Training JS #30: methods of arrayObject---reduce() and reduceRight().js

/*
Description:
Training JS #30:
methods of arrayObject---reduce() and reduceRight()

This lesson we learn two methods of array: reduce() and reduceRight(). The two method is approximate. It applies a function against an accumulator and each value of the array to reduce it to a single value. The only difference is: the order of reduce() is from left to right, but reduceRight() is from right to left.

For more information, please refer to:

Array.prototype.reduce()
Array.prototype.reduceRight()
Before looking at some examples, we need to understand that the callback function used for the reduce() can use up to 4 parameters: previousValue, currentValue, index and array. In general, we use only two parameters: previousValue and currentValue. index and array are optional, and we use it when necessary.

Here are some examples to help us understand the use of reduce() and reduceRight():

The first is a classic example, we can use reduce() method to get the sum of the number array:

var arr=[1,2,3,4,5];
console.log(arr.reduce((prev,curr)=>prev+curr)); //output: 15
//My habit is to use a and b
console.log(arr.reduce((a,b)=>a+b));             //output: 15
In addition to the callback function, reduce() also has an optional parameter: initialvalue. In the example above, the initialvalue is omitted. Then the initialvalue is the first element, and then start the cumulative operation from second element.

The following example uses the initialvalue:

var arr=[1,2,3,4,5];
console.log(arr.reduce((a,b)=>a+b,""));             //output: 12345
This example uses an empty string as the initialvalue, followed by these operation:

""+1-->"1","1"+2-->"12",......,"1234"+5-->"12345"
As long as you have enough imagination, reduce() can do a lot of work.

Calculated average:

function average(arr){
  return arr.reduce((a,b)=>a+b)/arr.length
}
console.log(average([1,2,3,4,5])); //output: 3
Count something

var arr=[1,2,3,4,5];
//count odd number:
console.log(arr.reduce((a,b)=>a+(b%2?1:0),0)); //output: 3
//count even number:
console.log(arr.reduce((a,b)=>a+(b%2?0:1),0)); //output: 2
The following is a complex example, which uses a callback function of the four parameters and the initialvalue:

var arr1=[1,2,3,4,5];
var arr2=[6,7,8,9,10];
var result=arr1.map(x=>x*x).reduce((a,b,i,ar)=>a.concat(ar[i],arr2[i]),[])
console.log(result); //output: [ 1, 6, 4, 7, 9, 8, 16, 9, 25, 10 ]
In the example above, Arr1 uses the map method to turn each element into its square.

[1,2,3,4,5]--->[1,4,9,16,25]
Then, using the reduce method. the two elements (ar[i] and arr2[i]) of the same index in turn are put into the initial value (an empty array).

[1,4,9,16,25]
                ---> [ 1, 6, 4, 7, 9, 8, 16, 9, 25, 10 ]
[6,7,8,9,10]
It should be noted that when reading the value of the first array, we used the ar[i] (fourth parameters of the callback function). This value is changed by the map() method([1,4,9,16,25]). If we used arr1[i] reading the value, will be the value of the original array([1,2,3,4,5])

Ok, lesson is over. let's us do some task.

#Task
Coding in function ```tailAndHead```. function accept 1 parameter ```arr```(a number array). 

We use an example to explain this task:

You got an array ```[123,456,789,12,34,56,78]```.

First, from left to right, the tail of an element and the head of the next element are added together. The results are put into an array. like this:
```
[123,456,789,1 2,3 4,5 6,78]
   3+4 6+7 9+1 2+3 4+5 6+7  
    |   |   |   |   |   |
   [7 ,13 ,10 , 5 , 9 ,13]
```
And then, Calculate their product and return it(```7x13x10x5x9x13=532350```)

So, ```tailAndHead([123,456,789,12,34,56,78]) should return 532350 ```

All elements of arr are positive integer. If a number is less than 10, its head and tail are the same. Please try to use ```reduce()``` solve this kata.

Some example:
```
tailAndHead([1,2,3,4,5]) should return 945
tailAndHead([111,2345,66,78,900]) should return 7293
tailAndHead([35456,782,569,2454,875]) should return 12012
```
*/
function tailAndHead(arr){
  let m=[];
  for(let i=0;i<arr.length-1;i++)
    m.push(arr[i]%10+Number((arr[i+1]+'')[0]))
  return m.reduce((a,b)=>a*b)
}