The difference between a junior and senior engineer staring at the same stack trace is rarely knowledge. It is method.
Read the whole thing, bottom first
The top line is where the error surfaced. The bottom is usually where it started. Most people read the first line, form a theory, and spend an hour defending it.
Read it backwards. Find the last line that is your code — that is where to put the first breakpoint.
Separate the three questions
Debugging goes wrong when these get mixed:
- What is happening? Observable facts only. No theories.
- Why is it happening? The mechanism.
- What should it do instead? The fix.
Jumping to 3 before finishing 1 produces the fix that makes the symptom disappear while the cause stays.
Make it happen on demand
An intermittent bug you cannot reproduce is not a bug you can fix — it is a bug you can only guess at. Before anything else, find the smallest input that triggers it reliably.
Often the act of shrinking the reproduction is the diagnosis.
Works with 1 item. Fails with 200. -> pagination or a limit
Works signed in as me. Fails for them. -> permissions or data shape
Works on retry. Fails the first time. -> a race or a cold cache
Believe the computer
When the evidence says something impossible, one of your assumptions is wrong. It is almost never the compiler.
The usual culprits:
- You are running different code than you think (stale build, wrong branch, cached bundle).
- You are hitting a different environment than you think.
- The value is a string and you are comparing it to a number.
"That can't happen" is the most expensive sentence in engineering. It usually means the check that would have caught it was never written.
Leave a trail
Once you have found it, the fix is the smaller half of the work. The larger half:
- Add the log line that would have made this obvious in one minute instead of an hour.
- Add the constraint or test that stops the class of bug, not just this instance.
- Write the commit message so the next person understands why, not just what.
Every bug is a free lesson about where your system is hard to see into. Spend it.