Very often, a subroutine is used to package an algorithm, which is simply an under‐
stood recipe for performing a given task. Let’s consider the algorithm for determining
whether the current date is in a leap year or not:
const
year
=
new
Date
().
getFullYear
();
if
(
year
%
4
!==
0
)
console.log(
`
${
year
}
is NOT a leap year.`
)
else
if
(
year
%
100
!=
0
)
console.log(
`
${
year
}
IS a leap year.`
)
else
if
(
year
%
400
!=
0
)
console.log(
`
${
year
}
is NOT a leap year.`
)
else
console.log(
`{$year} IS a leap year`
);
Imagine if we had to execute this code 10 times in our program…or even worse, 100
times. Now imagine further that you wanted to change the verbiage of the message
that gets printed to the console; you have to find all the instances that use this code
and change four strings! This is exactly the problem that subroutines address. In Java‐
Script, a function can fill this need:
function
printLeapYearStatus
() {
const
year
=
new
Date
().
getFullYear
();
if
(
year
%
4
!==
0
)
console.log(
`
${
year
}
is NOT a leap year.`
)
else
if
(
year
%
100
!=
0
)
console.log(
`
${
year
}
IS a leap year.`
)
else
if
(
year
%
400
!=
0
)
console.log(
`
${
year
}
is NOT a leap year.`
)
else
console.log(
`{$year} IS a leap year`
);
}
Now we’ve created a reusable subroutine (function) called
printLeapYearStatus
.
This should all be pretty familiar to us now.
Note the name we chose for the function:
printLeapYearStatus
. Why didn’t we call
it
getLeapYearStatus
or simply
leapYearStatus
or
leapYear
? While those names
would be shorter, they miss an important detail: this function simply prints out the
current leap year status. Naming functions meaningfully is part science, part art. The
name is not for JavaScript: it doesn’t care what name you use. The name is for other
people (or you in the future). When you name functions, think carefully about what
another person would understand about the function if all they could see was the
name. Ideally, you want the name to communicate exactly what the function does. On
the flip side, you can be too verbose in the naming of your functions. For example, we
might have named this function
calculateCurrentLeapYearStatusAndPrintToCon
sole
, but the extra information in this longer name has passed the point of diminish‐
ing returns; this is where the art comes in.
Functions as Subroutines That Return a Value
In our previous example,
printLeapYearStatus
is a subroutine in the usual sense of
the word: it just bundles some functionality for convenient reuse, nothing more. This
simple use of functions is one that you won’t find yourself using very often—and even
less so as your approach to programming problems becomes more sophisticated and
184 | Chapter 13: Functions and the Power of Abstract Thinking
