SumOfGeometricProgression.js

/*
 Returns the sum of a geometric progression
 Article on Geometric Progression: https://en.wikipedia.org/wiki/Geometric_series
 Examples:
 > sumOfGeometricProgression(2, 0.5, 6)
 3.9375
 > sumOfGeometricProgression(0.5, 10, 3)
 55.5
 > sumOfGeometricProgression(0.5, 10, Infinity)
 Error: The geometric progression is diverging, and its sum cannot be calculated
*/

/**
 *
 * @param {Number} firstTerm The first term of the geometric progression
 * @param {Number} commonRatio The common ratio of the geometric progression
 * @param {Number} numOfTerms The number of terms in the progression
 */
function sumOfGeometricProgression(firstTerm, commonRatio, numOfTerms) {
 if (!Number.isFinite(numOfTerms)) {
 /*
 If the number of Terms is Infinity, the common ratio needs to be less than 1 to be a convergent geometric progression
 Article on Convergent Series: https://en.wikipedia.org/wiki/Convergent_series
 */
 if (Math.abs(commonRatio) < 1) return firstTerm / (1 - commonRatio)
 throw new Error(
 'The geometric progression is diverging, and its sum cannot be calculated'
 )
 }

 if (commonRatio === 1) return firstTerm * numOfTerms

 return (
 (firstTerm * (Math.pow(commonRatio, numOfTerms) - 1)) / (commonRatio - 1)
 )
}

export { sumOfGeometricProgression }