TypeError: not a function 🟨 JavaScript

TypeError: x is not a function

You called something as a function, but its value is not a function (it’s undefined, a string, an object…).

Seen on: JavaScript Node.js

Meaning

The value exists (or is undefined) but isn’t callable. Common with wrong imports, calling array methods on non-arrays, or a variable shadowing a function.

Common causes

  • Wrong import (default vs named): import foo from vs import { foo } from
  • Calling .map/.forEach on an object or string instead of an array
  • Typo in method name, or method doesn’t exist in this library version
  • Variable reassigned/shadowed with a non-function
  • Missing semicolon before a line starting with ( (ASI issue)
  • Calling a jQuery plugin before it loaded

⚡ Quick fix

  1. Log typeof x right before the call
  2. Check the import style against the library docs
  3. Use Array.isArray(data) before array methods; for objects use Object.values(obj).map(...)

Detailed fix by platform

JavaScript

  1. Default vs named exports:
    javascript
    // utils.js: export function formatDate() {}
import formatDate from './utils.js';     // ❌ undefined → "formatDate is not a function"
import { formatDate } from './utils.js'; // ✅

Node.js

  1. CommonJS/ESM interop: const x = require('pkg').default may be needed for ESM default exports.

How to diagnose

  1. Value — What is typeof the thing being called?
  2. Import — Named or default export?
  3. Version — Does this method exist in the installed version?

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