3. React Basics and Working With Components - Udemy React Course

This section of the React Essentials Course will guide students through the creation of a demo web application using React, starting from scratch. It will cover vital React concepts such as components, JSX, props, and state. The course will teach students how to display data and make apps interactive by handling user events. By the end of the section, students will be able to build both static and dynamic interactive React applications, with no prior React knowledge required, although a basic understanding of JavaScript is necessary.

The core concept of React and its ecosystem is the use of Components. Components are reusable building blocks used to construct the user interface (UI) of React applications, regardless of their complexity. A well-designed React app is made by combining these Components, which encapsulate HTML, CSS, and JavaScript logic to define and control parts of the UI. This approach allows for easy management of complex UIs, enables code reuse, and ensures that related code is kept together, which simplifies development and reduces the likelihood of errors. Components also promote a separation of concerns, where different components handle different aspects of the UI. This pattern is beneficial not only in React but also in other front-end frameworks such as Angular, Vue, and Svelte, and extends beyond web development into areas like mobile development with frameworks like Flutter. The course will explore the creation and use of React Components in depth.

The content explains how to get started with React components by using a starting-project. There are two options for working on the project: using a provided CodeSandbox link for a browser-based environment or downloading an alternative zip file for local development.

For local development:

For CodeSandbox users:

In summary, the user is provided with a starting React project and guided on how to set up the development environment either locally or online using CodeSandbox to begin coding with React components.

The provided text talks about the initial setup of a React project. It mentions that the index.html file in the project is quite bare because React is responsible for rendering the content on the screen. This rendering is done through JavaScript files with .jsx extensions, which include index.jsx and App.jsx. The .jsx extension indicates that these files contain JSX (JavaScript Syntax Extension), which allows developers to write HTML within JavaScript files for creating user interfaces.

The index.jsx file doesn’t contain the actual content like images or titles, but it imports from the App.jsx file, which does contain the markup for what’s displayed on the website. However, the text highlights that JSX is not natively supported by browsers, so the code written by developers is transformed by the development server into browser-compatible code.

The App.jsx file is described as a React Component, which in React is essentially a JavaScript function with two key rules: the function name must start with an uppercase character, and it must return a renderable value, usually the HTML markup to be rendered, written in JSX. The author then suggests moving on to create a custom React Component following these principles.

When building React applications, you are encouraged to create your own components, potentially many of them, to structure your app. You can create a custom component by defining a new JavaScript function in the same JSX file as your main app component, though later you might place components in separate files. This function should be named with an uppercase first letter and will return JSX code that represents the component’s UI. The JSX code block should be wrapped in parentheses, especially if it spans multiple lines, and most code editors like VS Code or CodeSandbox will automatically format this for you.

To use the custom component within your app, you should reference it as a JSX tag (e.g., <Header />) in your app component’s JSX code. This tag can be self-closing but must include a forward slash before the closing bracket (e.g., <Header />). The creation and use of custom components are fundamental skills for a React developer, and while more complex features and concepts exist, this describes the basic process of building and integrating custom components in React.

This text explains how a React component gets rendered onto a website. When inspecting the source code of a website that uses React, you will not find the actual content like images or titles, but rather metadata and JavaScript files. The JavaScript file, typically named index.jsx, is where the transformed React code resides. This file imports the App component from App.jsx using standard JavaScript import/export syntax.

The App component is then passed as JSX code to the render method of the ReactDOM library, which is responsible for outputting the component’s content on the screen. This rendering process begins with the createRoot method that takes an existing HTML element as input (like a div with the id root) and sets it as the root for the React app. React then injects the App component into this element, which may contain nested components, forming a hierarchy or tree of components that gets rendered to the screen.

The rendered DOM, however, only shows default HTML elements and not the custom React components. React analyzes the component tree and generates the overall DOM from the JSX code, allowing developers to work with individual building blocks rather than a single large file. Custom component names must start with an uppercase letter to differentiate them from built-in elements, which start with lowercase letters. Custom components are executed as functions by React, and their returned JSX code is analyzed until React ends up with only built-in elements that are rendered to the screen. This process illustrates how React works with components and renders them onto the screen.

The content explains how to incorporate dynamic content into a React component. Specifically, it demonstrates how to randomly switch between different phrases—'Fundamental React concepts', 'Crucial React concepts', and 'Core React concepts'—within a Header component. The approach involves using JavaScript expressions inside curly braces within the JSX code to dynamically generate content. The provided code snippet introduces an array of possible phrases and a function to generate a random index, which is used to select a random phrase from the array each time the component is rendered. This dynamic expression can be placed directly in the JSX or extracted into a variable for cleaner code and is evaluated whenever the component is rendered, such as when the page is reloaded.

