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# How to get the standard deviation of an array of numbers using JavaScript ?

• Last Updated : 15 Jul, 2021

Given an array and the task is to calculate the standard deviation of it.

Example:

```Input:  [1, 2, 3, 4, 5]
Output: 1.4142135623730951

Input:  [23, 4, 6, 457, 65, 7, 45, 8]
Output: 145.13565852332775```

Please refer to Mean, Variance, and Standard Deviation for details.

Mean is average of element. Where 0 <= i < n

Mean of arr[0..n-1] = ∑(arr[i]) / n

Variance is the sum of squared differences from the mean divided by a number of elements.

Variance = ∑(arr[i] – mean)2 / n

Standard Deviation is the square root of the variance.

Standard Deviation = variance ^ 1/2

Approach: To calculate the standard deviation first we calculate the mean and then variance and then deviation. To calculate the mean we use Array.reduce() method and calculate the sum of all the array items and then divide the array with the length of the array.

To calculate the variance we use the map() method and mutate the array by assigning (value – mean) ^ 2 to every array item, and then we calculate the sum of the array, and then we divide the sum with the length of the array. To calculate the standard deviation we calculate the square root of the array.

Example:

## Javascript

 ``

Output:

```1.4142135623730951
145.13565852332775```

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