JavaScript/JScript: Control Structures I - Pearson Learning Solutions

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Chapter 14 JavaScript/JScript: Control Structures I 433
It is important to note here that when incrementing or decrementing a variable in a
statement by itself, the preincrement and postincrement forms have the same effect and the
predecrement and postdecrement forms have the same effect. It is only when a variable
appears in the context of a larger expression that preincrementing the variable and postincrementing
the variable have different effects. Predecrementing and postdecrementing
behave similarly.
Common Programming Error 14.10
Attempting to use the increment or decrement operator on an expression other than an lvalue
is a syntax error. An lvalue is a variable or expression that can appear on the left side of an
assignment operation. For example, writing ++(x + 1) is a syntax error because (x + 1)
is not an lvalue. 14.10
The chart in Fig. 14.15 shows the precedence and associativity of the operators introduced
up to this point. The operators are shown from top to bottom in decreasing order of
precedence. The second column describes the associativity of the operators at each level of
precedence. Notice that the conditional operator (?:), the unary operators increment (++)
and decrement (--) and the assignment operators =, +=, -=, *=, /= and %= associate from
right to left. All other operators in the operator precedence chart of Fig. 14.15 associate
from left to right. The third column names the groups of operators.
14.13 A Note on Data Types
Unlike its predecessor languages C, C++ and Java, JavaScript does not require variables to
have a type before they can be used in a program. A variable in JavaScript can contain a
value of any data type, and in many situations JavaScript automatically converts between
values of different types for you. For this reason, JavaScript is referred to as a loosely typed
language.
When a variable is declared in JavaScript but is not given a value, that variable has an
undefined value. Attempting to use the value of such a variable is normally a logic error.
When variables are declared, they are not assigned default values unless specified otherwise
by the programmer. To indicate that a variable does not contain a value, you can
assign the value null to the variable.
Operators Associativity Type
() left to right parentheses
++ -- right to left unary
* / % left to right multiplicative
+ - left to right additive
< >= left to right relational
== != left to right equality
?: right to left conditional
= += -= *= /= %= right to left assignment
Fig. 14.15 Precedence and associativity of the operators discussed so far.
e-Business and e-Commerce: How to Program, by Harvey M. Deitel, Paul J. Deitel, and Tem R. Nieto. Published by Prentice Hall.
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