feat: catch up on fcc dailies

src/freecodecamp/daily/2025-11-27/main.ts

@@ -0,0 +1,51 @@
+/**
+ * @copyright NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+/**
+ * Given the date of someone's birthday in the format YYYY-MM-DD, return the person's age as of November 27th, 2025.
+ * @param birthday - The date of someone's birthday in the format YYYY-MM-DD.
+ * @returns The person's age as of November 27th, 2025.
+ * @throws {TypeError} If the birthday date is invalid.
+ * @see https://www.freecodecamp.org/learn/daily-coding-challenge/2025-11-27
+ */
+export const calculateAge = (birthday: string): number => {
+  const [ year, month, day ] = birthday.split("-").map(Number);
+  if (
+    year === undefined
+    || month === undefined
+    || day === undefined
+    || Number.isNaN(year)
+    || Number.isNaN(month)
+    || Number.isNaN(day)
+  ) {
+    throw new TypeError("Invalid birthday date");
+  }
+
+  const parsed = new Date(`${birthday}T00:00:00Z`);
+  if (
+    Number.isNaN(parsed.getTime())
+    || parsed.getUTCFullYear() !== year
+    || parsed.getUTCMonth() + 1 !== month
+    || parsed.getUTCDate() !== day
+  ) {
+    throw new TypeError("Invalid birthday date");
+  }
+
+  const referenceYear = 2025;
+  const referenceMonth = 11;
+  const referenceDay = 27;
+
+  const isLaterMonth = month > referenceMonth;
+  const isSameMonth = month === referenceMonth;
+  const isLaterDay = day > referenceDay;
+  const isLaterDayInSameMonth = isSameMonth && isLaterDay;
+  const hasUpcomingBirthday = isLaterMonth || isLaterDayInSameMonth;
+  const birthdayAdjustment = hasUpcomingBirthday
+    ? 1
+    : 0;
+
+  return referenceYear - year - birthdayAdjustment;
+};