JavaScript Syntax

Posted by

JavaScript

Syntax


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

var x, y, z;       // Declare Variables
x = 5; y = 6;      // Assign Values
z = x + y;         // Compute Values

JavaScript Values

The JavaScript syntax defines two types of values:

  • Fixed values
  • Variable values

Fixed values are called

Literals

.

Variable values are called

Variables

.


JavaScript Literals

The two most important syntax rules for fixed values are:

1.

Numbers

are written with or
without decimals:

10.50
1001

2.

Strings

are text, written within double or single quotes:

“John Doe”
‘John Doe’





JavaScript Variables

In a programming language,

variables

are used to

store

data values.

JavaScript uses the

var

keyword to

declare

variables.

An

equal sign

is used to

assign values

to variables.

In this example, x is defined as a variable. Then, x is
assigned (given) the value
6:

var x;
x = 6;


JavaScript Operators

JavaScript uses

arithmetic operators

(

+


-


*


/

) to

compute

values:

(5 + 6) * 10

JavaScript uses an

assignment operator

(

=

) to

assign

values to variables:

var x, y;
x = 5;
y = 6;


JavaScript Expressions

An expression is a combination of values, variables, and operators,
which computes to a value.

The computation is called an evaluation.

For example, 5 * 10 evaluates to 50:

5 * 10

Expressions can also contain variable values:

x * 10

The values can be of various types, such as numbers and strings.

For example, “John” + ” ” + “Doe”, evaluates to “John Doe”:

“John” + ” ” + “Doe”


JavaScript Keywords

JavaScript

keywords

are used to
identify actions to be performed.

The

var

keyword tells the browser to create variables:

var x, y;
x = 5 + 6;
y = x * 10;


JavaScript Comments

Not all JavaScript statements are “executed”.

Code after double slashes

//

or between

/*

and

*/

is treated as a

comment

.

Comments are ignored, and will not be
executed:

var x = 5;   // I will be executed
// var x = 6;   I will
NOT be executed

You will learn more about comments in a later chapter.


JavaScript Identifiers

Identifiers are names.

In JavaScript, identifiers are used to name variables (and keywords, and
functions, and labels).

The rules for legal names are much the same in most programming languages.

In JavaScript, the first character must be a letter, or an underscore (_), or a
dollar sign ($).

Subsequent characters may be letters, digits, underscores, or dollar signs.

Numbers are not allowed as the first character.
This way JavaScript can
easily distinguish identifiers from numbers.


JavaScript is Case Sensitive

All JavaScript identifiers are

case sensitive

.

The variables

lastName

and

lastname

,
are two different variables:

var lastname, lastName;
lastName = “Doe”;
lastname = “Peterson”;

JavaScript does not interpret

VAR

or

Var

as the keyword

var

.


JavaScript and Camel Case

Historically, programmers have used different ways of joining multiple words into one variable name:


Hyphens:

first-name, last-name, master-card, inter-city.

Hyphens are not allowed in JavaScript. They are reserved for subtractions.


Underscore:

first_name, last_name, master_card, inter_city.


Upper Camel Case (Pascal Case):

FirstName, LastName, MasterCard, InterCity.


Lower Camel Case:

JavaScript programmers tend to use camel case that starts with a lowercase letter:

firstName, lastName, masterCard, interCity.


JavaScript Character Set

JavaScript uses the

Unicode

character set.

Unicode covers (almost) all the characters, punctuations, and symbols in the world.

For a closer look, please study our
Complete Unicode Reference
.