This blog will teach you how to find the sum of the elements that are on the diagonal of a 2-dimensional array, which is also called a matrix.

## 1. Calculating the Diagonal sum of a given array

let’s say we have an array called numbers and we want to find the diagonal sum of that array.

```
const numbers = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
];
```

As you know that there are two diagonal in this array(numbers) :

1. Top-Left to Bottom-Right.

2. Top-Right to Bottom-Left.

So we need to calculate two different sums here. The first sum is for the diagonal line that goes from the top-left corner to the bottom-right corner, and the second sum is for the diagonal line that goes from the top-right corner to the bottom-left corner.

### Top-Left to Bottom-Right

Let’s say we want to calculate the diagonal sum of the array(numbers).

This is how we will calculate the sum :

```
const numbers = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
];
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
sum = sum + numbers[i][i];
}
console.log(sum); // Output: 15
```

In this example, we have an array(numbers) with 3 sub-arrays, each containing 3 elements. To find the diagonal sum, we create a variable `sum`

and set it to 0. Then, we loop through the array using a `for`

loop and add the element at index

of each sub-array to the **i**`sum`

variable.

Note that the index

is used twice for each element, since we want to get elements that are on the diagonal of the array. Finally, we log the **i**`sum`

variable to the console, which outputs `15`

, the diagonal sum of the array.

## Top-Right to Bottom-Left

This is how we will calculate the sum :

```
const numbers = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
];
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
sum = sum + numbers[i][numbers.length - i - 1];
}
console.log(sum); // Output: 15
```

To calculate the sum , we first declare a variable

and initialize it to **sum****0**. Then, we use a `for`

loop to iterate over each row of the array. Inside the loop, we use the loop index

to calculate the column index for the element on the diagonal line.**i**

We do this by subtracting the current index

from the length of the array **i**

, and then subtracting 1 to account for zero-indexing. This gives us the correct column index for the element on the diagonal line from top right to bottom left.**numbers.length**

We then add the element at this index to the `sum`

variable. Finally, we log the `sum`

variable to the console, which should output

, the sum of the elements along the diagonal line from top right to bottom left.**15**

**ALSO SEE- ARRAY.FILTER() METHOD IN JAVASCRIPT.**