Operator Description
Example
--
Pre-decrement
--x // decrements x by one, and evaluates to the new
value
--
Post-decrement
x-- // decrements x by one, and evaluates to value of x
before the decrement
Remember that all numbers in JavaScript are doubles, meaning that if you perform an
arithmetic operation on an integer (such as
3/2
), the result will be a decimal number
(1.5).
Subtraction and unary negation both use the same symbol (the minus sign), so how
does JavaScript know how to tell them apart? The answer to this question is complex
and beyond the scope of the book. What’s important to know is that unary negation is
evaluated before subtraction:
const
x
=
5
;
const
y
=
3
-
-
x
;
// y is 8
The same thing applies for unary plus. Unary plus is not an operator you see used
very often. When it is, it is usually to force a string to be converted to a number, or to
align values when some are negated:
const
s
=
"5"
;
const
y
=
3
+
+
s
;
// y is 8; without the unary plus,
// it would be the result of string
// concatenation: "35"
// using unnecessary unary pluses so that expressions line up
const
x1
=
0
,
x2
=
3
,
x3
=
-
1.5
,
x4
=
-
6.33
;
const
p1
=
-
x1
*
1
;
const
p2
=
+
x2
*
2
;
const
p3
=
+
x3
*
3
;
const
p3
=
-
x4
*
4
;
Note that in these examples, I specifically used variables in conjunction with unary
negation and unary plus. That’s because if you use them with numeric literals, the
minus sign actually becomes part of the numeric literal, and is therefore technically
not an operator.
The remainder operator returns the remainder after division. If you have the expres‐
sion
x % y
, the result will be the remainder when dividing the dividend (
x
) by the
divisor (
y
). For example,
10 % 3
will be
1
(3 goes into 10 three times, with 1 left over).
Note that for negative numbers, the result takes on the sign of the dividend, not the
divisor, preventing this operator from being a true modulo operator. While the
remainder operator is usually only used on integer operands, in JavaScript, it works
82 | Chapter 5: Expressions and Operators
