JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

Variables
A variable stores data. Every variable has a name, called its identifier. Variables are declared in
JavaScript using var, a keyword that allocates storage space for new data and indicates to the
interpreter that a new identifier is in use. Declaring a variable is simple:
var x;
This statement tells the interpreter that a new variable x is about to be used. Variables can be
assigned initial values when they are declared:
var x = 2;
In addition, multiple variables can be declared with one var statement if the variables are
separated by commas:
var x, y = 2, z;
You should not use variables without first declaring them, although it is possible to do so in
certain cases. Using a variable on the right-hand side of an assignment without first declaring it
will result in an error.
Experienced programmers will notice that, unlike C, C++, and Java, there is only one way to
declare a variable in JavaScript. This highlights the fact that JavaScript‘s treatment of variable
data types is fundamentally different from many languages, including C, C++, and Java.
Basic Data Types
Every variable has a data type that indicates what kind of data the variable holds. The basic
data types in JavaScript are strings, numbers, and Booleans. A string is a list of characters, and
a string literal is indicated by enclosing the characters in single or double quotes. Strings may
contain a single character or multiple characters, including whitespace and special characters
such as \n (the newline). Numbers are integers or floating-point numerical values, and numeric
literals are specified in the natural way. Booleans take on one of two values: true or false.
Boolean literals are indicated by using true or false directly in the source code. An example of
all three data types follows.
var stringData = "JavaScript has strings\n It sure does";
var numericData = 3.14;
var booleanData = true;
JavaScript also supports two other basic types: undefined and null. All these data types as well
as the details of special characters are discussed in Chapter 3. However, one aspect of
JavaScript data types deserves special mention in this overview—weak typing.
Dynamic Typing
A major difference between JavaScript and many other languages readers might be familiar
with is that JavaScript is dynamically typed (or, by some definitions, weakly typed). Every
JavaScript variable has a data type, but the type is inferred from the variable‘s content. For
example, a variable that is assigned a string value assumes the string data type. A
consequence of JavaScript‘s automatic type inference is that a variable‘s type can change
during script execution. For example, a variable can hold a string at one point and then later be
assigned a Boolean. Its type changes according to the data it holds. This explains why there is
only one way to declare variables in JavaScript: there is no need to indicate type in variable
declarations.