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How Do I Fix “undefined is not a function” Error in JavaScript?

Introduction

If you’ve worked with JavaScript, chances are you’ve seen the error:

Uncaught TypeError: undefined is not a function

This error can be confusing, especially for beginners. It usually means you are trying to call something like a function, but JavaScript cannot find that function definition — in fact, the variable or object you are calling is undefined.

What Does “undefined is not a function” Mean?

In simple terms:

  • You’re trying to execute something as a function.

  • But that “something” is not defined as a function.

Example:

let greet;
greet(); // ❌ Error: undefined is not a function

Here, greet exists but is not assigned to a function. Calling it results in the error.

Common Causes of This Error

1. Calling a Function That Doesn’t Exist

sayHello(); // ❌ Error

✅ Fix:

function sayHello() {
  console.log("Hello, World!");
}
sayHello(); // ✅

2. Misspelled Function or Method Names

let str = "JavaScript";
console.log(str.toupperCase()); // ❌ Error

✅ Fix:

console.log(str.toUpperCase()); // ✅ JAVASCRIPT

3. Using Functions Before They’re Defined

sayHi(); // ❌ Error

const sayHi = function() {
  console.log("Hi!");
};

✅ Fix:

function sayHi() {
  console.log("Hi!");
}
sayHi(); // ✅

4. Accessing Object Methods That Don’t Exist

let user = {};
user.login(); // ❌ Error

✅ Fix:

let user = {
  login: function() {
    console.log("User logged in");
  }
};
user.login(); // ✅

5. Wrong Import/Export in Modules

// math.js
export function add(a, b) { return a + b; }

// app.js
import { addition } from './math.js';
addition(2, 3); // ❌ Error

✅ Fix:

import { add } from './math.js';
console.log(add(2, 3)); // ✅ 5

6. Asynchronous Code Issues

let data;
fetch("https://api.example.com")
  .then(res => res.json())
  .then(res => data = res);

console.log(data.print()); // ❌ Error

✅ Fix:

fetch("https://api.example.com")
  .then(res => res.json())
  .then(res => {
    console.log(res);
    // Safe to call methods on res here
  });

Debugging “undefined is not a function”

1. Check Console Output

console.log(greet); // undefined
greet(); // ❌

2. Use typeof

console.log(typeof greet); // 'undefined'

3. Check Imports/Exports

Make sure names match exactly.

4. Use ESLint/Prettier

Catch typos and mistakes before runtime.

Debugging Flowchart

START
   ↓
Did you get "undefined is not a function"?
   ↓
 YES
   ↓
Check the variable or method you are calling with ()
   ↓
Is the variable defined?
   ├── NO → Define the function or assign it properly → FIXED ✅
   └── YES
        ↓
Is it spelled correctly (case-sensitive)?
   ├── NO → Correct spelling → FIXED ✅
   └── YES
        ↓
Is it actually a function?
   ├── NO → Check object properties, imports, exports
   └── YES
        ↓
Is it called before being defined? (function expression/arrow)
   ├── YES → Move call after definition OR use function declaration
   └── NO
        ↓
Check async code (API, Promise, callback): Is data ready?
   ├── NO → Call inside .then() / async-await → FIXED ✅
   └── YES → Verify import/export or library dependencies

END (Error Fixed 🎉)

Best Practices to Avoid This Error

  • Always initialize functions before using them.

  • Double-check method names (JavaScript is case-sensitive).

  • Use default values for callbacks:

function runTask(callback = () => {}) {
  callback();
}
runTask(); // ✅

  • Validate objects before calling methods:

if (user && typeof user.login === "function") {
  user.login();
}

Conclusion

The “undefined is not a function” error in JavaScript happens when you try to call something that isn’t a function. The root causes include typos, missing imports, uninitialized variables, or incorrect async usage.

By carefully checking function definitions, verifying object methods, fixing import/export mismatches, and following best practices, you can quickly debug and prevent this error.

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