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‫All right, so we tackled the map

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‫and the filter method

3
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‫and so, now, it's time for the one method

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‫that rules them all, which is the reduced method.

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‫So, the reduced method is really the most versatile

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‫and the most powerful of all array methods in JavaScript.

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‫So, in fact, we could, probably, implement

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‫all the other methods, simply using the reduced method,

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‫but that would be a little bit too much work.

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‫And so, that's why we have all of these other methods

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‫which are a lot easier to use.

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‫So, I will really not go deep into the reduced method, here,

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‫because we could spend hours just with this method,

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‫but I will just show you one of the most common use cases,

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‫which is to add together numbers

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‫or, basically, to perform any operation,

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‫or mathematical operations, with numbers.

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‫So, let's say that, for some reason,

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‫we wanted to read all of the books in the array

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‫and we wanted to know how many pages we would have to read.

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‫So, the solution is to add together

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‫all of these pages properties

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‫of all of the books in the array, right?

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‫And so, for that, we can use the reduce method.

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‫So, let's say pagesAllBooks,

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‫and then books.reduce.

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‫Now, the reduce method also takes in a callback function

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‫which will be executed for each element of the array.

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‫And, as a second argument,

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‫it also takes in a starter value, basically.

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‫But before we go into these,

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‫let's, actually, understand what the reduce method does

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‫and what it's used for.

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‫So, it is called the reduce method

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‫because the goal of reduce is to reduce, basically,

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‫the entire array to just one value.

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‫So, to boil it down, and if you think about it,

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‫this is exactly what we are trying to do, now.

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‫So, we want to boil down the entire array

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‫to just one number, which is, in this case,

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‫the number of pages of all the books combined, right?

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‫And so, that second argument that I mentioned earlier,

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‫is, basically, the starter value

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‫for that final value that we want to obtain.

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‫So first, we will want to pass in a function.

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‫We will do that in a minute.

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‫And second, we want that starter value.

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‫And in this case, since we are adding together

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‫all of these values, we will start from zero, of course.

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‫It makes sense, right?

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‫And now, here, let's talk about the function

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‫that will get called.

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‫So, before we always had this callback function with a book

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‫and then, here, we did something, right?

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‫But, here, the callback function

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‫will be a little bit different.

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‫So, the function that we pass in, here,

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‫will not only get caught with the current element

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‫but also with the current value of the accumulator

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‫which starts at zero.

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‫So, that all sounds very confusing,

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‫but let me just call this, here,

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‫acc, which stands for accumulator,

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‫and then, we will see, in a second, why this makes sense.

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‫So here, what we want to return is this accumulator

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‫plus book.pages.

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‫And now, if we say pages or books,

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‫then we get this huge number.

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‫Okay, so let's understand what happened, here,

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‫by going over this array, element by element.

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‫So, the first element, again, here,

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‫is "The Lord of the Rings."

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‫And so, in this first iteration,

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‫this book, here, is, of course, this one,

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‫but what is the accumulator?

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‫Well, the accumulator is, basically,

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‫the current value of the final value

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‫that we want to boil the array down to.

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‫So, basically, in this case,

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‫the pages of all the books combined, which starts at zero.

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‫And so, in the first iteration

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‫the accumulator will still be zero.

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‫So, this function, here, will get called

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‫for the first element and then the accumulator will be zero

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‫and the book will be the current book.

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00:04:26,430 --> 00:04:29,580
‫And so, then we add together the current accumulator,

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‫so the current pages or books, so to say,

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‫which is still zero plus these 1,216.

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00:04:38,070 --> 00:04:40,890
‫And so, we can imagine that the second value, here,

90
00:04:40,890 --> 00:04:45,890
‫then becomes 1,216 after this first iteration.

91
00:04:45,930 --> 00:04:48,360
‫And so, therefore, in the next iteration,

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‫when we go to the second object,

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‫the current accumulator is already 1,216.

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‫So here, again, it will, in the second iteration,

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‫be already 1,216.

96
00:05:00,930 --> 00:05:04,770
‫And then, on top of that, we add 295.

