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The Ultimate Guide To Learning React.js

· 11 min read
Louis Petrik

This article serves as a broad overview of React.js. For more details, check out the linked pages.

React.js is a popular JavaScript library for building user interfaces, especially for single-page applications. It's used for handling the view layer in web and mobile apps. React allows you to design simple views for each state in your application, and it will efficiently update and render the components when your data changes. The core aspects of React that every developer should grasp include components, state and props, hooks, routing with React-Router, Context API, and higher order components.

Introduction to React.js​

React.js was developed by Facebook to address the need for a dynamic and high performing User Interface(UI). The main feature of React.js is the ability to break down complex UI into simpler components by utilizing the power of JSX. Not only does this make the code more readable and easier to maintain, but it also improves performance through efficient DOM manipulation.

import React from 'react';
import ReactDOM from 'react-dom';

function Hello() {
  return <h1>Hello, world!</h1>;
}

ReactDOM.render(
  <Hello />,
  document.getElementById('root')
);

This is a simple example of a functional component in React. We have a function Hello that returns a single <h1> element. The ReactDOM.render method then takes the Hello component and renders it to the DOM at the root div.

Understanding React.js Components​

Components are the building blocks of any React application, and a single app usually consists of multiple components. A component is essentially a JavaScript class or function that accepts inputs (called props) and returns a React element that describes how a section of the UI should appear.

import React from 'react';

functino Welcome() {
    return <h1>Welcome, {this.props.name}</h1>;
}

In the above code snippet, Welcome is a simple React class component that takes in a prop (in this case, name) and outputs an <h1> element. You can use the Welcome component like any other HTML tag, <Welcome name="John Doe" />, and it will output <h1>Welcome, John Doe</h1>.

Grasping React.js State and Props​

In React, both state and props are JavaScript objects. While both of them hold information that influences the output of render, they are different in their functionality with respect to component. Props (short for properties) are a way of passing data from parent to child components, whereas state is a data structure that allows a component to control its own rendering.

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={handleClick}>
        Click me
      </button>
    </div>
  );
}

In this code snippet, we have a simple Counter component that increments a value every time a button is clicked. The state is initialized in the constructor and then updated using the setState method.

Introduction to React Hooks​

React Hooks are a new addition in React 16.8 that lets you use state and other React features without writing a class. Hooks are functions that let you "hook into" React state and lifecycle features from function components. You can even write your own, custom hooks. The state of the component can be manipulated using the useState hook.

import { useState } from "react";

function FavoriteColor() {
  const [color, setColor] = useState("red");

  return (
    <>
      <h1>My favorite color is {color}!</h1>
      <button
        type="button"
        onClick={() => setColor("blue")}
      >Blue</button>
    </>
  )
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<FavoriteColor />);

In the above example, we first import the useState hook from the react library. Then, in the FavoriteColor component, we declare a state variable color, and set its initial value to "red". The useState hook returns a pair: the current state value and a function that lets you update it. You can call this function (setColor) from an event handler or somewhere else to update the state. The component will re-render with the new state when the setState function is called. This example also demonstrates how to read state in the component and how to update it. The useState hook can be used to keep track of any type of data including strings, numbers, booleans, arrays, and objects.

Styling React components​

React.js is a go-to choice for many developers when creating user interfaces. But, what about styling those interfaces? The truth is, styling in React.js is as integral as any other aspect of your application, bringing your UI to life. Let’s explore two of the most popular methods for styling in React.js: CSS-in-JSX and Styled-Components.

CSS-in-JS​

In a typical HTML and CSS setup, you might have separate stylesheets. But with React.js, you can inject your CSS right into your JSX. This approach, known as CSS-in-JS, is a powerful tool in creating dynamic, visually appealing components.

const myStyle = {
  color: "white",
  backgroundColor: "DodgerBlue",
  padding: "10px",
  fontFamily: "Arial"
};

const MyComponent = () => {
  return <h1 style={myStyle}>Hello Style!</h1>;
};

In the above example, the myStyle object contains CSS properties and values, defined in camelCase notation. This style object is then passed to the style attribute of the HTML element in the JSX. The resulting h1 tag will have a white color text on a DodgerBlue background, styled with Arial font and a padding of 10px.