The provided text explains the correct way to include images in a React project for optimal loading and to ensure they are included during the deployment process. Instead of directly setting the source attribute with a path to the image file, it is recommended to import the image using an import statement, which allows the build process to handle the image as part of the code transformation and optimization. This import creates a JavaScript variable that holds the path to the image, which can then be used as the value for the src attribute inside the JSX code using curly braces without quotes. This method ensures images are bundled correctly and can benefit from additional optimization steps during deployment.

The text explains how to use and reuse components in React. Components are reusable pieces of UI, and while some may only be used once, others are designed to be used multiple times with different data. React introduces the concept of props, which allows passing data into components to render them with specific information.

A new React component called CoreConcept is created to display items with an image, title, and description. The App component is then modified to include a section for CoreConcept items, where each item receives different data via props.

Props are passed to components by adding custom attributes, and the values for these attributes can be strings, numbers, objects, or arrays. In the component function, a single parameter, typically named props, is used to access these values. React automatically fills this props object with all the key-value pairs specified by the custom attributes.

By using props, the CoreConcept component can be reused multiple times with different data for each instance. This way, dynamic and reusable UI elements are created in a React application. The remaining items and use of props will be further discussed in the following lecture.

The content explains how to dynamically import and use data in a React component. Specifically, it discusses how to use an array of objects containing properties like image, title, and description from a data.js file in an App.jsx file. The steps include importing the CORE CONCEPTS array using named imports with curly braces, accessing array items by index to set component props, and using the spread operator to pass all properties of an object as props to a component for cleaner code. Additionally, it covers object destructuring in the component’s parameter list to directly use the properties as variables, which results in less verbose and more readable code. The overall message is to illustrate different ways to pass data to components and how to simplify the code while maintaining functionality.

The content explains different ways to handle properties (props) in React components:

The text assures the reader that these concepts will be illustrated with concrete examples throughout the course.

The content discusses the organization of React components in a project. Initially, all the components (Header, CoreConcept, and App) are in a single App.jsx file. The text points out that while this setup technically works, it is not recommended for larger projects due to maintainability issues. The suggested approach is to create separate files for each component, typically within a Components subfolder inside the SRC directory.

Here’s a summary of the steps and best practices mentioned:

By following these steps, each component is now in its own file, which aligns with common best practices and improves the project’s structure and maintainability as it grows.

The content discusses organizing CSS styles for a React project by splitting them into separate files specific to individual components. It suggests moving the CSS rules related to the header component from the main index.css file to a new header.css file placed next to the header.jsx file. To apply these styles, the header.css file must be imported into the header.jsx component file using an import statement. The author points out that importing CSS in this way doesn’t scope the styles to just that component; the styles would affect any similar elements on the page. As a solution to this limitation, the author hints at a future discussion on style scoping.

The author also recommends creating a subfolder within the Components folder for each component, like the header, to keep related files organized. After this structural change, the import paths in the affected JSX files must be updated to reflect the new folder locations. The summary of this content is:

The provided text discusses the process of developing an interactive section for a website application within the App.jsx file. The author aims to create a new component for tab buttons and outlines the following steps:

In summary, the text explains how to create a TabButton component in React that utilizes the children prop for component composition to display an interactive tab button section on a website. The author emphasizes the flexibility of React components, allowing developers to choose between different methods of content delivery based on preference and use case.

The provided text discusses how to make the TabButton component in a React application interactive. The goal is to show different content when the buttons are clicked. In React, instead of using the imperative approach of vanilla JavaScript (like using querySelector and addEventListener), developers use a declarative approach. This involves adding an onClick prop to the button element within the TabButton component, which takes a function as its value. This function, named handleClick, will be defined within the component function and will execute when the button is clicked. The handleClick function initially contains a console.log("Hello World!") to test the interactivity. It’s important to pass the function itself as a value to the onClick prop without executing it with parentheses, allowing React to handle the execution upon a click event. This step is crucial in making the application more interactive by responding to user events.

The text explains how to update dynamic content in a React application based on button clicks using custom button components. The process involves passing a function as a prop (named onSelect or similar) from the parent App component to a custom button component. This function is designed to be triggered when the button is clicked. Inside the custom button component, this onSelect prop is then used to set the value for the native onClick event handler of the HTML button element. The result is that clicking the button activates the handleSelect function in the App component, which can then be used to update the dynamic content in the application. This pattern of passing event handler functions as props to child components is common in React projects and allows for the dynamic manipulation of content based on user interactions.

The text describes a process for handling button clicks in a React application. The main points are:

In summary, the text outlines a method for updating dynamic content in a React application based on button clicks, utilizing a function that takes an identifier parameter and arrow functions to correctly handle events with custom arguments.

