use recursion to count "1" in number as binary

count1InBin.js

function count1InBin(n){
  if(n<2)
		return n&1;
	return count1InBin(n>>1)+(n&1); // note: +'s priority is higher than &
}

console.log(count1InBin(0)); // output: 0
console.log(count1InBin(1)); // output: 1
console.log(count1InBin(2)); // output: 1
console.log(count1InBin(3)); // output: 2
console.log(count1InBin(15)); // output: 4