Errors and Debugging

Debugging is the process of narrowing the distance between what the program did and what you expected. Start from evidence, reproduce reliably, and change one assumption at a time.

Throw useful errors

01function progressPercent(completed, total) {
02  if (!Number.isFinite(total) || total <= 0) {
03    throw new TypeError("total must be a positive number");
04  }
05  return Math.round((completed / total) * 100);
06}

Throw when a function cannot fulfill its contract. Include the invalid concept and expected condition without exposing secrets or private data.

Catch where recovery is possible

01function readSettings(raw) {
02  try {
03    return JSON.parse(raw);
04  } catch (error) {
05    console.warn("Settings were invalid; defaults restored", error);
06    return { theme: "system" };
07  }
08}

Do not catch every error at every level. Catch where you can retry, use a fallback, clean up, or present actionable feedback.

Use the debugger

• Read the first error and its full stack trace.
• Reproduce the smallest failing case.
• Set a breakpoint before the incorrect value appears.
• Inspect scope, call stack, network requests, and DOM state.
• Add a focused test before fixing a regression.
• Verify the fix under the original conditions.

01console.table(courses);
02console.assert(total >= completed, "Completed lessons exceed total");

Keep this

Define contracts, preserve error context, catch only where recovery exists, and debug from observable evidence.