# 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
const averageBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
arr.length;

console.log(averageBy([{ a: 40 }, { a: 20 }, { a: 80 }, { a: 60 }], o => o.a));
console.log(averageBy([{ a: 4 }, { a: 2 }, { a: 8 }, { a: 6 }], 'a'));
```
```

Sample Output:

```50
5
```

Pictorial Presentation:

Flowchart:

Live Demo:

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

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## JavaScript: Tips of the Day

How to remove a specific item from an array?

Find the index of the array element you want to remove using indexOf, and then remove that index with splice.

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

```const array = [2, 5, 9];

console.log(array);

const index = array.indexOf(5);
if (index > -1) {
array.splice(index, 1);
}

// array = [2, 9]
console.log(array);```

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

For the reason of completeness, here are functions. The first function removes only a single occurrence (i.e. removing the first match of 5 from [2,5,9,1,5,8,5]), while the second function removes all occurrences:

```function removeItemOnce(arr, value) {
var index = arr.indexOf(value);
if (index > -1) {
arr.splice(index, 1);
}
return arr;
}

function removeItemAll(arr, value) {
var i = 0;
while (i < arr.length) {
if (arr[i] === value) {
arr.splice(i, 1);
} else {
++i;
}
}
return arr;
}
//Usage
console.log(removeItemOnce([2,5,9,1,5,8,5], 5))
console.log(removeItemAll([2,5,9,1,5,8,5], 5))
```

Ref: https://bit.ly/2N9nKRp