What can I use to add new value to previous value and repeat this a certain amount of times? (Javascript)

Question

I'm trying to get an array of numbers based on a calculation that keeps adding a set amount to the previous amount until this have repeated 20 times. The initial number is a negative number because the client pays an initial amount of money for a solar power system and then the calculation should subtract an amount each month based on how much the client saves by not having to pay for electricity. It needs to be an array (I think) because it needs to go into a chart. Here's a google worksheet that might make what I'm trying to more clear. The part of the sheet that is relevant to my question is in columns T and U in pink.

I did tonne of reading on loops, different array types (reduce and map). I'm new to this so it didn't seem any of those types of arrays will do what I need to be done. I found the below code somewhere and it seemed like this is the closest to what I need to happen but I could be completely off track (my adjusted version is further down):

// program to generate fibonacci series up to n terms

// take input from the user
const number = parseInt(prompt('Enter the number of terms: '));
let n1 = 0, n2 = 1, nextTerm;

console.log('Fibonacci Series:');

for (let i = 1; i <= number; i++) {
    console.log(n1);
    nextTerm = n1 + n2;
    n1 = n2;
    n2 = nextTerm;
}

I tried to adjust it to try get it to do what I need it to do but in console it shows one number and then the rest is NAN. I know this means not a number but I don't know why or how to fix it:

function runningNetProfit(n) {
        var profitSequence = [0];
        var nextYear = (monthlyEstimatedSavings * 12);


        for (var i = negSystemCost; i < n - 1; i++) {

            profitSequence.push(nextYear);

            nextYear = nextYear + profitSequence[i];
        }
        return profitSequence;
    }

    console.log(runningNetProfit(20))

I added what I did (all of the code) to a codepen as well, maybe it can make my question more clear, that can be found here The javascript relevant to this question is right at the bottom from line 145. Any advice would be much appreciated.

Answer 1

See if this code works for you:
It takes a given installation cost, the price they pay for electricity, and the number of months, then spits out an array with these numbers.

const installCost = 250000;
const electricityCost = 45000;
const numMonths = 20

const newArray = Array.from({length: numMonths})
const updatedArray = newArray.map((_, index) => index * electricityCost - installCost);

console.log(updatedArray) // returns [-250000,-205000,-160000,-115000,-70000,-25000,20000,65000,110000,155000,200000,245000,290000,335000,380000,425000,470000,515000,560000,605000]

Here's the code sandbox for it: https://codesandbox.io/s/blue-pine-l5rjc8?file=/src/index.js