Step 1: Define the Function

The first step is to define a function in JavaScript. We will name it calculateSurfaceAreaPyramid. This function will take two arguments: baseLength and slantHeight, which represent the base length and slant height of the pyramid respectively.

function calculateSurfaceAreaPyramid(baseLength, slantHeight) {
  // function body goes here
}

Step 2: Validate the Arguments

In the body of the function, we will first validate the arguments to make sure that they are both numbers and are positive. We do this because the base length and slant height of a pyramid must be positive numbers.

function calculateSurfaceAreaPyramid(baseLength, slantHeight) {
  if (typeof baseLength !== 'number' || typeof slantHeight !== 'number' || baseLength <= 0 || slantHeight <= 0) {
    return 'Invalid input';
  }
  // calculation code goes here
}

Step 3: Calculate the Base Area

In the next step, we calculate the base area of the pyramid. This can be done quite simply by squaring the base length.

function calculateSurfaceAreaPyramid(baseLength, slantHeight) {
  if (typeof baseLength !== 'number' || typeof slantHeight !== 'number' || baseLength <= 0 || slantHeight <= 0) {
    return 'Invalid input';
  }

  let baseArea = Math.pow(baseLength, 2);
  // more code will go here
}

Step 4: Calculate the Lateral Area

After calculating the base area, we will calculate the lateral area. This is done by multiplying the base length by the slant height and then dividing by 2.

function calculateSurfaceAreaPyramid(baseLength, slantHeight) {
  if (typeof baseLength !== 'number' || typeof slantHeight !== 'number' || baseLength <= 0 || slantHeight <= 0) {
    return 'Invalid input';
  }

  let baseArea = Math.pow(baseLength, 2);
  let lateralArea = (baseLength * slantHeight) / 2;
  // more code will go here
}

Step 5: Calculate and Return the Total Surface Area

The final step is to calculate the total surface area of the pyramid. This is done by adding the base area to four times the lateral area. The result is then returned.

function calculateSurfaceAreaPyramid(baseLength, slantHeight) {
  if (typeof baseLength !== 'number' || typeof slantHeight !== 'number' || baseLength <= 0 || slantHeight <= 0) {
    return 'Invalid input';
  }

  let baseArea = Math.pow(baseLength, 2);
  let lateralArea = (baseLength * slantHeight) / 2;

  let totalSurfaceArea = baseArea + 4 * lateralArea;
  return totalSurfaceArea;
}

Conclusion

Congratulations, you have just written a JavaScript function to calculate the surface area of a pyramid given the length of its base and its slant height. Ensuring the inputs are valid adds robustness to your function, and commenting your code helps others (and your future self) understand what it is doing.