Add sequence of numbers in Javascript

Question

Let’s assume that the user type the value 40391, so I need to add incrementally all values from 1 up to 40390, which results in the value 815696245.

Today I do it with a simple for but I realized that the performance is not the best... Is there any way to make it more performatic ?

Answer 1

Yes, just use a formula.

resultado = n(n+1)/2

See working:

var num = 40391;
var res = (num * (num - 1))/2; /*  adaptado o "-1" para o enunciado da pergunta  */
                               /*  que diz que input 40391 resulta em 815696245  */

document.write(res);

Out of curiosity, this sequence generates what we call a "triangular number":

Further reading:

https://oeis.org/A000217

https://en.wikipedia.org/wiki/Triangular_number