Typeerror: callback is not a function

The “typeerror: callback is not a function” is an error message that occurs in JavaScript.

This article will help you learn, understand, and fix this error.

Remember that it is crucial to understand the error first before proceeding to the steps to fix it.

So, without further ado, let us start with understanding this error.

What is typeerror: callback is not a function?

As mentioned in our first sentence, typeerror: callback is not a function is an error message that occurs in JavaScript.

This error occurs when a function’s callback argument is given, but the function is called without passing the callback as a parameter.

Here is a sample code that causes this error:

function sample(callback) {
  return callback();
}
sample();

Error:

index.js:2
  return callback();
         ^

TypeError: callback is not a function

To fix this error, follow the guide below.

Typeerror: callback is not a function – SOLUTION

Fixing the typeerror: callback is not a function may sound frustrating, but it is easy and quick.

See the sample code below to help you fix the error you are facing.

Sample code:

function sample(callback = () => {}){
 return callback();
}
sample();

Aside from the sample code above, you can also fix it using this code:

function sample(callback) {
 return callback();
}
sample(() => {
  console.log('This is a success!');
});

Output:

This is a success!

If you only want to test a code, use our online JavaScript compiler. In there, you can run JS code online for free.

FAQs

Python TypeError debugging checklist

• Read the full traceback. The bottom line is the error type + message. The line above shows the exact code that triggered it.
• Print types. Insert print(type(x), type(y)) before the error line to see what Python actually has.
• Use isinstance. Guard code with if isinstance(x, expected_type):.
• Type hints + mypy. Adding x: int lets mypy catch mismatches before you run the code.
• Break into a debugger. Insert breakpoint() before the failing line and inspect variables live.

Common root causes across all TypeError variants

• Silent None returns. A function that should have returned a value returned None instead.
• Mixing types across function boundaries. Legacy code passing str where int is expected (or vice versa).
• Shadowed builtins. Local variable named list, dict, set overriding the built-in.
• Optional[T] not handled. Callers not accounting for the None case.
• Third-party library API drift. New version renamed a kwarg or changed a return type.

Modern tooling to prevent TypeError

• Type hints (PEP 484+). Optional[X], Union[X,Y], List[T] make expected types explicit.
• mypy or Pyright. Runs your codebase through a type checker before you run it.
• Ruff. Fast linter that catches many TypeError-adjacent bugs.
• pydantic v2. Runtime validation with the same syntax as static types.
• pytest fixtures. Test each function with edge-case inputs to catch TypeError paths early.

Conclusion

In conclusion, the typeerror: callback is not a function occurs in JavaScript as an error message.

You can solve this error by using the arrow function to define the callback() function, then returning it to the callback() function.

By following the guide above, you will surely solve this error quickly.

That is all for this tutorial, IT source coders!

We hope you have learned a lot from this. Have fun coding!

Thank you for reading! 😊

Elijah Galero

Programmer & Technical Writer at PIES IT Solution

Elijah Galero is a programmer and writer at PIES IT Solution, author of 175+ tutorials at itsourcecode.com. Specializes in Python error debugging (AttributeError, TypeError, ModuleNotFoundError), Python programming tutorials, and Microsoft Excel how-to guides for BSIT students and productivity learners.

Expertise: Python · Python Errors · Python AttributeError · Python TypeError · ModuleNotFoundError · MS Excel · MS PowerPoint
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