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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Drawing a performance conclusion from a single manual Date.now() before/after measurement

// Wrong: unreliable, one-shot measurement
const start = Date.now();
myFunction();
console.log(Date.now() - start);

// Correct: statistically reliable, accounts for JIT warm-up
const bench = new Bench();
bench.add("myFunction", () => myFunction());
await bench.run();

The Solution //

A single execution is heavily influenced by V8's JIT compiler still warming up, and by system noise (background processes, garbage collection timing) — it doesn't represent steady-state performance. Use a proper microbenchmarking library like tinybench, which runs many iterations and accounts for warm-up.

The Error //

Comparing two implementations against different input data or under different conditions

// Wrong: not a fair, isolated comparison
bench.add("A", () => implA(dataset1));
bench.add("B", () => implB(dataset2)); // different data — invalid

// Correct: only the implementation differs
bench.add("A", () => implA(sharedDataset));
bench.add("B", () => implB(sharedDataset));

The Solution //

A valid benchmark comparison must change exactly one variable — the implementation itself. Testing implementation A against a small dataset and implementation B against a large one (or on different hardware, with other processes running) produces a comparison where the measured difference could be explained by the uncontrolled variable rather than the actual code being compared.