Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

5.2 Operator Overview

If
you are a C, C++, or Java programmer, most
of the JavaScript operators should already be familiar to you. Table 5-1 summarizes the operators; you can refer to
this table for reference. Note that most operators are represented by
punctuation characters such as + and
=. Some, however, are represented by keywords such
as delete and instanceof.
Keyword operators are regular operators, just like those expressed
with punctuation; they are simply expressed using a more readable and
less succinct syntax.

In this table, the column labeled "P" gives the operator

precedence and the column labeled
"A" gives the operator associativity, which can be L
(left-to-right) or R (right-to-left). If you do not already
understand precedence and associativity, the subsections that follow
the table explain these concepts. The operators themselves are
documented following that discussion.

Table 5-1. JavaScript operators

5.2.1 Number of Operands

Operators
can be categorized based on the number of operands they expect. Most
JavaScript operators, like the + operator we saw
earlier, are

binary operators that combine two
expressions into a single, more complex expression. That is, they
operate on two operands. JavaScript also supports a number of

unary operators , which convert a single
expression into a single, more complex expression. The
- operator in the expression -3
is a unary operator that performs the operation of negation on the
operand 3. Finally, JavaScript supports one

ternary operator , the conditional operator
?:, which combines the value of three expressions
into a single expression.

5.2.2 Type of Operands

When constructing JavaScript
expressions, you must pay attention to
the data types that are being passed to operators and to the data
types that are returned. Different operators expect their
operands' expressions to evaluate to values of a certain data
type. For example, it is not possible to multiply strings, so the
expression "a" *
"b" is not legal in JavaScript. Note, however,
that JavaScript tries to convert expressions to the appropriate type
whenever possible, so the expression "3"
* "5" is legal. Its value is
the number 15, not the string
"15". We'll consider JavaScript type
conversions in detail in Section 11.1.

Furthermore, some operators behave differently depending on the type
of the operands. Most notably, the
+ operator adds numeric operands but
concatenates string operands. Also, if passed one string and one
number, it converts the number to a string and concatenates the two
resulting strings. For example, "1"
+ 0 yields the string
"10".

Notice that the
assignment
operators, as well as a few other operators, expect their lefthand
expressions to be lvalues.

lvalue is a
historical term that means "an expression that can legally
appear on the lefthand side of an assignment expression." In
JavaScript, variables, properties of objects, and elements of arrays
are lvalues. The ECMAScript specification allows built-in functions
to return lvalues but does not define any built-in functions that
behave that way.

Finally, note that operators do not always return the same type as
their operands. The comparison operators (less than, equal
to, greater than, etc.) take operands of various types, but when
comparison expressions are evaluated, they always return a boolean
result that indicates whether the comparison is true or not. For
example, the expression a < 3 returns
true if the value of variable a
is in fact less than 3. As we'll see, the
boolean values returned by comparison operators are used in
if statements, while loops, and
for loops -- JavaScript statements that control
the execution of a program based on the results of evaluating
expressions that contain comparison operators.

5.2.3 Operator Precedence

In
Table 5-1, the column labeled "P"
specifies the

precedence of each operator.
Operator precedence controls the order in which operations are
performed. Operators with higher numbers in the "P"
column are performed before those with lower numbers.

Consider the following expression:

w = x + y*z;

The multiplication operator
* has a higher precedence than the addition
operator +, so the multiplication is performed
before the addition. Furthermore, the assignment operator = has
the lowest precedence, so the assignment is performed after all the
operations on the righthand side are completed.

Operator precedence can be overridden with the explicit use of
parentheses. To force the addition in the previous example to be
performed first, we would write:

w = (x + y)*z;

In practice, if
you are at all unsure about the precedence of your operators, the
simplest thing is to use parentheses to make the evaluation order
explicit. The only rules that are important to know are these:
multiplication and division are performed before addition and
subtraction, and assignment has very low precedence and is almost
always performed last.

5.2.4 Operator Associativity

In Table 5-1,
the column labeled "A" specifies the

associativity of the operator. A value of L
specifies left-to-right associativity, and a value of
R specifies right-to-left associativity. The
associativity of an operator specifies the order in which operations
of the same precedence are performed. Left-to-right associativity
means that operations are performed from left to right. For example,
the addition operator has left-to-right associativity, so:

w = x + y + z;

is the same as:

w = ((x + y) + z);

On the other hand, the following (almost nonsensical) expressions:

x = ~-~y;
w = x = y = z;
q = a?b:c?d:e?f:g;

are equivalent to:

x = ~(-(~y));
w = (x = (y = z));
q = a?b:(c?d:(e?f:g));

because the unary, assignment, and ternary conditional
operators have right-to-left associativity.