tally.test.js

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 }
 */

// Acceptance criteria:

// 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("tally counts the frequency of items in an array", () => {
  const input = ["a", "a", "b", "c"];
  const expectedOutput = { a: 2, b: 1, c: 1 };
  expect(tally(input)).toEqual(expectedOutput);
});
// Given an empty array
// When passed to tally
// Then it should return an empty object
test("tally on an empty array returns an empty object", () => {
  const input = [];
  const expectedOutput = {};
  expect(tally(input)).toEqual(expectedOutput);
});

// Given an array with duplicate items
// When passed to tally
// Then it should return counts for each unique item
test("tally counts duplicate items correctly", () => {
  const input = ["x", "y", "x", "z", "y", "x"];
  const expectedOutput = { x: 3, y: 2, z: 1 };
  expect(tally(input)).toEqual(expectedOutput);
});

// Given an invalid input like a string
// When passed to tally
// Then it should throw an error
test("tally throws an error for invalid input", () => {
  expect(() => tally("not an array")).toThrow("Input must be an array");
});

// Given an array with keywords that are also properties of Object.prototype, like "toString"
// When passed to tally
// Then it should count them correctly without being affected by the prototype chain

test("tally handles prototype keywords correctly", () => {
  const input = ["toString", "toString", "valueOf"];
  const expectedOutput = { toString: 2, valueOf: 1 };

  expect(tally(input)).toEqual(expectedOutput);
});