How To Fix Undefined is Not a Function Errors

The “undefined is not a function” error occurs when JavaScript expects a function but finds undefined. This is a common runtime error that can break applications if not handled properly. Understanding why this error occurs helps prevent it effectively.

1. Understand the Problem

Common causes:
- Calling a function before it is defined
- Typographical errors in function names
- Overwriting a function variable with a non-function value
- Trying to call a property of an undefined object as a function

Example:

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

2. Ensure Proper Function Declarations

Function declaration:

function myFunc() {
  console.log('Hello!');
}
myFunc(); // ✅ works

Function expression:

const myFunc = function() {
  console.log('Hello!');
};
myFunc(); // ✅ works

Arrow function:

const myFunc = () => console.log('Hello!');
myFunc(); // ✅ works

3. Avoid Overwriting Function Variables

Ensure that function references are not accidentally overwritten:

let myFunc = () => console.log('Hello!');
myFunc = 10; // ❌ now myFunc is not a function

Always check variable assignments to prevent overwriting functions.

4. Check Object Properties

When calling a method on an object, ensure the property exists and is a function:

const obj = {
  greet: () => console.log('Hi!')
};

obj.greet(); // ✅ works
obj.sayHi(); // ❌ undefined is not a function

Use optional chaining to prevent errors:

obj.sayHi?.(); // ✅ safely does nothing if method doesn’t exist

5. Debugging Tips

Use console.log(typeof variable) to check if it’s a function.
Double-check spelling and capitalization of function names.
Inspect objects to verify that the method exists before calling.
Ensure that asynchronous code does not call a function before it’s assigned.

6. Best Practices Summary

Use proper function declarations, expressions, or arrow functions.
Avoid overwriting function variables with non-function values.
Check object properties before calling them as functions.
Use optional chaining for safer function calls.
Debug with typeof and console logs.

Following these practices ensures your JavaScript code avoids the common “undefined is not a function” error.

Citations

Internal: https://savanka.com/category/savanka-helps/
External: https://developer.mozilla.org/en-US/docs/Web/JavaScript