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

context, the interpreter would first search the form‘s context, then the document‘s, then the
window‘s, the browser‘s, and eventually the global context.
The exact details of how this works comprise JavaScript‘s object model, a subject discussed in
later chapters. A comprehensive knowledge of the topic is not really required to program in
JavaScript, but helps tremendously in understanding where the objects available to your scripts
come from, and how they are related. It will also go a long way in setting you apart from the
typical JavaScript developer!
Summary
JavaScript provides five primitive data types: number, string, Boolean, undefined, and null. Of
the five, undefined and null are special types that are not used to store data. Support for
complex types includes the composite types (objects and arrays) and functions. Arrays and
functions are special kinds of objects. Each primitive type is associated with an object that
provides methods useful for manipulating that kind of data. Scoping for variables is static: if a
variable is not found in the execution context in which it is referenced, the interpreter recursively
searches enclosing contexts (as defined in the source code) for its value. Because JavaScript
is weakly typed, automatic type conversion is performed whenever two unequal data types are
operated upon. This feature is powerful, but can also lead to ambiguities and subtle errors.
Novice JavaScript programmers are always encouraged to define variables in a common place
and to keep data types consistent across execution of their scripts. The next chapter discusses
how to operate on data values in meaningful ways as well as how to alter program flow.
Chapter 4: Operators, Expressions, and Statements
This chapter provides an overview of the basic building blocks of every script: operators,
expressions, and statements. The data types introduced in the last chapter are used directly as
literals or within variables in combination with simple operators, such as addition, subtraction,
and so on, to create expressions. An expression is a code fragment that can be evaluated to
some data type the language supports. For example, 2+2 is an expression with the numeric
value 4. Expressions are in turn used to form statements—the most basic unit of script
execution. The execution of statements is controlled using conditional logic and loops.
For those readers new to programming, after reading this chapter, simple scripts should start to
make sense. For experienced programmers, there should be no surprises in this chapter,
because JavaScript is similar to so many other languages: arithmetic and logical operators are
part of the language, as are traditional imperative flow-control constructs such as if, while, and
switch. Seasoned programmers may only need to skim this chapter.
Statement Basics
A JavaScript program is made up of statements. For example, a common statement we saw in
the last chapter is one that assigns a value to a variable. The statements here use the keyword
var to define variables and the assignment operator (=) to set values for them.
var x = 5;
var y = 10;
Assignment uses the = operator and places the value on the right-hand side into the variable on
the left. For example,
x = y + 10;
adds 10 to y and places the result (20) in x.
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