on fractional operands as well. For example,
10 % 3.6
will be
3
(3.6 goes into 10
twice, with 2.8 left over).
The increment operator (
++
) is effectively an assignment and addition operator all
rolled into one. Likewise, the decrement operator (
--
) is an assignment and subtrac‐
tion operator. They are useful shortcuts, but should be used with caution: if you bury
one of these deep in an expression, it might be hard to see the “side effect” (the
change in a variable). Understanding the difference between the
prefix and postfix
operators is important as well. The prefix version modifies the variable and then eval‐
uates to the new value; the postfix operator modifies the variable and then evaluates
to the value before modification. See if you can predict what the following expressions
evaluate to (hint: increment and decrement operators are evaluated before addition,
and we evaluate from left to right in this example):
let
x
=
2
;
const
r1
=
x
++
+
x
++
;
const
r2
=
++
x
+
++
x
;
const
r3
=
x
++
+
++
x
;
const
r4
=
++
x
+
x
++
;
let
y
=
10
;
const
r5
=
y
--
+
y
--
;
const
r6
=
--
y
+
--
y
;
const
r7
=
y
--
+
--
y
;
const
r8
=
--
y
+
y
--
;
Go ahead and run this example in a JavaScript console; see if you can anticipate what
r1
through
r8
will be, and what the value of
x
and
y
is at each step. If you have prob‐
lems with this exercise, try writing the problem down on a piece of paper, and adding
parentheses according to the order of operations, then doing each operation in order.
For example:
let
x
=
2
;
const
r1
=
x
++
+
x
++
;
// ((x++) + (x++))
// ( 2 + (x++)) eval left to right; x now has value 3
// ( 2 + 3 ) x now has value 4
// 5 result is 5; x has value 4
const
r2
=
++
x
+
++
x
;
// ((++x) + (++x))
// ( 5 + (++x)) eval left to right; x now has value 5
// ( 5 + 6 ) x now has value 6
// 11 result is 11; x has value 6
const
r3
=
x
++
+
++
x
;
// ((x++) + (++x))
// ( 6 + (++x)) eval left to right; x now has value 7
// ( 6 + 8 ) x now has value 8
// 14 result is 14; x has value 8
//
// ... and so on
Arithmetic Operators | 83
