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Now let's go ahead
and make another constructor.

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00:00:03,184 --> 00:00:07,043
So this time,
I'm going to create a Dalmatian class....

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00:00:09,180 --> 00:00:12,553
And it's going to be a dog
with the color...

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00:00:15,770 --> 00:00:19,907
[slowly]
white with black spots.

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00:00:22,995 --> 00:00:25,050
But obviously, I haven't...

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00:00:25,050 --> 00:00:27,660
created any kind of relationship
between "Dalmation"

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00:00:27,660 --> 00:00:30,540
and "dog" just yet, so if I reload--

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00:00:31,631 --> 00:00:33,741
oh! Syntax error.

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00:00:36,347 --> 00:00:39,176
Let's see where that is....

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00:00:39,176 --> 00:00:42,060
Oh, I forgot my parentheses.

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00:00:42,060 --> 00:00:43,712
This is a function.

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00:00:48,767 --> 00:00:52,880
So obviously, if I set "spot"

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to new...

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00:00:57,529 --> 00:01:01,333
Dalmation, Dal-ma-tion....

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00:01:04,319 --> 00:01:06,665
Let's try that again.

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00:01:06,665 --> 00:01:09,000
So now "Spot" is a dog,

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00:01:09,000 --> 00:01:11,194
but only the color property
is set.

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00:01:11,194 --> 00:01:14,857
So I can say "spot.color,"
and that works fine,

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00:01:14,857 --> 00:01:17,753
but if I say "spot.speak,"

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00:01:18,729 --> 00:01:20,130
that doesn't work at all.

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00:01:20,130 --> 00:01:24,030
So, in order to define
the inheritance relationship,

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I need to do something
a little odd.

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00:01:26,040 --> 00:01:28,260
So let me give you
a little background first.

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00:01:28,260 --> 00:01:30,630
So, if you're used to a language

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00:01:30,630 --> 00:01:35,190
like C++ or Java or C#,

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00:01:35,190 --> 00:01:38,254
then the way that you define
an inheritance relationship

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00:01:38,254 --> 00:01:41,730
is you have a base class,
like dog,

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00:01:41,730 --> 00:01:44,040
and then you extend
that base class

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00:01:44,040 --> 00:01:47,387
by creating a subclass, like Dalmation.

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00:01:47,387 --> 00:01:49,470
So...

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00:01:49,470 --> 00:01:51,750
the way you do it in JavaScript

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00:01:51,750 --> 00:01:53,010
is a little different.

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00:01:53,010 --> 00:01:56,220
So JavaScript is known
as a prototypal

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00:01:56,220 --> 00:01:58,530
or "proto-tip-pal" language,

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00:01:58,530 --> 00:02:01,500
where inheritance is based
on prototypes.

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00:02:01,500 --> 00:02:02,908
So what is a prototype?

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00:02:02,908 --> 00:02:05,372
A prototype is an object.

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00:02:05,372 --> 00:02:08,220
So the way you define
an inheritance relationship

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00:02:08,220 --> 00:02:12,330
is you set a base prototype,
a base object,

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00:02:12,330 --> 00:02:15,797
that the sub-object
that you're creating

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00:02:15,797 --> 00:02:18,210
inherits all the properties from.

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00:02:18,210 --> 00:02:20,703
So instead of extending a class,

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00:02:20,703 --> 00:02:23,397
which is a blueprint
for a set of objects,

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00:02:23,397 --> 00:02:26,640
instead,
you're extending an object

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00:02:26,640 --> 00:02:29,730
by adding additional methods
and additional properties

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00:02:29,730 --> 00:02:32,400
to the base set of objects
and properties

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00:02:32,400 --> 00:02:34,530
that that object has.

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00:02:34,530 --> 00:02:36,210
So what I want to do, basically,

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00:02:36,210 --> 00:02:39,000
is I want to create a new dog,

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00:02:39,000 --> 00:02:42,330
which is the prototype for all Dalmations,

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00:02:42,330 --> 00:02:44,610
and then I want to inherit

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00:02:44,610 --> 00:02:47,010
that prototype

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00:02:47,010 --> 00:02:49,950
for all Dalmations that I create.

