# JavaScript: Compute the average of an array, after mapping each element to a value using the provided function

## JavaScript fundamental (ES6 Syntax): Exercise-57 with Solution

Write a JavaScript program to compute the average of an array, after mapping each element to a value using the provided function.

• Use Array.prototype.map() to map each element to the value returned by fn.
• Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
• Divide the resulting array by its length.

Sample Solution:

JavaScript Code:

``````//#Source https://bit.ly/2neWfJ2
// Define a function 'averageBy' to calculate the average value of an array based on a given function or property.
const averageBy = (arr, fn) =>
// Map each element of the array to the value returned by the function or the value of the property.
arr.map(typeof fn === 'function' ? fn : val => val[fn])
// Calculate the sum of all values using the 'reduce' method.
.reduce((acc, val) => acc + val, 0) /
// Divide the sum by the length of the array to get the average.
arr.length;

// Test cases
console.log(averageBy([{ a: 40 }, { a: 20 }, { a: 80 }, { a: 60 }], o => o.a)); // Output: 50 (average of 'a' property)
console.log(averageBy([{ a: 4 }, { a: 2 }, { a: 8 }, { a: 6 }], 'a'));          // Output: 5 (average of 'a' property)
```
```

Output:

```50
5
```

Visual Presentation:

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-57-1 by w3resource (@w3resource) on CodePen.

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