importance: 5

---

# Sum all numbers till the given one

Write a function `sumTo(n)` that calculates the sum of numbers `1 + 2 + ... + n`.

For instance:

```js no-beautify
sumTo(1) = 1
sumTo(2) = 2 + 1 = 3
sumTo(3) = 3 + 2 + 1 = 6
sumTo(4) = 4 + 3 + 2 + 1 = 10
...
sumTo(100) = 100 + 99 + ... + 2 + 1 = 5050
```

Make 3 solution variants:

1. Using a for loop.
2. Using a recursion, cause `sumTo(n) = n + sumTo(n-1)` for `n > 1`.
3. Using the [arithmetic progression](https://en.wikipedia.org/wiki/Arithmetic_progression) formula.

An example of the result:

```js
function sumTo(n) { /*... your code ... */ }

alert( sumTo(100) ); // 5050
```

P.S. Which solution variant is the fastest? The slowest? Why?

P.P.S. Can we use recursion to count `sumTo(100000)`?