Make one hypothesis face a counterexample

First, build a mental model

A hypothesis earns confidence by making a prediction that could be wrong. A regression test preserves the smallest counterexample after the fix.

1. Write competing explanations and choose a cheap experiment whose outcomes distinguish them. Repeating the same successful case adds little evidence.
2. Change the mechanism responsible for the contract failure. Avoid masking the symptom with broad catches, disabled validation or ignored errors.
3. Keep the original failing case and a nearby valid case. A fix that rejects every input can pass a rejection test while breaking the feature.
4. A regression test should fail against the defective behavior and pass against the corrected behavior for the intended reason.

function parseCount(raw) {
  const count = Number(raw);
  if (!Number.isSafeInteger(count) || count < 0) {
    throw new Error('Count must be a nonnegative integer');
  }
  return count;
}
// Critic's question: should '' really mean zero?

1. Hypothesis

   Numeric conversion alone may accept inputs the product considers missing.

2. Counterexample

   Number('') is 0. The shown function accepts a blank string.

3. Repair contract

   Decide whether blank input is invalid, then reject it explicitly before conversion and retain valid numeric input.