Implement tally function and tests

Author: Richiealx

Sprint-2/implement/tally.js

@@ -1,3 +1,33 @@
-function tally() {}
+/**
+ * tally()
+ *
+ * Counts how many times each item appears in an array.
+ *
+ * Example:
+ * tally(['a','a','b','c'])
+ * returns { a: 2, b: 1, c: 1 }
+ */
+
+function tally(items) {
+  // Validate input
+  if (!Array.isArray(items)) {
+    throw new Error("Expected an array");
+  }
+
+  const counts = {};
+
+  // Loop through each item in the array
+  for (const item of items) {
+    // If the item already exists in the object, increase the count
+    if (counts[item]) {
+      counts[item] += 1;
+    } else {
+      // Otherwise initialise it
+      counts[item] = 1;
+    }
+  }
+
+  return counts;
+}
 
 module.exports = tally;

Sprint-2/implement/tally.test.js

@@ -1,34 +1,27 @@
 const tally = require("./tally.js");
 
-/**
- * tally array
- *
- * In this task, you'll need to implement a function called tally
- * that will take a list of items and count the frequency of each item
- * in an array
- *
- * For example:
- *
- * tally(['a']), target output: { a: 1 }
- * tally(['a', 'a', 'a']), target output: { a: 3 }
- * tally(['a', 'a', 'b', 'c']), target output: { a : 2, b: 1, c: 1 }
- */
+describe("tally()", () => {
+  test("returns an empty object for an empty array", () => {
+    expect(tally([])).toEqual({});
+  });
 
-// Acceptance criteria:
+  test("counts a single item", () => {
+    expect(tally(["a"])).toEqual({ a: 1 });
+  });
 
-// Given a function called tally
-// When passed an array of items
-// Then it should return an object containing the count for each unique item
+  test("counts repeated items", () => {
+    expect(tally(["a", "a", "a"])).toEqual({ a: 3 });
+  });
 
-// Given an empty array
-// When passed to tally
-// Then it should return an empty object
-test.todo("tally on an empty array returns an empty object");
+  test("counts multiple different items", () => {
+    expect(tally(["a", "a", "b", "c"])).toEqual({
+      a: 2,
+      b: 1,
+      c: 1,
+    });
+  });
 
-// Given an array with duplicate items
-// When passed to tally
-// Then it should return counts for each unique item
-
-// Given an invalid input like a string
-// When passed to tally
-// Then it should throw an error
+  test("throws an error for invalid input", () => {
+    expect(() => tally("not-an-array")).toThrow();
+  });
+});