JavaScript (JS) is a lightweight, interpreted, or just-in-time compiled programming language with
first-class functions. While it is most well-known as the scripting language for Web pages, many
non-browser environments also use it, such as Node.js, Apache CouchDB and Adobe Acrobat.
- JavaScriptis a prototype-based, multi-paradigm, single-threaded, dynamic language, supporting
object-oriented,
imperative, and declarative (e.g. functional programming) styles.
- The standards for JavaScript are the ECMAScript Language Specification (ECMA-262) and the
ECMAScript
Internationalization API specification (ECMA-402)
- Do not confuse JavaScript with the Java programming language. Both "Java" and "JavaScript" are
trademarks or registered trademarks of Oracle in the U.S. and other countries. However, the two
programming languages have very different syntax, semantics, and use.
You don't need any previous JavaScript knowledge, but you should have some basic background of :
- A general understanding of the Internet and the World Wide Web (WWW).
- Good working knowledge of HyperText Markup Language (HTML).
- Some programming experience. If you are new to programming, try one of the tutorials linked on
the main page about JavaScript.
How to add JavaScript to your HTML Page
JavaScript can be added to your HTML file in two ways:
- Internal JS: "We can add JavaScript directly to our HTML file by writing the
code inside the script tag. The "script" tag can either be placed inside the "head" or the
"body" tag according to the requirement."
- External JS: We can write JavaScript code in other file having an extension .js
and then link this file inside the "head" tag of the HTML file in which we want to add this
code.
You use variables as symbolic names for values in your application. The
names of variables, called identifiers, conform to certain rules.
A JavaScript identifier must start with a letter, underscore (_), or
dollar sign ($); subsequent characters can also be digits (0-9). Because
JavaScript is case sensitive, letters include the characters "A" through
"Z" (uppercase) and the characters "a" through "z" (lowercase).
You can use ISO 8859-1 or Unicode letters such as å and ü in identifiers.
You can also use the Unicode escape sequences as characters in
identifiers. Some examples of legal names are Number_hits, temp99, and
_name.
You can declare a variable in three ways:
With the keyword var. For example, var x = 42. This syntax
can be used to declare both local and global variables.
By simply assigning it a value. For example, x = 42. This
always declares a global variable. It generates a strict JavaScript
warning. You shouldn't use this variant.
With the keyword let. For example, let y = 13. This syntax
can be used to declare a block scope local variable. See Variable scope
below.
When you declare a variable outside of any function, it is called a
global variable, because it is available to any other code in the
current document. When you declare a variable within a function, it is
called a local variable, because it is available only within that
function.
JavaScript before ECMAScript 2015 does not have block statement scope;
rather, a variable declared within a block is local to the function (or
global scope) that the block resides within. For example the following
code will log 5, because the scope of x is the function (or global
context) within which x is declared, not the block, which in this case
is an if statement.
if (true) { var x = 5; } console.log(x); // 5
This behavior changes, when using the let declaration introduced in
ECMAScript 2015.
if (true) { let y = 5; } console.log(y); // ReferenceError: y is not
defined
Global variables are in fact properties of the global object. In web
pages the global object is window, so you can set and access global
variables using the window.variable syntax.
Consequently, you can access global variables declared in one window or
frame from another window or frame by specifying the window or frame
name. For example, if a variable called phoneNumber is declared in a
document, you can refer to this variable from an iframe as
parent.phoneNumber.
You can create a read-only, named constant with the const keyword. The
syntax of a constant identifier is the same as for a variable
identifier: it must start with a letter, underscore or dollar sign and
can contain alphabetic, numeric, or underscore characters.
const PI = 3.14;
A constant cannot change value through assignment or be re-declared
while the script is running. It has to be initialized to a value.
The scope rules for constants are the same as those for let block scope
variables. If the const keyword is omitted, the identifier is assumed to
represent a variable.
You cannot declare a constant with the same name as a function or
variable in the same scope. For example:
// THIS WILL CAUSE AN ERROR function f() {}; const f = 5; // THIS WILL
CAUSE AN ERROR ALSO function f() { const g = 5; var g; //statements
}
However, object attributes are not protected, so the following statement
is executed without problems.
const MY_OBJECT = {"key": "value"}; MY_OBJECT.key = "otherValue";
The latest ECMAScript standard defines seven data types:
Although these data types are a relatively small amount, they enable you
to perform useful functions with your applications. Objects and functions
are the other fundamental elements in the language. You can think of
objects as named containers for values, and functions as procedures that
your application can perform.
