Implement getOrdinalNumber and add Jest tests usingtdd

Author: remimarcelle

Sprint-3/2-practice-tdd/get-ordinal-number.js

@@ -1,5 +1,41 @@
+/**
+ * Converts a number into its ordinal string form (e.g. 1 → "1st", 2 → "2nd").
+ *
+ * English ordinal rules:
+ * - Numbers ending in 11, 12, or 13 always use "th" (they are exceptions).
+ * - Numbers ending in 1 (but not 11) use "st".
+ * - Numbers ending in 2 (but not 12) use "nd".
+ * - Numbers ending in 3 (but not 13) use "rd".
+ * - Everything else uses "th".
+ *
+ * @param {number} num - A positive whole number (e.g. 1, 22, 113).
+ * @returns {string} The number with its ordinal suffix appended (e.g. "1st", "22nd").
+ *
+ * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Remainder
+ * @see https://www.freecodecamp.org/news/javascript-modulo-operator/
+ */
 function getOrdinalNumber(num) {
-  return "1st";
+  // % is the "remainder" operator (also called modulo).
+  // num % 100 gives the last two digits of any number.
+  // e.g. 113 % 100 = 13, so we catch the 11/12/13 exceptions correctly.
+  const lastTwoDigits = num % 100;
+
+  // num % 10 gives the last single digit of any number.
+  // e.g. 21 % 10 = 1, so we know it ends in 1 → "st".
+  const lastDigit = num % 10;
+
+  // Check the 11/12/13 exceptions FIRST.
+  // If we checked lastDigit first, 11 would wrongly get "st" instead of "th".
+  if (lastTwoDigits === 11 || lastTwoDigits === 12 || lastTwoDigits === 13) {
+    return num + "th";
+  }
+
+  if (lastDigit === 1) return num + "st";
+  if (lastDigit === 2) return num + "nd";
+  if (lastDigit === 3) return num + "rd";
+
+  // All remaining numbers (4–9, 0, and any not caught above) use "th".
+  return num + "th";
 }
 
-module.exports = getOrdinalNumber;
+module.exports = getOrdinalNumber;

Sprint-3/2-practice-tdd/get-ordinal-number.test.js

@@ -18,3 +18,43 @@ test("should append 'st' for numbers ending with 1, except those ending with 11"
   expect(getOrdinalNumber(21)).toEqual("21st");
   expect(getOrdinalNumber(131)).toEqual("131st");
 });
+
+// Case 2: Numbers ending with 2 (but not 12)
+// When the number ends with 2, except those ending with 12,
+// Then the function should return a string by appending "nd" to the number.
+test("should append 'nd' for numbers ending with 2, except those ending with 12", () => {
+  expect(getOrdinalNumber(2)).toEqual("2nd");
+  expect(getOrdinalNumber(22)).toEqual("22nd");
+  expect(getOrdinalNumber(132)).toEqual("132nd");
+});
+
+// Case 3: Numbers ending with 3 (but not 13)
+// When the number ends with 3, except those ending with 13,
+// Then the function should return a string by appending "rd" to the number.
+test("should append 'rd' for numbers ending with 3, except those ending with 13", () => {
+  expect(getOrdinalNumber(3)).toEqual("3rd");
+  expect(getOrdinalNumber(23)).toEqual("23rd");
+  expect(getOrdinalNumber(133)).toEqual("133rd");
+});
+
+// Case 4: Numbers ending with 11, 12, or 13
+// When the number ends with 11, 12, or 13,
+// Then the function should return a string by appending "th" to the number.
+test("should append 'th' for numbers ending with 11, 12, or 13", () => {
+  expect(getOrdinalNumber(11)).toEqual("11th");
+  expect(getOrdinalNumber(12)).toEqual("12th");
+  expect(getOrdinalNumber(13)).toEqual("13th");
+});
+
+// Case 5: All other numbers
+// When the number does not end with 1, 2, or 3 (and is not an exception),
+// Then the function should return a string by appending "th" to the number.
+test("should append 'th' for all other numbers", () => {
+  expect(getOrdinalNumber(4)).toEqual("4th");
+  expect(getOrdinalNumber(10)).toEqual("10th");
+  expect(getOrdinalNumber(20)).toEqual("20th");
+  expect(getOrdinalNumber(100)).toEqual("100th");
+  expect(getOrdinalNumber(111)).toEqual("111th");
+});
+
+