[Codewars] Sum of the first nth term of Series (7 kyu) / JavaScript

https://www.codewars.com/kata/555eded1ad94b00403000071

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DESCRIPTION:

Task:

Your task is to write a function which returns the sum of following series upto nth term(parameter).

Series: 1 + 1/4 + 1/7 + 1/10 + 1/13 + 1/16 +...

 

Rules:

• You need to round the answer to 2 decimal places and return it as String.
• If the given value is 0 then it should return 0.00
• You will only be given Natural Numbers as arguments.

 

Examples:(Input --> Output)

1 --> 1 --> "1.00"
2 --> 1 + 1/4 --> "1.25"
5 --> 1 + 1/4 + 1/7 + 1/10 + 1/13 --> "1.57"

 

코드

function SeriesSum(n)
{
  let sum = 0;
  let num = 1;
  for(let i = 0; i < n; i++){
    sum += 1 / num;
    num += 3;
  }
  return sum.toFixed(2);
}