Helpful failure output

A failing test should tell you everything

When a test fails, the message should answer: what did we expect, what did we get, and where? Bad failure messages cost minutes per debug session — multiplied by hundreds of failing CI runs per year, the cost compounds.

function assertEqual(actual, expected, msg) {
  if (actual !== expected) {
    throw new Error(
      \`${msg || "values differ"}\n  expected: ${JSON.stringify(expected)}\n  actual:   ${JSON.stringify(actual)}\n\`
    );
  }
}

Good vs bad

BAD:  Error: assert failed
GOOD: Error: cart total
        expected: 225
        actual:   220

The good version tells you WHICH assert, WHAT was expected, and WHAT actually came out — you can fix it without re-running in a debugger.