Function to add up all the natural numbers from 1 to num in JavaScript

We are required to write a JavaScript function that takes in a number, say num.

Then our function should return the sum of all the natural numbers between 1 and num, including 1 and num.

For example, if num is ?

const num = 5;

Then the output should be ?

const output = 15;

because,

1+2+3+4+5 = 15

The Mathematical Formula

We will use the efficient mathematical formula to solve this problem ?

Sum of all natural numbers up to n =

((n*(n+1))/2)

This formula is much more efficient than using loops, especially for large numbers.

Using the Mathematical Formula

The code for this will be ?

const num = 5;
const sumUpto = num => {
    const res = (num * (num + 1)) / 2;
    return res;
};
console.log(sumUpto(num));
console.log(sumUpto(7));
console.log(sumUpto(45));
console.log(sumUpto(2));
console.log(sumUpto(8));
console.log(sumUpto(99));

15
28
1035
3
36
4950

Alternative: Using a Loop

For educational purposes, here's how you could solve it using a loop:

const sumUptoLoop = num => {
    let sum = 0;
    for (let i = 1; i <= num; i++) {
        sum += i;
    }
    return sum;
};

console.log(sumUptoLoop(5));
console.log(sumUptoLoop(10));

15
55

Comparison

Conclusion

The mathematical formula approach is the most efficient way to sum natural numbers from 1 to n. It provides constant time complexity and is the preferred method for production applications.