Create main.js.md

Add main.js an explanation of the provided JavaScript code

Author: ShadBalti

main.js.md

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+
+### `calculateAge` Function
+
+The `calculateAge` function performs the following tasks:
+
+1. **Get User Input:**
+   - Retrieves the user's birthdate from an HTML input element with the id 'birthdate'.
+
+```javascript
+let userDate = document.getElementById('birthdate').value;
+```
+
+2. **Check Validity of User Input:**
+   - Checks if the entered value is not a number (NaN) or an empty string. If invalid, it displays an alert and stops further execution.
+
+```javascript
+if (userDate == NaN || userDate == '') {
+  return alert('Enter a valid date of birth. You may have not entered a complete date or entered a non-existent date information.');
+}
+```
+
+3. **Create Date Objects:**
+   - Converts the user's birthdate and the current date into `Date` objects.
+
+```javascript
+const birthdate = new Date(userDate);
+const currentDate = new Date();
+```
+
+4. **Reset Result and Error Display:**
+   - Hides the result and error elements on the HTML page.
+
+```javascript
+document.getElementById('result').style.display = 'none';
+document.getElementById('error').style.display = 'none';
+```
+
+5. **Check Birthdate Validity:**
+   - If the birthdate is greater than the current date, it displays an error message.
+
+```javascript
+if (birthdate > currentDate) {
+  document.getElementById('error').style.display = 'block';
+  document.getElementById('error').innerHTML = 'Please enter a valid birthdate.';
+  return;
+}
+```
+
+6. **Calculate Age Difference:**
+   - Calculates the age difference between the birthdate and the current date in various units (milliseconds, years, months, weeks, days, hours, minutes, seconds).
+
+```javascript
+const ageInMilliseconds = currentDate - birthdate;
+const millisecondsInYear = 1000 * 60 * 60 * 24 * 365.25;
+const millisecondsInMonth = millisecondsInYear / 12;
+const millisecondsInWeek = 7 * 24 * 60 * 60 * 1000;
+// ... (similar calculations for other time units)
+```
+
+7. **Display Age Information:**
+   - Updates the HTML elements with the calculated age values.
+
+```javascript
+document.getElementById('years').textContent = years;
+document.getElementById('months').textContent = months;
+// ... (similar updates for other time units)
+```
+
+8. **Calculate and Display Life Milestones:**
+   - Calculates and displays life milestones like retirement age, half-life, and specific ages.
+
+```javascript
+const retirementAge = 65;
+const halfLife = years / 2;
+const age18 = 18;
+const age21 = 21;
+const age30 = 30;
+
+document.getElementById('retirementAge').textContent = retirementAge;
+document.getElementById('halfLife').textContent = halfLife.toFixed(2); // Show with two decimal places
+// ... (similar updates for other milestones)
+```
+
+9. **Additional Notes:**
+   - The code provides flexibility to add more milestones as needed.