Sprint 1: sync exercises with error-handling prep

Add mean and describe-median katas to implement, and change the existing
fix/implement exercises to throw on bad input instead of ignoring it,
matching the updated Sprint 1 prep.

Author: abdishakoor-dev

Sprint-1/fix/median.js

@@ -2,8 +2,8 @@
 // Start by running the tests for this function
 // If you're in the Sprint-1 directory, you can run `npm test -- fix` to run the tests in the fix directory
 
-// Hint: Please consider scenarios when 'list' doesn't have numbers (the function is expected to return null)
-// or 'list' has mixed values (the function is expected to sort only numbers).
+// Hint: Please consider scenarios when 'list' isn't an array, is empty,
+// or contains values that aren't numbers (the function is expected to throw - see the tests).
 
 function calculateMedian(list) {
   const middleIndex = Math.floor(list.length / 2);

Sprint-1/fix/median.test.js

@@ -33,18 +33,39 @@ describe("calculateMedian", () => {
     expect(list).toEqual([3, 1, 2]);
   });
 
-  [ 'not an array', 123, null, undefined, {}, [], ["apple", null, undefined] ].forEach(val =>
-    it(`returns null for non-numeric array (${val})`, () => expect(calculateMedian(val)).toBe(null))
+  // There is no median of an empty array, so calculateMedian should throw
+  it("throws when given an empty array", () => {
+    expect(() => calculateMedian([])).toThrow(
+      new Error("calculateMedian requires a non-empty array")
+    );
+  });
+
+  // Non-array input should throw (no argument at all counts, as 'list' is then undefined)
+  [ 'banana', 123, null, undefined, {}, ["apple", null, undefined], ["ten", "twenty"] ].forEach(val =>
+    it(`throws for non-numeric input (${val})`, () =>
+      expect(() => calculateMedian(val)).toThrow(
+        new Error("calculateMedian requires an array of numbers")
+      ))
   );
 
+  it("throws when called with no argument", () => {
+    expect(() => calculateMedian()).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  // Arrays containing any non-number value should throw, rather than filtering them out
   [
-    { input: [1, 2, "3", null, undefined, 4], expected: 2 },
-    { input: ["apple", 1, 2, 3, "banana", 4], expected: 2.5 },
-    { input: [1, "2", 3, "4", 5], expected: 3 },
-    { input: [1, "apple", 2, null, 3, undefined, 4], expected: 2.5 },
-    { input: [3, "apple", 1, null, 2, undefined, 4], expected: 2.5 },
-    { input: ["banana", 5, 3, "apple", 1, 4, 2], expected: 3 },
-  ].forEach(({ input, expected }) =>
-    it(`filters out non-numeric values and calculates the median for [${input}]`, () => expect(calculateMedian(input)).toEqual(expected))
+    [1, 2, "3", null, undefined, 4],
+    ["apple", 1, 2, 3, "banana", 4],
+    [1, "2", 3, "4", 5],
+    [1, "apple", 2, null, 3, undefined, 4],
+    [3, "apple", 1, null, 2, undefined, 4],
+    ["banana", 5, 3, "apple", 1, 4, 2],
+  ].forEach((input) =>
+    it(`throws for an array containing a non-number [${input}]`, () =>
+      expect(() => calculateMedian(input)).toThrow(
+        new Error("calculateMedian requires an array of numbers")
+      ))
   );
 });

Sprint-1/implement/describe-median.js

@@ -0,0 +1,25 @@
+// Don't change this function.
+function calculateMedian(list) {
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMedian requires an array of numbers");
+  }
+  if (list.length === 0) {
+    throw new Error("calculateMedian requires a non-empty array");
+  }
+  for (const item of list) {
+    if (typeof item !== "number") {
+      throw new Error("calculateMedian requires an array of numbers");
+    }
+  }
+  const middleIndex = Math.floor(list.length / 2);
+  const sorted = [...list].sort((a, b) => a - b);
+  if (sorted.length % 2 === 0) {
+    return (sorted[middleIndex - 1] + sorted[middleIndex]) / 2;
+  }
+  return sorted[middleIndex];
+}
+
+// Implement this function. See describe-median.test.js for the acceptance criteria.
+function describeMedian(list) {}
+
+module.exports = describeMedian;

Sprint-1/implement/describe-median.test.js

@@ -0,0 +1,36 @@
+/* Describe the median, or explain why there isn't one
+
+calculateMedian is given to you at the top of describe-median.js. It throws
+when it can't calculate a median. Don't change it.
+
+Implement describeMedian, which calls calculateMedian and returns a sentence
+instead of crashing.
+
+E.g. describeMedian([1, 2, 3]), target output: "The median is 2"
+E.g. describeMedian([]), target output: "Could not calculate a median: calculateMedian requires a non-empty array"
+
+You'll need a try/catch block for this. The catch block must use the error's
+message, so the caller can see what went wrong.
+*/
+
+const describeMedian = require("./describe-median.js");
+
+// Acceptance criteria:
+
+// Given an array of numbers
+// When passed to describeMedian
+// Then it should return "The median is " followed by the median
+// Delete this test.todo and replace it with a test.
+test.todo('given [1, 2, 3], returns "The median is 2"');
+
+// Given an empty array
+// When passed to describeMedian
+// Then it should return "Could not calculate a median: calculateMedian requires a non-empty array"
+
+// Given something that isn't an array of numbers, e.g. "banana"
+// When passed to describeMedian
+// Then it should return "Could not calculate a median: calculateMedian requires an array of numbers"
+
+// Given an empty array
+// When passed to describeMedian
+// Then it should not throw

