Additional for Loop Patterns
By using the comma operator (which we’ll learn more about in
combine multiple assignments and final expressions. For example, here’s a
for
loop
to print the first eight Fibonacci numbers:
for
(
let
temp
,
i
=
0
,
j
=
1
;
j
<
30
;
temp
=
i
,
i
=
j
,
j
=
i
+
temp
)
console
.
log
(
j
);
In this example, we’re declaring multiple variables (
temp
,
i
, and
j
), and we’re modify‐
ing each of them in the final expression. Just as we can do more with a
for
loop by
using the comma operator, we can use nothing at all to create an infinite loop:
for
(;;)
console
.
log
(
"I will repeat forever!"
);
In this
for
loop, the condition will simply evaluate to
undefined
, which is falsy,
meaning the loop will never have cause to exit.
While the most common use of
for
loops is to increment or decrement integer indi‐
ces, that is not a requirement: any expression will work. Here are some examples:
let
s
=
'3'
;
// string containing a number
for
(;
s
.
length
<
10
;
s
=
' '
+
s
);
// zero pad string; note that we must
// include a semicolon to terminate
// this for loop!
for
(
let
x
=
0.2
;
x
<
3.0
;
x
+=
0.2
)
// increment using noninteger
console
.
log
(
x
);
for
(;
!
player
.
isBroke
;)
// use an object property as conditional
console
.
log
(
"Still playing!"
);
Note that a
for
loop can always be written as a
while
loop. In other words:
for([initialization]; [condition]; [final-expression])
statement
is equivalent to:
[initialization]
while([condition]) {
statement
[final-expression]
}
However, the fact that you can write a
for
loop as a
while
loop doesn’t mean you
should. The advantage of the
for
loop is that all of the loop control information is
right there on the first line, making it very clear what’s happening. Also, with a
for
loop, initializing variables with
let
confines them to the body of the
for
loop (we’ll
learn more about this in
); if you convert such a
for
statement to a
while
statement, the control variable(s) will, by necessity, be available outside of the
for
loop body.
Control Flow Statements in JavaScript | 71
