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This playground aims to show you how to write a React application quickly by covering what I consider to be the most important aspects of the library in a straight to the point manner. This chapter is an introduction to React.
Why learn React?
The fact that React is "just" a library makes it very compelling to learn: the API surface is quite small, and you can quickly decide if you like it or not. The functional nature of React components, with local state, immutable parameters, and no side effects, combined with a unidirectional data flow, lead to better code and fewer bugs and is interesting to learn if you haven't used functional programming before. Finally, the community around React is very active and it has a rich ecosystem that you can benefit from and contribute to.
Principles of React
A user interface in React is built with components. A component has its own mutable state, and it cannot modify the state of other components. It now seems logical, but remember that when React was released the most used framework was AngularJS with 2-way binding by default, and components written as directives modifying the DOM (components were only introduced in AngularJS 1.5 released in early 2016). A React component can include other components and specify values based on its own state.
React uses a virtual DOM rather than the DOM directly to optimize rendering. This is because operations on the DOM usually involve many steps (layout, styling) in addition to the tree operations themselves. React determines when a component needs rendering
This is not a goal that is explicitly stated on their website, but I believe that this is a core aspect of React. By choosing to be only about the view, it ends up being a relatively small library with few concepts to grasp. Once you know React, you can then choose other libraries to handle other things than rendering; or you may stick with React only, if you prefer. Your choice!
Your first React component
The first code snippet on the right is a minimalist React application with one
Counter component that we will use as an example. This component can be used to store a number of items and increment/decrement it. Think of the shopping cart for your favorite e-commerce website.
Click the Run button below to compile the example given in the file
counter-basic.js and render it:
After importing the
ReactDOM modules, we define a
Counter function. Notice anything strange?
The function seems to return HTML directly! Actually, it does not return HTML but a tree of React elements (the so-called virtual DOM), and this is not HTML but JSX.
React.createElement to create elements with children and attributes.
You can also see JSX in the last line here, as the first argument to
ReactDOM.render(<Counter />, document.getElementById('root'));
In fact, a JSX element can be any classic HTML-like element like
<div>, or a React component, like our
<Counter /> element here. This allows components to instantiate other components.
There are three things to keep in mind when using JSX:
- a JSX template is a tree and as such it must have a single root element. If your rendering function returns several elements, you'll have to put them in a single top-level element like a
returnstatement and the JSX expression for readability, you must surround the JSX expression with parentheses. Otherwise, as stated in the ECMAScript standard, Automatic Semicolon Insertion will insert a semicolon before your expression, because there should not be a line terminator between a
returnstatement and its value (if it has one).
- when you generate a list of elements, each element should have a
keyproperty with a distinct value, for performance reasons. If you don't do that, the code will still compile, but you'll see warnings in the console.
Fix the code of the file
counter-jsx.js on the second code tab to to follow these rules:
There are also restrictions regarding attribute names (you cannot use an attribute named
First, if you prefer to isolate the view in a separate file, go for it: put the rendering function in its own file, and bam! there you go. Then, ask yourself: does it really change anything, other than having twice as many files?
Then again, remember that you're not really writing HTML; JSX is just easier to use than creating elements by hand. The thing is, if you are targeting a browser (you don't have to by the way, you could build a mobile app using React native instead), at some point this is going to require one of two things: either you give the browser some fixed HTML, or you manipulate the DOM dynamically. React does the latter, it just happens to be convenient to describe the view with HTML-like syntax (JSX). If you're not happy with JSX you could very well write a completely different language that would be more like a Pug (formerly Jade) template or an S-expression such as Racket's X-expression:
(html (body ((bgcolor "red")) (h1 "Hi!") (br) "Bye!"))