TypeError: x is not a function

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

Runtime TypeError when the code attempts to call a value that is not callable. Stack points at the call site.

Context

Import/require mismatches (default vs named), destructuring a non-function, overwriting a method, or calling something that resolved to undefined/null/object.

Root cause

The expression before () evaluated to a non-callable value (undefined, null, number, string, plain object, etc.). Common causes are wrong import style or a missing export.

Solutions

1. Inspect the import and the runtime value (check-import-and-value) · no feedback yet · rev 1

Applies to: Node and browsers · Environment: node · browser

Log the value immediately before the call: console.log(typeof fn, fn).
If it is an import, check whether the module uses default or named exports. ESM: import foo from './m' vs import { foo } from './m'. CJS interop: const foo = require('./m'); may need foo.default or foo.foo.
Confirm the export actually exists: node -e "console.log(require('./m'))" or inspect the package's package.json exports field.
If the name was shadowed (e.g. a parameter with the same name as an imported function), rename one of them.

Verify:

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

Expected: prints 1, exit 0

Lint diagnostics: creates_files

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 require('./m') always returns the function when the module does module.exports = { fn } (you get the object).
Using import { default as foo } when the package only has a CJS default export and the bundler/runtime interop differs.
Calling a class without new when the class does not support it (or vice versa).

Boundaries

Does not cover Proxy apply traps that intentionally throw.
Does not cover bound functions or Function.prototype.call/apply misuse.

Provenance

Not recorded.

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