calculateSimpleInterest()

Step 1: Define the Function

We will start by creating a simple JavaScript function named calculateSimpleInterest. This function will take three parameters: principal, rate, and time. The function needn't perform any operations at this stage.

function calculateSimpleInterest(principal, rate, time) {

}

Step 2: Input Validation

Next, we'll add some input validation to our function. We want to make sure that all parameters are numbers and are greater than 0. If not, we'll return an error message.

function calculateSimpleInterest(principal, rate, time) {
  if (typeof principal !== 'number' || principal <= 0 || typeof rate !== 'number' || rate <= 0 || typeof time !== 'number' || time <= 0) {
    return 'Invalid inputs, all inputs must be numbers greater than 0';
  }
}

Step 3: Calculate Simple Interest

Now, we can calculate the simple interest using the formula: Interest = (Principal * Rate * Time) / 100. We'll return the calculated value from our function.

function calculateSimpleInterest(principal, rate, time) {
  if (typeof principal !== 'number' || principal <= 0 || typeof rate !== 'number' || rate <= 0 || typeof time !== 'number' || time <= 0) {
    return 'Invalid inputs, all inputs must be numbers greater than 0';
  }
  let interest = (principal * rate * time) / 100;
  return interest;
}

Step 4: Testing the Function

Let's test our function with some sample inputs to ensure it works correctly. For instance, if we call our function with a principal of 1000, a rate of 10, and a time of 2, it should return 200.

calculateSimpleInterest(1000, 10, 2);  // 200

Conclusion

We've created a function in JavaScript to calculate simple interest given a principal amount, a rate of interest, and a period of time. We also validated the inputs to make sure they're all numbers and greater than zero. Using this function, we can easily calculate simple interest in various situations.