JSX in ReactJS: a quick intro with examples

What is JSX file 

  • A JSX file can have HTML code in a JavaScript file. 
  • JSX elements are treated as JavaScript expressions. 
  • JSX element looks exactly like HTML, this can be put in a JS file instead of an HTML file. 
  • JSX element can be saved a variable, can be passed in a function or can also be stored in an object or an array.
  • JSX is not a JavaScript, so browsers can’t read it.
  • A JSX compiler is needed, to compile to JSX to covert to native JS
  • We can give attribute in JSX elements just like HTML
    e.g.; <p id="test"></p>
  • Use parentheses to wrap multi-line JSX expressions
  • JSX outer element should be exactly one, if not wrap all the code in a <div> </div>
  • ReactDOM is the JSX renderer.
  • ReactDOM.render() is the most common way to render JSX. It takes a JSX expression, creates a corresponding tree of DOM nodes, and adds that tree to the DOM. 
  • The first argument to ReactDOM.render is a JSX element or a variable having the JSX expression, the second argument is the target.
  • One special thing about ReactDOM.render() is that it only updates DOM elements that have changed.  This is significant! Only updating the necessary DOM elements is a large part of what makes React so successful. React does this by something called Virtual DOM 

More JSX

  • JSX and HTML grammar are mostly the same except class attribute, in JSX you will have to use ClassName, this is because JSX is get converted to native JS and class is reserved there 
  • Self-closing tags — One has to add a mandatory slash (/) e.g.; <br /> in HTML slash is optional.
  • We can also write native JavaScript inside a JSX file
  • Curly braces are used to embed the JS code inside JSX expression, any JS code inside the curly braces will be executed.


HTML inside JSX -> output: 2 + 3
import React from 'react';    
import ReactDOM from 'react-dom';    
ReactDOM.render(<h1>2 + 3</h1>,document.getElementById('app'));

JS inside JSX-> output: 5
import React from 'react';    
import ReactDOM from 'react-dom';    
ReactDOM.render(<h1>{2 + 3}</h1>,document.getElementById('app'));

More about JSX and React on my next post.

Keep learning!

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