Calculate percent change over multiple dynamic values with Javascript

Question

I have a table displaying values for each month over a year. It looks something like:

Month |  2015  |
----------------
Jan   |  4856  |
Feb   |  6521  |
Mar   |  ....  |
Apr   |  ....  |
May   |  ....  |
Jun   |  ....  |
Jul   |  ....  |
Aug   |  ....  |
Sep   |  ....  |
Oct   |  ....  |
Nov   |  ....  |
Dec   |  ....  |
----------------
SUM   |  48956 |

I have an array collecting all the SUM values like this:

sum = [48956]

The user can then choose to add more years of values to the table like:

Month |  2013  |  2014  |  2015  |
----------------------------------
Jan   |  6587  |  8954  |  4856  |
Feb   |  1254  |  1125  |  6521  |
Mar   |  ....  |  ....  |  ....  |
Apr   |  ....  |  ....  |  ....  |
May   |  ....  |  ....  |  ....  |
Jun   |  ....  |  ....  |  ....  |
Jul   |  ....  |  ....  |  ....  |
Aug   |  ....  |  ....  |  ....  |
Sep   |  ....  |  ....  |  ....  |
Oct   |  ....  |  ....  |  ....  |
Nov   |  ....  |  ....  |  ....  |
Dec   |  ....  |  ....  |  ....  |
----------------------------------
SUM   |  35780 |  96448 |  48956 |
----------------------------------
diff  |        | %change| %change|

The array now looks like:

sum = [35780, 96448, 48956]

Now i want the "diff" row to show the increase or decrease in percentage over the years.
2014 compared with 2013, 2015 compared with 2014 and so on..

How can i grab ex. 96448 and 35780 in the array and calculate the diff value for 2014, and then the same for each new year added to the table?

96448 - 35780 = 60668 / 96448 = 0,629 * 100 = 62,9% 

Ofcourse the first year (2013 in this case) has no diff.

Answer 1

const values = [10000, 20000, 25000, 5000]; // values per year sorted ascending

const increases = values.map((currVal, index) => {
    if (index === 0) {
        return;
    }

    const prevVal = values[index - 1];
    return ((currVal - prevVal) / prevVal) * 100;
}).filter(Boolean);

console.log(increases); // [100, 25, -80] values in percent

This will return a list with your increase (%) from year to year.

Answer 2

The solution using Array.reduce function:

var sum = [35780, 96448, 48956],
    diff = ['-'];  // first column is empty '-'

sum.reduce(function (a, b) {
    if (a === 0) return b;
    var isNegative = b - a < 0;
    diff.push(parseInt((b - a) / (isNegative? a : b) * 100));
    return b;
}, 0);

console.log(diff);  // ["-", 62, -49]