We’ve seen that % (the remainder operator) can be used to test whether a number is even or odd by using % 2 to check whether it’s divisible by two. Here’s another way to define whether a positive whole number is even or odd: Zero is even. One is odd. For any other number N, its evenness is the same as N - 2. Define a recursive function isEven co…

iseven.js

// Recursive function to determine if a number is even
function isEven( checkNum ) {

  	// if the number is negative then convert it to positive
  	if ( checkNum < 0 )
      checkNum *= -1;
  
  	// if the number is zero then it's even
  	if ( checkNum == 0 ) {
      result = true;
    }
  
    // if the number is odd then it's odd
    else if ( checkNum == 1 ) {
      result = false; 
    }
  
    // if the number isn't 0 or 1 then keep subtracting until it is
    else {
      checkNum = checkNum - 2;
      isEven( checkNum );
    }
  
  return result;
}

console.log(isEven(50));
// → true
console.log(isEven(75));
// → false
console.log(isEven(-1));
// → false