TypeError: callback is not a function in JavaScript [Solved]

# TypeError: callback is not a function in JavaScript

The "TypeError: callback is not a function" error occurs when we define a callback parameter to a function but invoke the function without passing a callback.

To solve the error, specify a function as a default parameter, or always provide a parameter when calling the function.

Here is an example of how the error occurs.

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

// ⛔️ TypeError: callback is not a function
example();

# Always provide a callback when calling the function

To solve the error, make sure to always provide a callback when calling the function.

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

// ✅ Works
example(() => {
  console.log('success');
});

We passed an arrow function callback when calling the example function, but you can also use a named function.

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

function logMessage() {
  console.log('success');
}

// ✅ Works
example(logMessage);

# Define a default value for the callback parameter

Alternatively, you can define a default value for the callback parameter. This way you won't get an error even if no parameter is provided when calling the function.

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function example(callback = () => {}) {
  return callback()
}

// ✅ Works
example();

We used a function that doesn't do anything as the default value for the callback parameter.

When the function is called without a callback being provided, the default value of an empty function is used.

You can also use any other function as the default parameter depending on your use case.

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function example(callback = defaultFunc) {
  return callback();
}

function defaultFunc() {
  console.log('success');
}

// ✅ Works
example(); // 👉️ success

# Check if the callback function was provided before calling it

Perhaps, an even better approach is to check if the callback was provided before calling it in the function.

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function example(callback) {
  if (typeof callback === 'function') {
    return callback()
  }
}

// ✅ Works
example();

The typeof operator allows us to check if the callback variable stores a function.

We only call the callback if it is provided and has a type of function.

Make sure that the variable stores a function before calling it.

You might be declaring a variable named callback that shadows the value of a function you're trying to invoke.

Make sure you're spelling callback correctly, as variable names are case-sensitive.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• How to call a Function inside another Function in JS
• How to call a Function in an Object in JavaScript