We can use the bitwise AND& operator for this problem. The & operates on its operand and treats them as binary values and performs the AND operation.

function isEven(num) {
  if (num & 1) {
    return false;
  } else {
    return true;
  }
};

0 in binary is 000.
1 in binary is 001.
2 in binary is 010.
3 in binary is 011.
4 in binary is 100.
5 in binary is 101.
6 in binary is 110.
7 in binary is 111.
and so on...

So when we console.log this expression 5 & 1 it returns 1. Ok, first the & operator converts both numbers to binary so 5 turns to 101 and 1 turns to 001.
Then it compares every bit (0's and 1's) using the bitwise AND operator. 101 & 001. As we can see from the table the result can be only 1 if a AND b are 1.

• So first we compare the left most bit 1&0 the result should be 0.
• Then we compare the middle bit 0&0 the result should be 0.
• Then we compare the last bit 1&1 the result should be 1.
• Then the binary result 001 will be converted to a decimal number which will be 1.

If we console.log this expression 4 & 1 it will return 0. Knowing the last bit of 4 is 0 and 0 & 1 will be 0. If you have a hard time understand this we could use a recursive function to solve this problem.

function isEven(num) {
  if (num < 0 || num === 1) return false;
  if (num == 0) return true;
  return isEven(num - 2);
}