Question: ```js
var num = prompt("Enter a number");

for (var sum = 0; sum <= num; sum++) {
  sum = sum + 1;
}
document.write(sum);
```

example when I enter 6 in the prompt it will sum 1+2+3+4+5+6 =21. but as of right now i can only print 123456 instead of 21.

Answer: 
* Input received by you is a string and that's why it's contacting rather than adding to sum.
* This is the best optimum solution as its time complexity is O(3) times only.
* so, it's fast. rather than with brute force which is o(n);

```js
var num = prompt("Enter a number");

function total(n) {
  return n * (n + 1) / 2;
}
document.write(total(parseInt(num)));
```
here is some change in your code.  

**note**: (+) is used to convert string-number type to number type

```js
var num = prompt("Enter a number");
const sum = Array.from(Array(+num + 1).keys()).reduce((prev, curr) => prev += curr, 0);
document.write(sum);
```