JSFiddle - React, Tailwind, and code Playground

JavaScript

for(n=prompt(o=0);n=n/2|0;o+=n%2);alert(o);

/*
// ungolfed

var n = 123;
var o = 0;
while (n > 0)
{
    o += n % 2;
    n = n / 2|0;
    alert(n);
}
alert(o);

//*/