and filesystem support, are not available in the browser for security reasons (can you
imagine the damage hackers could do if they could delete your files from the
browser?). Others, such as the ability to create a web server, simply aren’t very useful
in a browser.
It’s important to understand what’s JavaScript, and what’s part of an API. A program‐
mer who has always written browser-based code might reasonably assume that
window
and
document
are simply part of JavaScript. However, those are APIs provided
in the browser environment (which were covered in
). In this chapter, we’ll
cover the APIs provided in Node.
If you haven’t already, make sure Node and npm are installed (see
).
Modules
Modules are a mechanism for packaging and namespacing code. Namespacing is a
way to prevent name collisions. For example, if Amanda and Tyler both write a func‐
tion called
calculate
, and you simply cut and paste their functions into your pro‐
gram, the second one will replace the first. Namespacing allows you to somehow refer
to “Amanda’s
calculate
” and “Tyler’s
calculate
.” Let’s see how Node modules solve
this problem. Create a file called amanda.js:
function
calculate
(
a
,
x
,
n
) {
if
(
x
===
1
)
return
a
*
n
;
return
a
*
(
1
-
Math
.
pow
(
x
,
n
))
/
(
1
-
x
);
}
module
.
exports
=
calculate
;
And a file called tyler.js:
function
calculate
(
r
) {
return
4
/
3
*
Math
.
PI
*
Math
.
pow
(
r
,
3
);
}
module
.
exports
=
calculate
;
We could legitimately make the argument that Amanda and Tyler were both lazy in
naming their functions something so nondescript, but we’ll let it slide for the sake of
this example. The important line in both of these files is
modules.export = calcu
late
.
module
is a special object that Node makes available to implement modules.
Whatever you assign to its
exports
property will be what is exported from the mod‐
ule. Now that we’ve written a couple of modules, let’s see how we use them in a third
program. Create a file called app.js, and we’ll import these modules:
const
amanda_calculate
=
require
(
'./amanda.js'
);
const
tyler_calculate
=
require
(
'./tyler.js'
);
282 | Chapter 20: Node
