JavaScript Syntax
JavaScript Programs
A computer program is a list of "instructions" to
be "executed" by the computer.
In a programming language, these program instructions are
called statements.
JavaScript is a programming language.
JavaScript statements are separated by semicolons:
Example
var x, y, z;
x = 5;
y = 6;
z = x + y;
»
In HTML, JavaScript programs are executed by the web browser.
JavaScript Statements
JavaScript statements are composed of:
Values, Operators, Expressions, Keywords, and
Comments.
JavaScript Values
The JavaScript syntax defines two types of values: Fixed
values and variable values.
Fixed values are called literals. Variable values are called
variables.
JavaScript Literals
The most important rules for writing fixed values are:
Numbers are written with or without decimals:
10.50
1001
»
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, 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. It is 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
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