implement isProperFraction with console.assert tests for all cases

Author: remimarcelle

Sprint-3/1-implement-and-rewrite-tests/implement/2-is-proper-fraction.js

@@ -12,6 +12,10 @@
 
 function isProperFraction(numerator, denominator) {
   // TODO: Implement this function
+  // a denominator of 0 is not a proper fraction - it is invalid. divison by 0 undefined.
+  if (denominator === 0) return false;
+  // Maths.abs handles negative numbers, -> e.g. -1/2 is a proper fraction, but 1/-2 is not.
+  return Math.abs(numerator) < Math.abs(denominator);
 }
 
 // The line below allows us to load the isProperFraction function into tests in other files.
@@ -31,3 +35,25 @@ function assertEquals(actualOutput, targetOutput) {
 
 // Example: 1/2 is a proper fraction
 assertEquals(isProperFraction(1, 2), true);
+
+// TODO: Write tests to cover all cases.
+// What combinations of numerators and denominators should you test?
+// Using inline style as the result doesn't need to be stored,
+// we only need to check if it equals the expected value
+
+// Example: 1/2 is a proper fraction
+assertEquals(isProperFraction(1, 2), true);
+
+// Proper fractions: numerator smaller than denominator
+assertEquals(isProperFraction(3, 4), true); // normal proper fraction
+assertEquals(isProperFraction(-1, 2), true); // negative numerator - still proper
+assertEquals(isProperFraction(0, 5), true); // zero numerator - 0/5 = 0, which is proper
+
+// Improper fractions: numerator greater than or equal to denominator
+assertEquals(isProperFraction(2, 1), false); // numerator bigger than denominator
+assertEquals(isProperFraction(5, 3), false); // normal improper fraction
+assertEquals(isProperFraction(4, 4), false); // boundary: equal - 4/4 = 1, a whole number not proper
+
+// Invalid: zero denominator - division by zero is undefined
+assertEquals(isProperFraction(1, 0), false);
+