Debugging and tests: finding and checking errors

Reproduce the problem first

Record steps, inputs, expected output and actual output. Reduce the example while keeping the failure. For a wrongly initialized counter, examine the isolated calculation before investigating the whole interface.

Console and breakpoints

The console displays messages and errors; a stack trace shows the execution path. A DevTools breakpoint pauses execution so you can inspect variables. Step through instructions to find where the value becomes wrong. Excessive logging can obscure useful evidence.

A minimal Node.js test

Use a Node.js version with the built-in test runner. Save this as add.test.mjs; .mjs enables ES modules. Run node --test add.test.mjs in that folder.

import test from "node:test";
import assert from "node:assert/strict";

function add(a, b) {
  return a + b;
}

test("adds two numbers", () => {
  assert.equal(add(4, 7), 11);
});

test("zero preserves the value", () => {
  assert.equal(add(4, 0), 4);
});

Coverage and limits

These tests check two numerical outputs, not a UI, API or every possible input. The function is inline to keep this example self-contained; in a project, import the real function so tests detect implementation changes.

Choose useful cases

Include a typical case and relevant limits such as zero or an empty collection. Define invalid-input behaviour first. Check expected behaviour, rather than only whether execution throws.

Regression exercise

1. Temporarily replace addition with subtraction and verify failure.
2. Restore addition and verify success.
3. Add a negative-number case and calculate its expectation manually.