JavaScript

JavaScript is the programming language of HTML and the Web.
JavaScript is used to program the behavior of web pages. It can:
- Change HTML content.
- Change HTML attribute values.
- Change the style of an HTML element.
- Hide HTML elements.
- Show hidden HTML elements.

https://www.w3schools.com/js/js_examples.asp

Adding JavaScript to HTML

In HTML, JavaScript code must be inserted between <script> and </script> tags.
You can place any number of scripts in an HTML document.
Scripts can be placed in the <body>, or in the <head> section of an HTML page, or in both.
To use an external script, put the name of the script file in the src (source) attribute of a <script> tag:

https://www.w3schools.com/js/js_whereto.asp

Examples

<!DOCTYPE html>
<html>
  <head>
  <!-- myFunction1 is in <head> and run before <body> is rendered -->
    <script>
      function myFunction1() {
        document.getElementById('demo').innerHTML = 'Button 1 was clicked.';
      }
    </script>
    <!-- myFunction2 is in external file -->
    <script src="script.js"></script>
  </head>
  <body>
    <h2>JavaScript in Head</h2>

    <p id="demo">Click a button.</p>

    <button type="button" onclick="myFunction1()">1</button>
    <button type="button" onclick="myFunction2()">2</button>
    <button type="button" onclick="myFunction3()">3</button>

    <!-- myFunction3 is last tag inside <body>, but before ending </body> tag. Scripts will run after <body> is rendered -->
    <script>
      function myFunction3() {
        document.getElementById('demo').innerHTML = 'Button 3 was clicked.';
      }
    </script>
  </body>
</html>

http://next.plnkr.co/edit/25j2P5c6q9L4RQYK?preview

Syntax

JavaScript syntax is the set of rules, how JavaScript programs are constructed

var x, y, z;       // How to declare variables
x = 5; y = 6;      // How to assign values
z = x + y;         // How to compute values

Variables
- Used to store data values.
- var keyword to declare variables.
- An equal sign is used to assign values to variables.
Operators
- JavaScript uses arithmetic operators ( + - * / ) to compute values.
- JavaScript uses an assignment operator ( = ) to assign values to variables.

function myFunction(p1, p2) {
  return p1 * p2;   // The function returns the product of p1 and p2
}

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().
Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).
The parentheses may include parameter names separated by commas: (parameter1, parameter2, ...)
The code to be executed, by the function, is placed inside curly brackets: {}

https://www.w3schools.com/js/js_functions.asp

https://www.w3schools.com/js/js_syntax.asp