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JORDAN HAYASHI: Hello, and
welcome for Lecture Two-- | 17.27 | 2.37 |
React, Props, and State. | 19.64 | 2.4 |
So last week, we talked about a bunch
of different topics, one being ES6 | 22.04 | 3.72 |
and beyond and the syntax
that comes with each of those. | 25.76 | 2.69 |
We talked about closures,
the process by which | 28.45 | 2.62 |
a function can reference variables
declared in a parent function. | 31.07 | 5.13 |
We talked about IIFEs, Immediately
Invoked Function Expressions. | 36.2 | 3.68 |
We talked about using functions
as first-class citizens. | 39.88 | 3.675 |
We talked about the execution
stack and event loop, | 43.555 | 2.125 |
and how JavaScript actually
executes in browsers. | 45.68 | 2.82 |
We talked about callbacks, promises, and
async/await, all of the different ways | 48.5 | 3.3 |
to handle asynchronous actions. | 51.8 | 1.89 |
And last, we talked about this
and the way that this is bound. | 53.69 | 3.96 |
This week, we're going to
start with classes, which | 57.65 | 3.23 |
is a syntax I was introduced in ES6. | 60.88 | 3.639 |
It simplifies the defining of complex
objects that have their own prototypes. | 64.519 | 4.967 |
And with that, you have
two different things. | 69.486 | 1.874 |
You have classes and instances. | 71.36 | 1.53 |
Classes is basically an abstract
thing that you can declare, | 72.89 | 3.73 |
basically saying, hey, by
the way, any of these objects | 76.62 | 2.78 |
that you create will have these
methods associated with them. | 79.4 | 3.03 |
Or they might have these things
attached to them that you can use. | 82.43 | 4.14 |
And then when you actually
turn an abstract class | 86.57 | 2.64 |
into an instance of that object,
that is called an instance. | 89.21 | 4.42 |
An example of that would be the
difference between the date, | 93.63 | 6.12 |
which is a function-- it's a class-- | 99.75 | 2.66 |
or a new date. | 102.41 | 1.23 |
So if you do something like
const d equals new date, | 103.64 | 4.54 |
then now you have a date object itself. | 108.18 | 3.38 |
And so Date with a capital D
would be a class in that case, | 111.56 | 3.137 |
and the lowercase D
would be an instance. | 114.697 | 1.708 |
These instances have things attached
to them, like methods and properties, | 120.61 | 3.54 |
and the classes have
things like static methods. | 124.15 | 2.4 |
And so methods are
basically anything that's | 126.55 | 2.07 |
a function that can be invoked
on any of the instances. | 128.62 | 4.12 |
You can think of that as a
function on the classes prototype. | 132.74 | 6.24 |
A static method, on the
other hand, is basically | 138.98 | 2.6 |
a method that doesn't really care about
the particular instance of a class. | 141.58 | 3.3 |
Instead, it cares about all instances of
the class, so something like Date.now, | 144.88 | 10.14 |
where you don't really care about
a specific instance of a date | 155.02 | 3.78 |
if you just want to get the time. | 158.8 | 1.92 |
Whereas something like
turning a date to a string-- | 160.72 | 2.62 |
in this class, d.toString-- | 163.34 | 3.625 |
in that case, you do care
about the particular date | 166.965 | 2.125 |
object that you're working on. | 169.09 | 1.47 |
And so when you do
capital Date.now, that's | 170.56 | 2.91 |
considered a static method,
since it's attached to the class. | 173.47 | 2.97 |
And if you do something like date-- | 176.44 | 2.52 |
lowercase d-- dot toString, that really
matters which instance you're attached | 178.96 | 4.38 |
to, and therefore, that's a method. | 183.34 | 2.71 |
Lastly, we have properties,
which are like methods. | 186.05 | 3.86 |
But rather than being functions,
they're actually just values, | 189.91 | 3.68 |
and they are associated with a
particular instance of a class. | 193.59 | 4.55 |
And so with classes come
a few different keywords. | 198.14 | 2.27 |
We have new, which you saw me type
over here, which is basically saying, | 200.41 | 3.91 |
hey, give me an instance
of this particular class. | 204.32 | 2.99 |
You invoke the class like an object, in
case you want to pass anything into it. | 207.31 | 3.32 |
And so say we want to do new
const d2 equals new date, | 210.63 | 5.362 |
and we actually want
to pass in some number. | 215.992 | 1.833 |
That gives us a new date
from a very long time ago. | 220.69 | 3.93 |
So d.toString. | 224.62 | 1.29 |
Oh, d2.toString. | 228.784 | 1.596 |
Since we passed in the number 1,
2, 3, 4, that's basically saying, | 233.8 | 3.78 |
give me a date that's 1,234 milliseconds
after date 0, which is back in 1969. | 237.58 | 13.23 |
So a constructor is
basically something that you | 250.81 | 2 |
define within a class that says,
hey, when you create a new class, | 252.81 | 3.719 |
invoke this method such that you
create a new instance of a class. | 256.529 | 5.911 |
So let's actually practice
this a little bit. | 262.44 | 2.46 |
So is everybody here familiar with
a data structure called a set? | 264.9 | 3.075 |
So basically what a
set is, is it's a list, | 273.85 | 3.12 |
a data structure that supports things
like add, delete, and inclusion, | 276.97 | 4.5 |
where you cannot have multiple
things of the same value. | 281.47 | 3.62 |
Or in other words, it's
a list of unique values. | 285.09 | 2.84 |
And the methods that
it should support are | 287.93 | 2.51 |
add, which is basically
add to this list; | 290.44 | 2.46 |
delete, which is basically get
rid of something from this; | 292.9 | 3.06 |
or inclusion, which is saying, hey,
does this list have a particular value? | 295.96 | 5.97 |
And it should also have the
ability to get its size. | 301.93 | 2.54 |
And so down at the bottom of
the file, I defined a few tests | 304.47 | 4 |
that we're going to run after we
implement this, such as line 7 here, | 308.47 | 4.65 |
we have const s gets a new
set with an array from 1 to 5, | 313.12 | 4.17 |
which is basically saying, give me a
new set with five values, 1, 2, 3, 4, | 317.29 | 3.21 |
and 5. | 320.5 | 1.09 |
We're going to try to
do S.add1, and so we're | 321.59 | 2.21 |
going to try to add 1
three times to the set. | 323.8 | 3.37 |
And when we do S.size, we want it
actually to only have five members, | 327.17 | 4.31 |
because you shouldn't be
able to add 1 multiple times. | 331.48 | 4.17 |
Down here, we do S.add6,
and then we try S.has6, | 335.65 | 5.07 |
and it should contain the number 6. | 340.72 | 2.67 |
That thing should not be there. | 343.39 | 3.18 |
We try to see the size of it, and
it should have added another member. | 346.57 | 3.752 |
And then down here, we tried
to delete that and do a couple | 350.322 | 2.458 |
of associated checks with that. | 352.78 | 1.85 |
And so how are we going to go
about implementing this class? | 354.63 | 3.22 |
So as per the slide over here, we
use a method called constructor | 361.22 | 3.8 |
in order to go ahead and construct
an instance of this class. | 365.02 | 4.18 |
And so within the class, we should
have a method called constructor, | 369.2 | 6.31 |
which takes a single argument. | 375.51 | 3.55 |
It should take an array or some
sort of list to be created with. | 379.06 | 4.324 |