Accumulation of numbers in loops in JavaScript

Let's use the loop to find the sum of integers from 1 to 100. To solve such a problem, numbers are iterated over in loop and their sum is sequentially written to some variable:

let res = 0;

for (let i = 1; i <= 100; i++) {
	res = res + i;
}

console.log(res); // the required sum

You can simplify the solution through the operator +=:

let res = 0;

for (let i = 1; i <= 100; i++) {
	res += i;
}

console.log(res);

Find the sum of even numbers from 2 to 100.

Find the sum of odd numbers from 1 to 99.

Find the product of integers from 1 to 20.