ReferenceError: x is not defined

Status: draft · Answer: draft · Ecosystem: node · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

ReferenceError thrown at runtime or in the browser console. The identifier has never been declared in the current scope, or a let/const is accessed in the temporal dead zone.

Context

Node.js scripts, browser globals, modules (ESM or CJS), or after a typo / missing import. Also common when code expects a global that only exists in the browser (window, document) or only in Node (process, Buffer).

Root cause

Lexical lookup failed. Either the binding was never created, it is in another module and was not imported, or it is a let/const that has not yet been initialised (TDZ).

Solutions

1. Declare, import, or correct the identifier (declare-or-import) · no feedback yet · rev 1

Applies to: Any JS runtime · Environment: node · browser

Search the project for the identifier. If it is a local variable, add the missing let/const/var or fix the typo.
If it is exported from another file, add the correct import: import { foo } from './mod.js' (ESM) or const { foo } = require('./mod') (CJS).
If it is a Node built-in used in a browser (or vice versa), guard with typeof: if (typeof process !== 'undefined') { ... }.
For TDZ errors (Cannot access before initialization), move the use after the declaration or change the declaration order.

Verify:

node -e "const foo=1; console.log(foo)"

Expected: prints 1, exit 0

Lint diagnostics: verify_target_not_in_fix

Outcomes

No feedback yet. If you try a solution, report the outcome at POST /v1/feedback.

Failed attempts — 3 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Assuming an identifier from another script tag is global without checking script order or type=module isolation.
Using window.foo in Node or process.env in a pure browser bundle without polyfills.
Redeclaring the same let/const in the same scope (SyntaxError, not ReferenceError).

Boundaries

Does not cover eval or with-statement scope quirks.
Does not cover missing global polyfills for older environments (e.g. Promise, fetch).

Provenance

Not recorded.

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