# JavaScript: Get the standard deviation of an array of numbers

## JavaScript fundamental (ES6 Syntax): Exercise-225 with Solution

Write a JavaScript program to get the standard deviation of an array of numbers.

• Use Array.prototype.reduce() to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation.
• Omit the second argument, usePopulation, to get the sample standard deviation or set it to true to get the population standard deviation.

Sample Solution:

JavaScript Code:

`````` // Define a function 'standardDeviation' to calculate the standard deviation of an array
const standardDeviation = (arr, usePopulation = false) => {
// Calculate the mean of the array
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;

// Calculate the sum of squared differences from the mean
const sumOfSquaredDifferences = arr.reduce((acc, val) => acc.concat((val - mean) ** 2), [])
.reduce((acc, val) => acc + val, 0);

// Calculate the standard deviation
return Math.sqrt(sumOfSquaredDifferences / (arr.length - (usePopulation ? 0 : 1)));
};

// Log the standard deviation of the array with and without using population adjustment
console.log(standardDeviation([10, 2, 38, 23, 38, 23, 21]));
console.log(standardDeviation([10, 2, 38, 23, 38, 23, 21], true));
```
```

Output:

```13.284434142114991
12.29899614287479
```

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-225-1 by w3resource (@w3resource) on CodePen.

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