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00:02:49,950 --> 00:02:52,170
The way that works is the constructor,

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00:02:52,170 --> 00:02:53,850
the Dalmation function,

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00:02:54,390 --> 00:02:57,540
has a prototype property, which is an object.

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00:02:57,990 --> 00:02:59,700
So whenever you create a function,

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00:02:59,700 --> 00:03:01,470
there's going to be a prototype property

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00:03:01,470 --> 00:03:03,570
for that function, let's take a look at it.

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00:03:03,960 --> 00:03:05,190
So Dalmation,

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00:03:06,540 --> 00:03:07,920
which is a function.prototype,

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00:03:11,820 --> 00:03:12,960
is an object

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00:03:14,040 --> 00:03:16,350
with a constructor function, and actually,

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00:03:16,350 --> 00:03:18,780
this constructor function, if I open this up,

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00:03:19,650 --> 00:03:21,420
the function is Dalmation.

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00:03:21,450 --> 00:03:25,110
So an object whose function, constructor function,

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00:03:25,110 --> 00:03:28,440
is a Dalmation, is the prototype

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00:03:28,530 --> 00:03:30,660
for all Dalmation objects.

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00:03:31,350 --> 00:03:33,450
And there aren't any other properties

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00:03:33,450 --> 00:03:36,180
that are really interesting or useful on this object,

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00:03:36,180 --> 00:03:39,090
because I haven't set any, but for example,

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00:03:39,360 --> 00:03:50,610
if I set Dalmation.prototype.legs = 4...

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00:03:52,140 --> 00:03:54,810
And then I said, spot.legs,

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00:03:56,010 --> 00:03:58,800
all of a sudden, Spot, an existing Dalmation,

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00:03:58,800 --> 00:04:01,350
inherits the number of legs

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00:04:01,650 --> 00:04:05,130
from the prototype of the Dalmatian class.

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00:04:06,330 --> 00:04:11,460
So I can do this for other properties as well.

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00:04:11,490 --> 00:04:14,940
So dog.sound is bark,

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00:04:17,550 --> 00:04:20,730
spot.sound is now bark as well.

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00:04:21,810 --> 00:04:24,510
And this will be true for any other dog that I create.

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00:04:24,540 --> 00:04:33,870
So for example, Rover is new Dalmation as well.

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00:04:33,870 --> 00:04:35,760
rover.legs is four,

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00:04:36,840 --> 00:04:39,270
rover.sound is bark,

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00:04:39,900 --> 00:04:42,960
but I still haven't set a property

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00:04:44,100 --> 00:04:47,130
for speak, so that's still going to be an error.

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00:04:48,780 --> 00:04:52,170
So this suggests a way of doing inheritance,

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00:04:52,170 --> 00:04:55,260
which is to make sure that all of the properties

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00:04:55,260 --> 00:04:57,690
defined for dog are also set

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00:04:58,200 --> 00:05:01,650
for the prototype object for the Dalmatian class,

90
00:05:03,060 --> 00:05:05,220
and there's a really easy way of doing that,

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00:05:05,460 --> 00:05:06,510
which is...

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00:05:07,710 --> 00:05:15,480
Dalmation.prototype = new Dog.

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00:05:18,000 --> 00:05:20,760
So Dalmation is going to have a prototype object

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00:05:20,760 --> 00:05:22,020
because it's a function,

95
00:05:23,130 --> 00:05:27,120
but I'm overriding that default generic object

96
00:05:27,180 --> 00:05:29,850
with a new dog.

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00:05:30,810 --> 00:05:32,940
So any new dog that I create

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00:05:33,000 --> 00:05:35,070
is going to have all of the properties

99
00:05:35,070 --> 00:05:36,390
that are defined for dog.

100
00:05:37,080 --> 00:05:39,660
So let's go ahead and-- actually let me save this...

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00:05:40,320 --> 00:05:41,220
Save...

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00:05:42,120 --> 00:05:43,470
and then reload,

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00:05:44,640 --> 00:05:49,710
and now, I'm going to create Spot is new Dalmation.