when to use reduce and reduceRight?

Question

Can you describe this for me?

var arr, total; arr = [1, 2, 3, 4, 5]; total = arr.reduce(function(previous, current) { return previous + current; }); // total is 15 

Answer 1

The order for reduce is from left to right, and it's from right to left for reduceRight, as the following piece of code shows:

var arr = ["1", "2", "3", "4", "5"];  total1 = arr.reduce(function(prev, cur) {     return prev + cur; });  total2 = arr.reduceRight(function(prev, cur) {     return prev + cur; });  console.log(total1); // => 12345 console.log(total2); // => 54321 

Answer 2

In some cases, the difference between reduce and reduceRight does matter:

However, because adding two integers is commutative in JavaScript, it doesn't matter in your example, just like how 1 + 2 is equal to 2 + 1 in math.

var arr = ["A", "B", "C", "D", "E"];    console.log(  arr.reduce((previous, current)      => previous + current)  )  console.log(  arr.reduceRight((previous, current) => previous + current)  )