Error map
UnhandledPromiseRejection Node.js: [UnhandledPromiseRejection] — This error originated either by throwing inside of an async function without a catch block
Node.js · layer 4, Application & API
Centre: this error. First ring: errors it is linked with (11). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (11)
TypeError: Failed to fetch / NetworkError when attempting to fetch resource
The browser couldn’t complete the request at the network level — CORS, DNS, offline, blocked mixed content, or the server refused…
Warning: An update to Component inside a test was not wrapped in act(...)
A component updated state after the test’s synchronous assertions — async work wasn’t awaited.
Error [ERR_REQUIRE_ESM]: require() of ES Module not supported
CommonJS code used `require()` on a package that is ES-module-only (node-fetch 3, chalk 5, nanoid 4, got 12…).
SyntaxError: await is only valid in async functions and the top level bodies of modules
await was used inside a function that isn’t async (or in a classic, non-module script).
AbortError: The user aborted a request. / signal is aborted without reason
A fetch was cancelled through an AbortController (or the browser cancelled it).
TypeError: Failed to execute 'json' on 'Response': body stream already read
A fetch Response body was consumed twice (e.g. calling .text() then .json()).
RuntimeWarning: coroutine 'fetch' was never awaited
An async function was called without await, so it never ran.
Error: connect ECONNREFUSED 127.0.0.1:5432
Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.
RuntimeError: Event loop is closed
Async code tried to use an event loop that has already been shut down.
RxJS: EmptyError: no elements in sequence (firstValueFrom / lastValueFrom on an empty Observable)
`firstValueFrom`, `lastValueFrom`, `first()` or `last()` waited for a value but the Observable completed without emitting.
Angular SSR: ReferenceError: window is not defined / document is not defined / localStorage is not defined
Code that only works in a browser ran during server-side rendering, where `window`, `document` and `localStorage` do not exist.
Two steps away (33)
SyntaxError: Unexpected token < in JSON at position 0
`JSON.parse()` / `response.json()` received something that isn’t JSON — very often an HTML error page.
CORS Error: No 'Access-Control-Allow-Origin' header is present
The browser blocked a cross-origin request because the server’s response didn’t include the CORS headers allowing your site’s ori…
SSL: CERTIFICATE_VERIFY_FAILED / unable to get local issuer certificate
The client couldn’t verify the server’s TLS certificate chain, so it refused the HTTPS connection.
Warning: validateDOMNesting(...): <div> cannot appear as a descendant of <p>.
The JSX produces invalid HTML nesting, which also causes hydration mismatches in SSR.
Error: useNavigate() may be used only in the context of a <Router> component.
A React Router hook ran in a component that isn’t rendered inside a Router provider.
Error: No QueryClient set, use QueryClientProvider to set one
TanStack Query hooks ran outside a QueryClientProvider (or a duplicate copy of the library is installed).
Error: Cannot find module 'x' (MODULE_NOT_FOUND)
Node can’t resolve the module path — the package isn’t installed, or a relative path/extension is wrong.
SyntaxError: Cannot use import statement outside a module
An `import` statement ran in a context treated as CommonJS (Node without "type": "module", a browser script without type="module"…
TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts"
Node tried to run a TypeScript file as ESM without a TypeScript loader.
IndentationError: unexpected indent / expected an indented block
Python’s block structure is broken — inconsistent indentation or mixed tabs and spaces.
SyntaxError: Unexpected token '<' (HTML served instead of JavaScript)
The browser loaded a .js file but received HTML (an error page or the SPA index.html).
SyntaxError: Unexpected end of input
The parser reached the end of the code (or JSON) while still expecting more — a missing } ) ] or empty JSON.
TypeError: Reduce of empty array with no initial value
Array.reduce() ran on an empty array without an initial value.
RangeError: Invalid array length
An array was created or resized with a negative, fractional or enormous length.
URIError: URI malformed
decodeURIComponent/decodeURI received an invalid percent-encoded sequence.
java.net.SocketException: Connection reset
The remote side closed the TCP connection abruptly while Java was reading or writing.
DOMException: Failed to execute 'querySelector' on 'Document': 'x' is not a valid selector
The CSS selector string is invalid — often an ID starting with a digit or containing special characters.
SecurityError: Blocked a frame with origin "x" from accessing a cross-origin frame.
Script tried to read or modify an iframe (or window) from another origin.
error: externally-managed-environment
Your system Python (Debian 12+, Ubuntu 23.04+, Homebrew) refuses pip installs into the OS-managed environment (PEP 668).
Python was not found; run without arguments to install from the Microsoft Store
Windows ran the Microsoft Store “App execution alias” instead of a real Python installation.
RuntimeError: Working outside of application context.
Code used current_app, g, or a Flask extension (like SQLAlchemy) without an active Flask application context.
MySQL ERROR 2002 (HY000): Can't connect to local MySQL server through socket
The client couldn’t reach the MySQL server — it isn’t running, or the client is looking for the socket/port in the wrong place.
Error: listen EADDRINUSE: address already in use :::3000
Another process is already listening on that port.
API Request Timed Out (ETIMEDOUT / ReadTimeout / timeout of 30000ms exceeded)
The client gave up waiting — either for the connection to open or for the response to arrive.
TypeError: f() takes 1 positional argument but 2 were given
A function was called with more positional arguments than it accepts — very often a method missing `self`.
TypeError: f() missing 1 required positional argument: 'x'
A function was called without a required parameter.
IndexError: list index out of range
Code accessed a list position that doesn’t exist.
Angular: Error: Cannot find control with name: 'email' (reactive forms)
A `formControlName` points at a control that does not exist in the bound `FormGroup`.
Angular: NG02100: InvalidPipeArgument: '[object Object]' for pipe 'AsyncPipe'
The `async` pipe received a plain object or array instead of an Observable or Promise.
RxJS: TypeError: You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, ReadableStream, Array, AsyncIterable, or Iterable.
An RxJS operator (`switchMap`, `mergeMap`, `forkJoin`, `subscribe`) received `undefined` — a function forgot to `return` an Obser…
Error: Hydration failed because the initial UI does not match what was rendered on the server
The HTML rendered on the server differs from the first client render, so React (Next.js/Remix) can’t attach to it.
ReferenceError: Cannot access 'x' before initialization
A let/const/class was used before its declaration line ran (temporal dead zone) — often a circular import.
Angular SSR: NG0500: During hydration Angular expected <div> but found <span> (NG0501, NG0502)
The HTML rendered on the server does not match what the browser's Angular app tried to render, so hydration failed and the page w…
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