JavaScript Program to Find Average of Array

To find the average of an array in JavaScript, sum all elements using reduce and then divide by the array's length like this: const average = arr.reduce((a, b) => a + b, 0) / arr.length;.

Examples

Input[5]

Output5

Input[1, 2, 3, 4, 5]

Output3

Input[]

OutputNaN

How to Think About It

To find the average, first add all numbers in the array together, then divide that total by how many numbers there are. This gives the central value representing the array.

Algorithm

1

Get the input array of numbers.

2

Add all the numbers together to get the sum.

3

Count how many numbers are in the array.

4

Divide the sum by the count to get the average.

5

Return or print the average.

Code

javascript

const arr = [1, 2, 3, 4, 5];
const sum = arr.reduce((a, b) => a + b, 0);
const average = sum / arr.length;
console.log(average);

Output

3

Dry Run

Let's trace the array [1, 2, 3, 4, 5] through the code

1

Start with array

arr = [1, 2, 3, 4, 5]

2

Calculate sum using reduce

sum = 1 + 2 + 3 + 4 + 5 = 15

3

Calculate average

average = sum / arr.length = 15 / 5 = 3

4

Print average

Output: 3

Iteration	Accumulator (a)	Current Value (b)	Sum So Far
1	0	1	1
2	1	2	3
3	3	3	6
4	6	4	10
5	10	5	15

Why This Works

Step 1: Sum all elements

The reduce method adds each number in the array to an accumulator, starting from 0.

Step 2: Count elements

The length of the array tells us how many numbers there are to average.

Step 3: Divide sum by count

Dividing the total sum by the number of elements gives the average value.

Alternative Approaches

for loop sum

javascript

const arr = [1, 2, 3, 4, 5];
let sum = 0;
for (let i = 0; i < arr.length; i++) {
  sum += arr[i];
}
const average = sum / arr.length;
console.log(average);

Uses a simple loop instead of reduce; easier to understand for beginners but more code.

for...of loop

javascript

const arr = [1, 2, 3, 4, 5];
let sum = 0;
for (const num of arr) {
  sum += num;
}
const average = sum / arr.length;
console.log(average);

Cleaner loop syntax; good readability and same performance.

using array length check

javascript

const arr = [1, 2, 3, 4, 5];
if (arr.length === 0) {
  console.log('Array is empty');
} else {
  const sum = arr.reduce((a, b) => a + b, 0);
  const average = sum / arr.length;
  console.log(average);
}

Adds safety for empty arrays to avoid dividing by zero.

Complexity: O(n) time, O(1) space

Time Complexity

The program loops through the array once to sum all elements, so it takes time proportional to the number of elements, O(n).

Space Complexity

It uses a fixed amount of extra space for sum and average variables, so space complexity is O(1).

Which Approach is Fastest?

All approaches (reduce, for loop, for...of) have the same time and space complexity; choice depends on readability and style.

Approach	Time	Space	Best For
reduce method	O(n)	O(1)	Concise code, functional style
for loop	O(n)	O(1)	Beginners learning loops
for...of loop	O(n)	O(1)	Readable and modern syntax