1
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Now let's go ahead
and make another constructor.

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00:00:03,184 --> 00:00:06,962
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 --> 00:00:19,781
[speaking while typing]
"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 --> 00:00:31,631
and "dog" just yet,
so if I reload--

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00:00:31,631 --> 00:00:33,540
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 --> 00:00:43,790
This is a function.

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00:00:48,767 --> 00:00:51,482
So obviously, if I set...

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00:00:51,482 --> 00:00:53,940
[speaking while typing]
"spot"

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00:00:53,940 --> 00:00:55,712
to "new...

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00:00:57,546 --> 00:01:01,333
"Dalmation," Dal-ma-tion....

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00:01:04,328 --> 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 --> 00:01:20,163
that doesn't work at all.

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00:01:20,163 --> 00:01:24,030
So, in order to define
the inheritance relationship,

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00:01:24,030 --> 00:01:26,040
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

35
00:01:56,220 --> 00:01:58,530
or "proto-tip-pal" language,

36
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.

39
00:02:05,372 --> 00:02:08,220
So the way you define
an inheritance relationship

40
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,

44
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

47
00:02:29,730 --> 00:02:32,400
to the base set of objects
and properties

48
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,

51
00:02:39,000 --> 00:02:42,330
which is the prototype
for all Dalmations,

52
00:02:42,330 --> 00:02:44,610
and then I want to inherit

53
00:02:44,610 --> 00:02:47,010
that prototype

54
00:02:47,010 --> 00:02:49,950
for all Dalmations that I create.

55
00:02:49,950 --> 00:02:52,170
The way that works
is the constructor,

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00:02:52,170 --> 00:02:54,394
the Dalmation function,

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00:02:54,394 --> 00:02:58,014
has a prototype property,
which is an object.

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00:02:58,014 --> 00:02:59,700
So whenever you create a function,

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

60
00:03:01,470 --> 00:03:03,960
for that function;
let's take a look at it.

61
00:03:03,960 --> 00:03:06,546
So Dalmation,

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00:03:06,546 --> 00:03:10,728
which is a function.prototype,

63
00:03:11,820 --> 00:03:14,040
is an object

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00:03:14,040 --> 00:03:15,872
with a constructor function.

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00:03:15,872 --> 00:03:17,642
And actually,
this constructor function,

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00:03:17,642 --> 00:03:19,696
if I open this up,

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00:03:19,696 --> 00:03:21,468
the function is Dalmation.

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00:03:21,468 --> 00:03:25,067
So an object whose function,
constructor function,

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00:03:25,067 --> 00:03:27,123
is a Dalmation,

70
00:03:27,123 --> 00:03:28,530
is the prototype

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00:03:28,530 --> 00:03:31,350
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 --> 00:03:38,016
because I haven't set any.

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00:03:38,016 --> 00:03:39,360
But for example,

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00:03:39,360 --> 00:03:40,470
if I set

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00:03:40,470 --> 00:03:42,263
[speaking while typing]
"Dalmation

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00:03:43,917 --> 00:03:45,763
".prototype

79
00:03:46,913 --> 00:03:49,551
".legs

80
00:03:49,551 --> 00:03:52,148
"= 4...."

81
00:03:52,148 --> 00:03:53,056
And then I said,

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00:03:53,056 --> 00:03:56,010
[speaking while typing]
"spot.legs,"

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

84
00:03:58,800 --> 00:04:01,650
inherits the number of legs

85
00:04:01,650 --> 00:04:05,336
from the prototype
of the Dalmatian class.

86
00:04:06,370 --> 00:04:11,490
So I can do this
for other properties as well.

87
00:04:11,490 --> 00:04:15,518
So dog.sound is "bark,"

88
00:04:16,990 --> 00:04:21,810
"spot.sound"
is now "bark" as well.

89
00:04:21,810 --> 00:04:24,540
And this will be true
for any other dog that I create.

90
00:04:24,540 --> 00:04:32,870
So for example,

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00:04:32,870 --> 00:04:33,870
[speaking while typing]
Rover is new Dalmation as well.

92
00:04:33,870 --> 00:04:35,760
rover.legs is four,

93
00:04:36,840 --> 00:04:39,270
rover.sound is bark,

94
00:04:39,900 --> 00:04:42,960
but I still haven't set a property

95
00:04:44,100 --> 00:04:47,130
for speak, so that's still going to be an error.

96
00:04:48,780 --> 00:04:52,170
So this suggests a way of doing inheritance,

97
00:04:52,170 --> 00:04:55,260
which is to make sure that all of the properties

98
00:04:55,260 --> 00:04:57,690
defined for dog are also set

99
00:04:58,200 --> 00:05:01,650
for the prototype object for the Dalmatian class,

100
00:05:03,060 --> 00:05:05,220
and there's a really easy way of doing that,

101
00:05:05,460 --> 00:05:06,510
which is...

102
00:05:07,710 --> 00:05:15,480
Dalmation.prototype = new Dog.

103
00:05:18,000 --> 00:05:20,760
So Dalmation is going to have a prototype object

104
00:05:20,760 --> 00:05:22,020
because it's a function,

105
00:05:23,130 --> 00:05:27,120
but I'm overriding that default generic object

106
00:05:27,180 --> 00:05:29,850
with a new dog.

107
00:05:30,810 --> 00:05:32,940
So any new dog that I create

108
00:05:33,000 --> 00:05:35,070
is going to have all of the properties

109
00:05:35,070 --> 00:05:36,390
that are defined for dog.

110
00:05:37,080 --> 00:05:39,660
So let's go ahead and-- actually let me save this...

111
00:05:40,320 --> 00:05:41,220
Save...

112
00:05:42,120 --> 00:05:43,470
and then reload,

113
00:05:44,640 --> 00:05:49,710
and now, I'm going to create Spot is new Dalmation.