Use the if statement to execute a statement if a logical condition is
true. Use the optional else clause to execute a statement if the condition
is false. An if statement looks as follows:
if (condition) { statement_1; } else { statement_2; }
condition can be any expression that evaluates to true or false. See
Boolean for an explanation of what evaluates to true and false. If
condition evaluates to true, statement_1 is executed; otherwise,
statement_2 is executed. statement_1 and statement_2 can be any statement,
including further nested if statements.
You may also compound the statements using else if to have multiple
conditions tested in sequence, as follows:
if (condition_1) { statement_1; } else if (condition_2) { statement_2;
} else if (condition_n) { statement_n; } else { statement_last; }
In the case of multiple conditions only the first logical condition which
evaluates to true will be executed. To execute multiple statements, group
them within a block statement ({ ... }) . In general, it's good practice
to always use block statements, especially when nesting if statements:
if (condition) { statement_1_runs_if_condition_is_true;
statement_2_runs_if_condition_is_true; } else {
statement_3_runs_if_condition_is_false;
statement_4_runs_if_condition_is_false; }
It is advisable to not use simple assignments in a conditional expression,
because the assignment can be confused with equality when glancing over
the code. For example, do not use the following code:
if (x = y) { /* statements here */ } If you need to use an
assignment in a conditional expression, a common practice is to put
additional parentheses around the assignment. For example:
if ((x = y)) { /* statements here */ }
A while statement executes its statements as long as a specified condition
evaluates to true. A while statement looks as follows:
while (condition) statement If the condition becomes false,
statement within the loop stops executing and control passes to the
statement following the loop.
The condition test occurs before statement in the loop is executed. If
the condition returns true, statement is executed and the condition is
tested again. If the condition returns false, execution stops and
control is passed to the statement following while.
To execute multiple statements, use a block statement ({ ... }) to group
those statements.
Example:
The following while loop iterates as long as n is less than three:
var n = 0; var x = 0; while (n < 3) { n++; x += n; }
With each iteration, the loop increments n and adds that value to x.
Therefore, x and n take on the following values:
- After the first pass: n = 1 and x = 1
- After the second pass: n = 2 and x = 3
- After the third pass: n = 3 and x = 6
After completing the third pass, the condition n < 3 is no longer
true, so the loop terminates.
A function definition (also called a function declaration, or function
statement) consists of the function keyword, followed by:
- The name of the function.
-
A list of arguments to the function, enclosed in parentheses and
separated by commas.
-
The JavaScript statements that define the function, enclosed in curly
brackets, { }.
For example, the following code defines a simple function named square:
function square(number) { return number * number; }
The function square takes one argument, called number. The function
consists of one statement that says to return the argument of the
function (that is, number) multiplied by itself. The return statement
specifies the value returned by the function.
return number * number;
Primitive parameters (such as a number) are passed to functions by
value; the value is passed to the function, but if the function changes
the value of the parameter, this change is not reflected globally or in
the calling function.
What can be build using JS
JavaScript is a widely-used programming language. Given below are some domains/products that can be
built using JavaScript:
- Websites: JavaScript helps us to add behavior of our website. It helps users to
interact with the website. For eg. clicking on buttons, saving details, uploading details on the
website, etc.
- Web servers: We can make robust server applications using JavaScript. To be
precise we use JavaScript frameworks like Node.js and Express.js to build these servers.
- Game Development: In Game Development industry, JavaScript is used widely.
With the addition of HTML5 Canvas, it’s now possible to make 2D and 3D games in JavaScript very
efficiently.
- 3D Drawings: JavaScript in addition with HTML Canvas is used to make
three-dimensional graphics.
- Mobile Apps: Mobile applications are the most popular modes of communicating
these days. JavaScript also used to design mobile applications. There are many JavaScript
frameworks using which we can make android, IOS, and hybrid apps.
- Smartwatch Apps: The popular smartwatch maker Pebble has created Pebble.js, a
small JavaScript framework that allows a developer to create an application for the Pebble line
of watches in JavaScript.
Why to learn JavaScript ?
JavaScript is the most popular and hence the most loved language around the globe. Apart from this,
there are abundant reasons to learn it. Below are a listing of few important points:
- No need of compilers: Since JavaScript is an interpreted language, therefore it
does not need any compiler for compilations.
- Used both Client and Server-side:Earlier JavaScript was used to build
client-side applications only, but with the evolution of its frameworks namely Node.js and
Express.js, it is now widely used for building server-side applications too.
- Helps to build a complete solution:As we saw, JavaScript is widely used in both
client and server-side applications, therefore it helps us to build an end-to-end solution to a
given problem.
- Used everywhere: JavaScript is so loved because it can be used anywhere. It can
be used to develop websites, games or mobile apps, etc.
- Huge community support:JavaScript has a huge community of users and mentors who
love this language and take it’s legacy forward.