Learn How to Fix 'TypeError: X is not a function' in JavaScript

How to Fix 'TypeError: X is not a function' in JavaScript

DodaTech Updated Jun 15, 2026 3 min read

The Error

TypeError: user.getName is not a function
TypeError: (intermediate value).someFunction is not a function
TypeError: "hello".toUppercase is not a function  // note the typo

This error means JavaScript expected a function at a certain position, but found something else — usually undefined, null, or a value of the wrong type.

What Causes It

1. Variable is undefined or null

let greet;
greet(); // TypeError: greet is not a function

const options = null;
options.execute(); // TypeError: options is not a function

Fix: Always check that your function variable exists before calling it:

if (typeof greet === 'function') {
  greet();
}

2. Wrong Import or Missing Export

// module.js
export const sayHello = () => console.log('Hi');

// app.js
import { sayHelo } from './module.js';
sayHelo(); // TypeError: sayHelo is not a function (it's undefined)

Fix: Verify import names match their exports. Use an editor with import autocompletion.

3. Async Function Before Definition

// ❌ Called before declaration
execute(); // TypeError if variable is in TDZ

const execute = () => console.log('running');

Fix: Use function declarations (which are hoisted) or call functions after their definition:

// Function declaration — hoisted, works
execute();
function execute() {
  console.log('running');
}

4. Object Property Typo

const person = {
  getName() { return 'Alice'; }
};

person.getname(); // TypeError: person.getname is not a function (wrong case)

Fix: JavaScript is case-sensitive. getName and getname are different properties. Use autocomplete to avoid typos.

5. Method Extracted from Object Loses Context

const user = {
  name: 'Alice',
  greet() {
    console.log(`Hello, ${this.name}`);
  }
};

const greetFn = user.greet; // Function reference only
greetFn(); // TypeError: this.name is undefined — but not "not a function"

Note: This usually causes a different error (this is undefined), but can cause “not a function” if you extract a non-existent method.

How to Fix It

Step 1: Log the Value

Always check what you’re actually calling:

console.log('typeof myFunc:', typeof myFunc);
console.log('myFunc value:', myFunc);

If typeof is 'undefined', 'string', 'number', 'object', or 'boolean' — it’s not a function.

Step 2: Check the Import

// Did you import correctly?
import { add } from './math.js'; // Is 'add' exported from math.js?

// Is the path correct?
import { add } from './math.js'; // Or is it './utils/math.js'?

Step 3: Verify Method Names

// Common mistakes
str.toUppercase()   // Should be toUpperCase()
arr.forEech()       // Should be forEach()
arr.puhs()          // Should be push()

Step 4: Check for Property Shadowing

const data = {
  value: 42,
  process: function() { /* ... */ }
};

// If someone overwrites:
data.process = 123; // Now it's a number, not a function
data.process(); // TypeError!

Prevention

Use TypeScript — catches “not a function” errors during compilation.
Name consistently — use autocomplete and avoid manual typing for method names.
Add null checks — typeof fn === 'function' before calling.
Check exports — verify export and import match exactly.
Use default function parameters safely:

function execute(callback = () => {}) {
  callback(); // Safe — callback is always a function
}

Summary

Cause	Check	Fix
Undefined variable	`typeof fn`	Initialize the function
Wrong import	Import names	Fix import/export mismatch
Async timing	Hoisting	Use function declarations
Typo	Method spelling	Use autocomplete
Overwritten property	Object state	Avoid reassigning methods

Built by the developers of Doda Browser, DodaZIP, and Durga Antivirus Pro.

Previous How to Fix SyntaxError in JavaScript — Unexpected Token, Missing Brackets & JSON Errors Next How to Fix 'Unhandled Promise Rejection' in JavaScript

Built by the developers of DodaTech

Doda Browser, DodaZIP & Durga Antivirus Pro

Home Browse JavaScript Tutorials