Leveraging the Power of Styled-Components​

While CSS-in-JSX provides a quick way to style components, styled-components bring an extra level of power and flexibility. Styled-components is a library for React and React Native that allows you to use component-level styles in your application that are written with a mixture of JavaScript and CSS.

import styled from 'styled-components';

const StyledButton = styled.button`
  background-color: coral;
  color: white;
  font-size: 1em;
  padding: 0.25em 1em;
  border: 2px solid palevioletred;
  border-radius: 3px;
`;

const MyComponent = () => {
  return <StyledButton>Click me</StyledButton>;
};

In this code snippet, we first import the styled object from the styled-components library. We then create a StyledButton component using the styled.button syntax. The CSS for this component is written within a template literal, enclosed in backticks. The resulting StyledButton component can be used just like any other React component.

Routing with React-Router​

React Router is a standard library system built on top of the React and used to create routing in the React application using React Router Package. It provides the synchronous URL on the browser with data that will be displayed on the web page. It maintains the standard structure and behavior of the web application and is used for developing single page web applications.

import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import Home from "./Home";
import About from "./About";

function App() {
  return (
    <Router>
      <Switch>
        <Route path="/about">
          <About />
        </Route>
        <Route path="/">
          <Home />
        </Route>
      </Switch>
    </Router>
  );
}

In the above example, the BrowserRouter is used as a <Router> component. The <Switch> component is used to render only the first <Route> or <Redirect> that matches the location. The <Route> component is used to define the mapping between the URL path and the component that should be rendered. The <Route path="/about"> means that the About component will be rendered when the URL path is '/about'.

Context API​

React's Context API is a feature for component tree-wide state management. A context in React is a way to share values between components without having to explicitly pass a prop through every level of the tree.

const colors = {
  blue: "#03619c",
  yellow: "#8c8f03",
  red: "#9c0312"
};

export const ColorContext = React.createContext(colors.blue);

// In your App component
import React from 'react';
import { ColorContext } from "./ColorContext";

function App() {
  return (
    <ColorContext.Provider value={colors}>
      <Home />
    </ColorContext.Provider>
  );
}

// Using the Consumer
return (
  <ColorContext.Consumer>
    {colors => <div style={colors.blue}>Hello World</div>}
  </ColorContext.Consumer>
);

// Using useContext Hook
import React, { useContext } from "react";
import ColorContext from './ColorContext';

const MyComponent = () => {
  const colors = useContext(ColorContext);

  return <div style={{ backgroundColor: colors.blue }}>Hello World</div>;
};

React's Context API allows you to share specific data from all levels of your application, thereby solving problems related to prop-drilling. The above example demonstrates how to create a context using the createContext method and pass data as a prop【22†source】. The Provider component enables the data in your context throughout your entire application. It wraps the context of your function and facilitates its functionality throughout【23†source】. The Consumer component allows you to subscribe to a context’s changes, it will update and adjust your application based on the modification【24†source】. Lastly, the useContext Hook is used in functional components to access the context within a functional component and works with a Provider and Consumer in one call【26†source】.

Higher Order Components​

Higher-Order Component (HOC) is an advanced technique in React for reusing component logic. It's not a feature built into React, but rather a pattern that emerges from React’s compositional nature. A higher-order component is a function that takes a component and returns a new component. More details on HOCs in plain JavaScript.

function withExtraPropAdded(component) {
  const Component = component;
  return function(props) {
    return <Component {...props} extraProp="someValue" />;
  }
}

// Use this HOC in another component
const ComponentWithExtraProp = withExtraPropAdded(SomeComponent);

In this example, withExtraPropAdded is a higher-order component that takes a component and returns a new component with an extra prop. The SomeComponent is wrapped by the HOC and the resulting component ComponentWithExtraProp has an additional prop extraProp.

Conclusion​

React.js offers a robust solution for building user interfaces in JavaScript. Its component-based architecture allows developers to build complex UIs from isolated and reusable pieces of code. Features like state, props, context, and hooks enable us to manage data and side-effects in our application. Libraries like React Router aid in creating single-page applications with multiple views and linking. By understanding these core concepts, you can leverage the full potential of React.js in your projects and create efficient, scalable, and maintainable web applications.

FAQ​

What is React.js and what are its core concepts?​

React.js is a popular JavaScript library for building user interfaces, especially for single-page applications. It's used for handling the view layer in web and mobile apps. React allows you to design simple views for each state in your application, and it will efficiently update and render the components when your data changes. The core aspects of React that every developer should grasp include components, state and props, hooks, routing with React-Router, Context API, and higher order components.

There are two popular methods for styling in React.js: CSS-in-JSX and Styled-Components. CSS-in-JS is a powerful tool in creating dynamic, visually appealing components by injecting your CSS right into your JSX. On the other hand, styled-components is a library for React and React Native that allows you to use component-level styles in your application that are written with a mixture of JavaScript and CSS.