Class and Instance Creation
Prior to ES6, creating a class in JavaScript was a fussy, unintuitive affair. ES6 introdu‐
ces some convenient new syntax for creating classes:
class
Car
{
constructor
() {
}
}
This establishes a new class called
Car
. No instances (specific cars) have been created
yet, but we have the ability to do so now. To create a specific car, we use the
new
keyword:
const
car1
=
new
Car
();
const
car2
=
new
Car
();
We now have two instances of class
Car
. Before we make the
Car
class more sophisti‐
cated, let’s consider the
instanceof
operator, which can tell you if a given object is an
instance of a given class:
car1
instanceof
Car
// true
car1
instanceof
Array
// false
From this we can see that
car1
is an instance of
Car
, not
Array
.
Let’s make the
Car
class a little more interesting. We’ll give it some data (make,
model), and some functionality (shift):
class
Car
{
constructor
(
make
,
model
) {
this
.
make
=
make
;
this
.
model
=
model
;
this
.
userGears
=
[
'P'
,
'N'
,
'R'
,
'D'
];
this
.
userGear
=
this
.
userGears
[
0
];
}
shift
(
gear
) {
if
(
this
.
userGears
.
indexOf
(
gear
)
<
0
)
throw
new
Error(
`Invalid gear:
${
gear
}
`
);
this
.
userGear
=
gear
;
}
}
Here the
this
keyword is used for its intended purpose: to refer to the instance the
method was invoked on. You can think of it as a placeholder: when you’re writing
your class—which is abstract—the
this
keyword is a placeholder for a specific
instance, which will be known by the time the method is invoked. This constructor
allows us to specify the make and model of the car when we create it, and it also sets
up some defaults: the valid gears (
userGears
) and the current gear (
gear
), which we
initialize to the first valid gear. (I chose to call these user gears because if this car has
an automatic transmission, when the car is in drive, there will be an actual mechani‐
150 | Chapter 9: Objects and Object-Oriented Programming
