🔴 Reading Error Messages

Option B correctly identifies both the immediate error (calling .map() on undefined) and the likely root cause (the data was expected but missing). This is the most useful explanation because it tells you what went wrong and why it typically happens. Option A confuses a type error with a syntax/language error. Option C misreads "reading 'map'" as a problem inside the map function — it's the property name being read, not the function. Option D is wrong — map is a built-in array method, not an import. Pattern: TypeError: Cannot read properties of undefined (reading 'X') always means you tried to access property X on undefined.

Option C gives the most precise and actionable explanation. It includes: (1) where it crashed — line 47, function process_order, (2) what it was trying to do — access key "price", (3) why it failed — the key didn't exist. This is the root cause. Option A is completely wrong. Option B confuses a KeyError with a SyntaxError. Option D makes a plausible-sounding guess but the error says 'price' is missing, not quantity. When reading tracebacks, always: (1) go to the bottom for the error type and message, (2) find the last file + line number — that's where the crash occurred.

Option B is the most accurate and useful explanation. It: (1) recognises 127.0.0.1 as localhost (the same machine), (2) identifies port 5432 as the default PostgreSQL port, (3) correctly interprets ECONNREFUSED as the connection being actively rejected — which typically means the service isn't listening. Option A confuses "refused" with "permission denied." Option C is possible but ECONNREFUSED specifically means the OS rejected the connection (the port isn't listening), not blocked by a firewall — firewalls usually cause a timeout (ETIMEDOUT), not a refusal. Option D describes EADDRINUSE, not ECONNREFUSED. Key: ECONNREFUSED = nothing is listening; ETIMEDOUT = blocked/no response.

Option C is the most complete and accurate explanation. It tells the junior developer: (1) what the operation was — inserting a new user, (2) what value caused the conflict, (3) why it failed — the unique constraint on the email column, (4) what a unique constraint means. Option A confuses this with a primary key violation (different thing). Option B is backwards — the constraint is defined, that's exactly why it's failing. Option D describes the business meaning but not the technical cause — a good engineer explains both. This error pattern: "duplicate key value violates unique constraint" always means you're trying to insert or update a value that already exists in a column marked as UNIQUE.