97
00:05:04,770 --> 00:05:07,200
‫So, that's then the current book pages.

98
00:05:07,200 --> 00:05:09,780
‫And so, then, in the next iteration

99
00:05:09,780 --> 00:05:14,700
‫the accumulator will already be 1,216 plus 295

100
00:05:14,700 --> 00:05:16,890
‫which is like 1,500 something.

101
00:05:16,890 --> 00:05:20,520
‫And then, on top of that, we add this number of pages,

102
00:05:20,520 --> 00:05:23,760
‫and so it goes, all the way, until the end.

103
00:05:23,760 --> 00:05:26,940
‫So, we can imagine this accumulator here, basically,

104
00:05:26,940 --> 00:05:30,630
‫as a pile, onto which we put more and more and more,

105
00:05:30,630 --> 00:05:33,180
‫until, in the end, we reach the final result

106
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‫that we were looking for.

107
00:05:34,680 --> 00:05:37,050
‫So, it's like an intermediary value,

108
00:05:37,050 --> 00:05:41,040
‫in this case, onto which we keep adding the number of pages

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00:05:41,040 --> 00:05:45,573
‫until, in the end, we reach 3,227 pages.

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00:05:46,800 --> 00:05:51,510
‫Okay, so that was a bit confusing, once again,

111
00:05:51,510 --> 00:05:53,910
‫but it's also not super important

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00:05:53,910 --> 00:05:55,860
‫to understand how it works,

113
00:05:55,860 --> 00:05:58,170
‫especially if all we're doing

114
00:05:58,170 --> 00:06:01,890
‫is, like, this simple adding of numbers.

115
00:06:01,890 --> 00:06:04,320
‫So, this always works the same in the end.

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00:06:04,320 --> 00:06:07,440
‫And so, yeah, the most important use case, usually,

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‫of reduce is just this.

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00:06:09,900 --> 00:06:13,290
‫But we can also do very, very complex things,

119
00:06:13,290 --> 00:06:14,730
‫as I said, in the beginning,

120
00:06:14,730 --> 00:06:16,740
‫because this initial value, here,

121
00:06:16,740 --> 00:06:19,140
‫doesn't have to be a number.

122
00:06:19,140 --> 00:06:22,530
‫It could also be an array or it could be an object.

123
00:06:22,530 --> 00:06:24,360
‫And so, what this means

124
00:06:24,360 --> 00:06:26,040
‫is that we could do, really,

125
00:06:26,040 --> 00:06:28,500
‫anything that we do with map or filter

126
00:06:28,500 --> 00:06:31,230
‫also with a reduce.

127
00:06:31,230 --> 00:06:35,029
‫But I won't go into that, right now, because, yeah,

128
00:06:35,029 --> 00:06:38,310
‫(laughs) for that we have already map and filter.

129
00:06:38,310 --> 00:06:40,890
‫Now, here, I just like to, in this case,

130
00:06:40,890 --> 00:06:43,893
‫to replace the accumulator value with sum,

131
00:06:45,690 --> 00:06:46,980
‫so calling it just sum,

132
00:06:46,980 --> 00:06:50,010
‫because that is what it's actually doing.

133
00:06:50,010 --> 00:06:51,930
‫But just keep in mind that,

134
00:06:51,930 --> 00:06:53,730
‫well, it is an accumulator

135
00:06:53,730 --> 00:06:56,850
‫because we keep adding up and up and up

136
00:06:56,850 --> 00:06:59,850
‫onto this temporary value, so to say.

137
00:06:59,850 --> 00:07:02,130
‫In this case, it's simply the sum of the pages.

138
00:07:02,130 --> 00:07:05,403
‫And so, here, I like to then call it sum.

139
00:07:06,810 --> 00:07:09,930
‫Okay, and that's, actually, all I had to show you, here,

140
00:07:09,930 --> 00:07:11,250
‫about reduce,

141
00:07:11,250 --> 00:07:13,680
‫because, yeah, many times we just use it

142
00:07:13,680 --> 00:07:15,720
‫for simple stuff like this.

143
00:07:15,720 --> 00:07:18,123
‫And so, now, you know how to do that.