The provided text describes an issue encountered while working with a React application. The author explains that they’ve attempted to update the displayed content based on which tab button is pressed using a variable called tabContent. Initially, the variable is set to prompt the user to click a button. However, despite updating tabContent when a button is pressed, the UI does not reflect any changes.

The reason for this behavior is that React components re-render only when their state changes. Since tabContent is a regular variable and not part of the component’s state, React does not recognize the need to re-render the component. As a result, the UI remains unchanged even when tabContent is updated.

The author demonstrates this by adding console.log statements, showing that while the handleSelect function executes and updates tabContent, the main component function does not re-execute, and therefore the UI is not updated.

The issue highlights the need for using state in React components to ensure the UI updates when data changes. The author suggests that they will explore the concept of state next as a solution to this problem.

React components do not re-execute when regular variables change, so they cannot be used to update the UI. Instead, React provides a concept called state, which allows components to re-render when the state changes. State in React is managed using the useState hook, which is imported from the React library. When using useState, you initialize it with a default value and it returns an array with two elements: the current state value and a function to update that state. This function, when called, will also signal React to re-render the component.

The state-related variables and updater functions must be called at the top level of a React component function, not inside loops, conditions, or nested functions. This is a rule of React Hooks. The first element of the array returned by useState is the managed state value, and the second is the function to update that state.

When the state update function is called, React schedules the update and re-executes the component function to reflect the new state. However, the updated state value is only available after the component function re-executes, not immediately after the state update function is called.

In summary, useState allows developers to create stateful components in React, enabling interactive and dynamic user interfaces where the UI can respond to user actions and state changes.

The provided text discusses the implementation of a feature that displays content based on button clicks in a React application. The application has been updated to include a data file that exports an examples object containing keys that match button identifiers (state, props, JSX, components). Each key has an object with a title, description, and code to be displayed.

To implement this, a div with the ID tab-content is added to the App.jsx file, containing an h3 element for the title, a paragraph for the description, and a pre tag with a code element to showcase the code example. The examples object is imported into the app.jsx file, and the content is dynamically rendered based on the state that stores the selected button identifier.

Initially, there was an error because the default state did not match any of the keys in the examples object. To fix this, the initial state is set to "components," which is a valid key, eliminating the error and allowing the correct information to display upon initial page load. Now, when different buttons are clicked, the content changes to display the relevant information for each topic.

The content discusses different methods of conditionally rendering content in a React application, specifically when dealing with a situation where you want to display different data based on the button pressed on a webpage.

Initially, the page is set to display "Components" data, but the preference is to show a message like "please select a topic" before any selection is made. To implement this behavior, the initial state is set to an undefined value to indicate no topic is selected.

Several approaches to conditionally render content in React are described:

The content emphasizes that as a React developer, it’s essential to understand these different approaches, as they will be encountered in real-world projects. It’s up to the developer to choose the method they find most readable and understandable.

The text discusses how to dynamically style HTML elements in React, specifically how to indicate which tab is active in a tabbed interface. It explains that in React, you should use the className prop instead of class to set CSS classes, which is JSX-specific. The example provided involves adding an "active" class to a button to highlight it when it’s selected. To do this dynamically, a new isSelected boolean prop is introduced for the TabButton component, which determines if the button is active. This prop is used with a ternary expression inside curly braces to set the className to "active" when the button is selected and to an empty string or undefined when it is not. This dynamic styling is based on the component’s state (selectedTopic), which changes when different tabs are clicked. The App component passes a true or false value to the isSelected prop of each TabButton based on whether its selectedTopic state matches the button’s corresponding identifier. This results in the selected tab being visually indicated to the user.

The demo application is functionally complete, but the code can be improved, particularly in the way the core concept components are rendered. Currently, the components are manually repeated, which creates two issues: unnecessary typing and potential breaks if the data source changes (e.g., if an array element is removed but the code still tries to output four components).

The suggested improvement is to dynamically generate the core concept components based on the number of items in the core concepts array. This can be done using JSX’s ability to render arrays of renderable data, including arrays of JSX elements. The transformation of the array of JavaScript objects (which JSX cannot directly render) into an array of JSX elements is achieved using the JavaScript map method. This method allows for iterating over each item in the array and converting it into the desired JSX code.

When implementing this, developers must be aware of the React warning regarding unique key props for list items. This warning can be resolved by adding a unique key prop to each list item component, which helps React to efficiently render and update the list. A unique identifier, such as the title of the item, can be used for the key prop.

In summary, the app’s code can be optimized by using the map method to dynamically render list items based on data, which also resolves issues with manual repetition and potential data source changes. Adding unique key props to each list item is essential for efficient rendering and to prevent React warnings.

This section of the course covered the essentials of creating an interactive demo web app using React. Key concepts included:

The section prepared learners to start building basic React apps and set the foundation for diving deeper into React’s essentials and advanced features in the course.