Sprint-1/implement/max.test.js

@@ -3,7 +3,7 @@
 In this kata, you will need to implement a function that find the largest numerical element of an array.
 
 E.g. max([30, 50, 10, 40]), target output: 50
-E.g. max(['hey', 10, 'hi', 60, 10]), target output: 60 (sum ignores any non-numerical elements)
+E.g. max(['hey', 10, 'hi', 60, 10]) throws Error("findMax requires an array of numbers") (max can't compare non-numerical elements, so it shouldn't guess)
 
 You should implement this function in max.js, and add tests for it in this file.
 
@@ -34,10 +34,10 @@ test.todo("given an empty array, returns -Infinity");
 // When passed to the max function
 // Then it should return the largest decimal number
 
-// Given an array with non-number values
+// Given an array containing a value that isn't a number
 // When passed to the max function
-// Then it should return the max and ignore non-numeric values
+// Then it should throw Error("findMax requires an array of numbers")
 
-// Given an array with only non-number values
+// Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the max function
-// Then it should return the least surprising value given how it behaves for all other inputs
+// Then it should throw Error("findMax requires an array of numbers")

Sprint-1/implement/mean.js

@@ -0,0 +1,3 @@
+function calculateMean(list) {}
+
+module.exports = calculateMean;

Sprint-1/implement/mean.test.js

@@ -0,0 +1,50 @@
+/* Calculate the mean of an array of numbers
+
+In this kata, you will need to implement a function that calculates the mean of an array of numbers.
+
+E.g. calculateMean([1, 2, 3]), target output: 2
+E.g. calculateMean([]) throws Error("calculateMean requires a non-empty array")
+E.g. calculateMean("banana") throws Error("calculateMean requires an array of numbers")
+
+There is no mean of an empty array, and a string isn't an array of numbers, so
+calculateMean should throw rather than return a value. This is the same problem
+calculateMedian solves in the prep and in the fix directory.
+
+You should implement this function in mean.js, and add tests for it in this file.
+
+We have set things up already so that this file can see your function from the other file.
+*/
+
+const calculateMean = require("./mean.js");
+
+// Acceptance criteria:
+
+// Given an array of numbers
+// When passed to calculateMean
+// Then it should return their mean
+// Delete this test.todo and replace it with a test.
+test.todo("given [1, 2, 3], returns 2");
+
+// Given an array with a single number
+// When passed to calculateMean
+// Then it should return that number
+
+// Given an array containing negative or decimal numbers
+// When passed to calculateMean
+// Then it should return the correct mean
+
+// Given an empty array
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires a non-empty array")
+
+// Given a value that isn't an array, e.g. "banana", 42, null or {}
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")
+
+// Given no argument at all
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")
+
+// Given an array containing a non-number value, e.g. [1, "2", 3]
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")

Sprint-1/implement/sum.test.js

@@ -3,7 +3,7 @@
 In this kata, you will need to implement a function that sums the numerical elements of an array
 
 E.g. sum([10, 20, 30]), target output: 60
-E.g. sum(['hey', 10, 'hi', 60, 10]), target output: 80 (ignore any non-numerical elements)
+E.g. sum(['hey', 10, 'hi', 60, 10]) throws Error("sum requires an array of numbers") (sum can't add non-numerical elements, so it shouldn't guess)
 */
 
 const sum = require("./sum.js");
@@ -27,10 +27,10 @@ test.todo("given an empty array, returns 0")
 // When passed to the sum function
 // Then it should return the correct total sum
 
-// Given an array containing non-number values
+// Given an array containing a value that isn't a number
 // When passed to the sum function
-// Then it should ignore the non-numerical values and return the sum of the numerical elements
+// Then it should throw Error("sum requires an array of numbers")
 
-// Given an array with only non-number values
+// Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the sum function
-// Then it should return the least surprising value given how it behaves for all other inputs
+// Then it should throw Error("sum requires an array of numbers")

Sprint-1/readme.md

@@ -31,6 +31,8 @@ Here is a recommended order:
 1. `max.test.js`
 2. `sum.test.js`
 3. `dedupe.test.js`
+4. `mean.test.js`
+5. `describe-median.test.js`
 
 Again, you can run just the test for the `implement` directory if you're in the `Sprint-1` directory by running `npm test -- implement`.