findGCD()

Step 1: Define the Problem

In this step, we try to identify our problem. Our problem here is to find the Greatest Common Divisor (GCD) of two numbers. The GCD of two or more integers is the largest positive integer that divides each of the integers without a remainder.

Step 2: Start Coding Function

In JavaScript, we define our function findGCD that takes two parameters num1 and num2, which are the two numbers we're finding the GCD of.

function findGCD(num1, num2) {
}

Step 3: Implement Euclidean Algorithm

We could use the Euclidean algorithm to find the GCD. First, we check if num2 is zero. If true, num1 is our GCD. Otherwise, we recursively call findGCD with num2 and the remainder of num1 divided by num2 as the parameters.

function findGCD(num1, num2) {
  if(num2 === 0) return num1;
  return findGCD(num2, num1 % num2);
}

Step 4: Include Input Check

It's good practice to check the inputs. Here, we'll check if both inputs are numbers otherwise return a message stating that both inputs must be numbers.

function findGCD(num1, num2) {
  if(typeof num1 !== 'number' || typeof num2 !== 'number') return 'Both inputs must be numbers';
  if(num2 === 0) return num1;
  return findGCD(num2, num1 % num2);
}

Step 5: Full and Final Code

Now, we have our final function that first checks if inputs are numbers, then calculates the GCD using the Euclidean algorithm.

function findGCD(num1, num2) {
  if(typeof num1 !== 'number' || typeof num2 !== 'number') return 'Both inputs must be numbers';
  if(num2 === 0) return num1;
  return findGCD(num2, num1 % num2);
}