does it mean when you read that React is declarative?
Immutability One concept you will likely meet when programming in React is immutability and its opposite, mutability. An immutable variable can never be changed. To update its value, you create a new variable. The same applies to objects and arrays. An object is never updated, but copied before changing it. This applies to React in many places.
In Redux, you never mutate the state directly, but only through reducers, which are functions. The question is, why? There are various reasons, the most important of which are: Mutations can be centralized, like in the case of Redux, which improves your debugging capabilities and reduces sources of errors. Code looks cleaner and simpler to understand. You never expect a function to change some value without you knowing, which gives you predictability.
This gives you performance. All functional components are pure components: For example, think about using map to create a new array from an initial set, and then filtering the result using filter: Create specialized version of a component Use an outer component to expand and specialize a more generic component: The component needs to output props.
First, what is the DOM? What happens next is: In React this means that: Thanks to one-way bindings, data cannot flow in the opposite way as would happen with two-way bindings, for example , and this has some key advantages: A JSX primer Here is how you define a h1 tag containing a string: Inside a JSX expression, attributes can be inserted very easily: By doing a process called transpiling. For example the following two constructs are equivalent: For example here's how to use a constant value defined elsewhere: Curly braces accept any JS code: In JSX, they are renamed to their camelCase equivalent: This means you define properties in an object: JSX allows components defined in React for example to completely encapsulate their style.
Is this the go-to solution? Inline styles in JSX are good until you need to write media queries style animations reference pseudo classes e.
This also applies to the textarea field, e. This means that you might run into issues when using an HTML entity in a string expression. To fix this you can either move the entities outside the expression: Rule 1: Instead of doing it manually, e. React makes it very simple: Even plain HTML tags are component on their own, and they are added by default. DOM offers by typing it in the Browser Console: Custom components There are 2 ways to define a component in React.
A function component: There is also a third syntax which uses the ES5 syntax, without the classes: State Setting the default state of a component In the Component constructor, initialize this. For example the BlogPostExcerpt component might have a clicked state: Why you should always use setState The reason is that using this method, React knows that the state has changed. Unidirectional Data Flow A state is always owned by one Component. This is the reason the state is often moved up in the Component tree.
Moving the State Up in the Tree Because of the Unidirectional Data Flow rule, if two components need to share state, the state needs to be moved up to a common ancestor.
The state is passed down to the components that need that value via props: Change it! In a function component, props is all it gets passed, and they are available by adding props as the function argument: It gets complicated when: How props are passed When initializing a component, pass the props in a way similar to HTML attributes: Children A special prop is children.
That contains the value of anything that is passed in the body of the component, for example: Presentational vs container components In React, components are often divided into 2 big buckets: Each of those have their unique characteristics.
Presentational components are mostly concerned with generating some markup to be outputted. It gets data from its props, and just focuses on showing an element: The state can be initialized by props. Props can be used to set the internal state based on a prop value in the constructor, like this: Flow and TypeScript help a lot, but React has a way to directly help with props types, and even before running the code, our tools editors, linters can detect when we are passing the wrong values: In the previous article about the State you saw this example: What this means is that this is not defined unless you define methods as arrow functions: Hooks allow function components to access them too, in a different way.
First, there are 3 phases in a React component lifecycle: Mounting When mounting you have 4 lifecycle methods before the component is mounted in the DOM: Constructor The constructor is the first method that is called when mounting a component. You usually use the constructor to set up the initial state using this. Returns an object with the updated elements of the state or null if the state does not change render From the render method you return the JSX that builds the component interface.