Explain This Error

A JavaScript error. Paste Uncaught TypeError: Cannot read properties of undefined (reading 'map').
Language: JavaScript (the word “Uncaught” and the class name TypeError). Error type: TypeError.
Rules: all 54 are tested and one matches — the signature cannot read propert(y|ies) of (undefined|null) finds “Cannot read properties of undefined”.
Detail: the rule’s extract pattern captures map, so the card says “You read .map of something that was undefined/null.” The array you called .map on did not exist yet — typically data that had not loaded. First fix: items?.map(…) or a default of [].

Two rules match. Paste PHP Parse error: syntax error, unexpected token "}" in /var/www/html/index.php on line 27.
It matches the PHP parse-error rule (its longest signature, syntax error, unexpected, is 24 characters) and the JavaScript unexpected-token rule (16 characters). The longer, more specific signature wins: Parse error: syntax error (PHP), with Unexpected token listed under Also possible.
File and line: /var/www/html/index.php, line 27. PHP reports the line where it gave up, so check line 26 too — a missing ; or ) on the line above is the usual cause.

The common mistake: pasting the browser’s symptom instead of the server’s cause. The console shows GET https://example.com/api/users 500 (Internal Server Error). That matches only the HTTP 500 rule, whose advice is “the real cause is in its logs” — true, but not an answer. The server’s error log for the same request says PHP Fatal error: Uncaught Error: Call to undefined function mysqli_connect() in /var/www/api/users.php:14. That matches Call to undefined function (PHP), names “mysqli_connect()” as the missing function and points at users.php, line 14: the mysqli extension is not enabled in php.ini. A 5xx code always means “go and read the server log”; paste that line instead.

Most error formats are older than the languages that print them. Unix gave system calls a numeric errno with a short name and a sentence — ENOENT “No such file or directory”, EACCES “Permission denied” — and those exact phrases still appear in Node.js, Python and shell output today. POSIX shells add exit statuses on top: 127 when a command is not found, 126 when it cannot be executed, and 128 plus the signal number when a process is killed, which is why an out-of-memory kill shows up as 137.

HTTP status codes were grouped into five classes by the first digit in HTTP/1.0 (RFC 1945, 1996) and are now defined in RFC 9110 (2022). SQL-92 standardised the five-character SQLSTATE, whose first two characters give the class, and databases kept their own numbers alongside it — MySQL’s 1045, 1062 and 1064 map to SQLSTATE 28000, 23000 and 42000. JavaScript gained try/catch and the standard error classes such as TypeError and ReferenceError in ECMAScript 3 (1999).

Wording changes too. In 2021 V8, the engine in Chrome and Node.js, replaced “Cannot read property 'map' of undefined” with “Cannot read properties of undefined (reading 'map')”, so the rules here recognise both forms.

This tool reads a pasted error message and explains it in plain English: what went wrong, the usual causes, and a short checklist of fixes. It covers 54 common errors across browser/network, HTTP, JavaScript, SQL, PHP, Linux/server and SSL/DNS, and when nothing specific matches it still names the family and gives that family’s general guidance.

The rules are the same data as the Error Reference, so every explanation links to a full page for that error. Alongside the match it reads the language, the error type and the file and line the message points to, and Show Work lists every rule that matched and why the winner won.

Everything runs in your browser. The error text is never uploaded or sent to an AI, so it is safe to paste messages that contain paths, table names or internal hostnames. You can link to a prefilled error with ?q= (and &family=php to fix the family), but typing never changes the address bar.

Related tools: HTTP Status Codes, JSON Validator, and Regex Tester.