# JavaScript: Get the sum of a given array using the provided function

## JavaScript fundamental (ES6 Syntax): Exercise-32 with Solution

Write a JavaScript program to get the sum of a given 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.

Sample Solution:

JavaScript Code:

``````//#Source https://bit.ly/2neWfJ2
// Define a function named sumBy that calculates the sum of the values returned by applying a function to each element in an array.
const sumBy = (arr, fn) =>
// Map the array to extract the values based on the provided function or key.
arr.map(typeof fn === 'function' ? fn : val => val[fn])
// Reduce the mapped values to calculate their sum.
.reduce((acc, val) => acc + val, 0);

// Test the function with different arrays and functions/keys.
console.log(sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n)); // Output: 20
console.log(sumBy([{ n: -4 }, { n: -2 }, { n: 8 }, { n: 6 }], 'n'));    // Output: 8
```
```

Output:

```20
8
```

Visual Presentation:

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-1-32 by w3resource (@w3resource) on CodePen.

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