HTML and CSS Reference
In-Depth Information
An assignment statement evaluates the expression on the right side of the equal sign and
assigns the result to the variable on the left side of the equal sign. The equal sign is the
assignment operator.
var total = 5 + 4;
var friend = "Tony";
total
=
5 + 4
variable
assignment
expression
friend
=
"Tony"
When an expression contains a number of operators and operands, such as
5 * 4 + 3 / -2.2
,
and the order of evaluation is ambiguous, then JavaScript must determine what to do.
This is where the precedence and associative rules come in. They tell JavaScript how to
evaluate such an expression. Precedence refers to the way in which the operator binds
to its operand, such as, should addition be done before division or should assignment
come before multiplication? The precedence of one operator over another determines
what operation is done first. As shown in Table 5.1, the operators are organized as a hier-
archy, the operators of highest precedence at the top, similar to a social system where
those with the most power (or money) are at the top. In the rules of precedence, the
multiplication operator is of higher precedence than the addition operator, technically
meaning the operator of higher precedence binds more tightly to its operands. The
assignment operators are low in precedence and thus bind loosely to their operand. In
the expression
sum = 5 + 4
the equal sign is of low precedence, so the expression
5 + 4
is
evaluated first and then the result is assigned to
sum
. Parentheses are of the highest pre-
cedence. An expression placed within parentheses is evaluated first; thus, in the expres-
sion
2 * (10 - 4)
, the expression within the parentheses is evaluated first and that result
is multiplied by
2
. When parentheses are nested, the expression contained within the
innermost set of parentheses is evaluated first.
Associativity refers to the order in which an operator evaluates its operands: left to right
in no specified order, or right to left. When all of the operators in an expression are of equal
precedence, normally the association is left to right; in the expression
5 + 4 + 3
, the evalu-
ation is from left to right. In Example 5.1, how is the expression evaluated? Is addition,
multiplication, or division done first? And in what order, right to left or left to right?
