Calculates standard deviation

standard-deviation.js

/**
 * Gets the standard deviation from a set of numbers.
 * @example
 * const { mean, stdv } = getStandardDeviation([10, 10, 10, 10, 5]);
 * console.log(mean); // 9
 * console.log(stdv); // 2
 * @param {[number]} numbers - An array of numbers. 
 * @returns Object containing `mean` and `stdv`.
 */
function getStandardDeviation(numbers) {
    // Calculates the mean of all numbers
    function calcMean(numbers) {
        if (numbers.length === 0) {
            return 0;
        }
        
        return numbers.reduce((sum, number) => sum + number, 0) / numbers.length;
    }
    
    // Calculates the standard deviation of all numbers
    function calcStandardDeviation(numbers, mean) {
        const deviationsSum = numbers.reduce((sum, number) => {
            const diff = number - mean;
            
            return sum + (diff * diff);
        }, 0);
        
        return Math.sqrt(deviationsSum / numbers.length);
    }
    
    const mean = calcMean(numbers);
    const stdv = calcStandardDeviation(numbers, mean);
    
    return {
        mean,
        stdv,
    };
}