JavaScript Variables can be declared in 4 ways:
varletconstIn this first example, x, y, and z are undeclared variables.
They are automatically declared when first used:
x = 5;
y = 6;
z = x + y;
JavaScript comments can be used to explain JavaScript code, and to make it more readable.
JavaScript comments can also be used to prevent execution, when testing alternative code.
Single line comments start with //.
Any text between // and the end of the line will be ignored by JavaScript (will not be executed).
This example uses a single-line comment before each code line:
// Change heading:
document.getElementById("myH").innerHTML = "My First Page";
// Change paragraph:
document.getElementById("myP").innerHTML = "My first paragraph.";
Unicode is a universal character set that defines all the characters needed for writing the majority of living languages in use on computers.
Unicode aims to be (and already is) a superset of all other encoded computer character sets.
The Unicode Standard covers (almost) all characters, punctuations, and symbols in the world and enables processing, storage, and transport of text independent of platform and language.
The Unicode Consortium develops the Unicode Standard. The goal is to replace existing character sets with UTF (Unicode Transformation Format).
The Unicode Standard is implemented in HTML, XML, JavaScript, E-mail, PHP, Databases and in all modern operating systems and browsers.
Unicode can be implemented by different character sets. The most commonly used encodings are UTF-8 and UTF-16:
| Charset | Description |
|---|---|
| UTF-8 | A variable-length character encoding (1 to 4 bytes long). UTF-8 is backwards compatible with ASCII and the preferred encoding for e-mail and web pages. |
| UTF-16 | A variable-length character encoding. UTF-16 is used in all major operating systems like Windows, IOS, and Unix. |
The first 128 characters of UTF-8 have the same binary values as ASCII, making ASCII text valid UTF-8.
The default character set in HTML-4 (ISO-8859-1) were limited in size and not compatible in multilingual environments.
The default character encoding in HTML-5 is UTF-8.
HTML5 pages using a different character set than UTF-8 must specify this a <meta> tag:
<meta charset=”ISO-8859-1″>
JavaScript syntax is the set of rules, how JavaScript programs are constructed:
// How to create variables:
var x;
let y;
// How to use variables:
x = 5;
y = 6;
let z = x + y;
The JavaScript syntax defines two types of values:
Fixed values are called Literals.
Variable values are called Variables. Continue reading JavaScript Syntax
let x, y, z; // Statement 1
x = 5; // Statement 2
y = 6; // Statement 3
z = x + y; // Statement 4
In JavaScript you cannot use these reserved words as variables, labels, or function names:
| abstract | arguments | await* | boolean |
| break | byte | case | catch |
| char | class* | const* | continue |
| debugger | default | delete | do |
| double | else | enum* | eval |
| export* | extends* | false | final |
| finally | float | for | function |
| goto | if | implements | import* |
| in | instanceof | int | interface |
| let* | long | native | new |
| null | package | private | protected |
| public | return | short | static |
| super* | switch | synchronized | this |
| throw | throws | transient | true |
| try | typeof | var | void |
| volatile | while | with | yield |
Words marked with* was new in ECMAScript 5 and ECMAScript 6.
JavaScript can “display” data in different ways:
innerHTML.document.write().window.alert().console.log().To access an HTML element, JavaScript can use the document.getElementById(id) method.
The id attribute defines the HTML element. The innerHTML property defines the HTML content:
<!DOCTYPE html>
<html>
<body>
<h1>My First Web Page</h1>
<p>My First Paragraph</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = 5 + 6;
</script>
</body>
</html>
In HTML, JavaScript code is inserted between <script> and </script> tags.
<script>
document.getElementById("demo").innerHTML = "My First JavaScript";
</script>
JavaScript is the programming language of the web.
It can update and change both HTML and CSS.
It can calculate, manipulate and validate data.
JavaScript is one of the 3 languages all web developers must learn :
1. HTML to define the content of web pages
2. CSS to specify the layout of web pages
3. JavaScript to program the behavior of web pages