How to Fix the ‘undefined is not a function’ Error in JavaScript

2 min readJun 16, 2024

JavaScript is a versatile and widely-used programming language, but it is not without its quirks and challenges. One common and perplexing error that developers often encounter is the ‘undefined is not a function’ error. This article aims to demystify this error and provide practical solutions to fix it.

Understanding the Error

At its core, the ‘undefined is not a function’ error occurs when the code attempts to execute a variable or property as a function, but it is actually undefined. This usually means that either the function name is misspelled, the function is not in scope, or the function has not been initialized yet.

Common Causes

1. Typos

One of the simplest and most common causes is a typo. For example, you might have declared a function named doSomething(), but somewhere in your code, you mistakenly call it as doSomehting(). JavaScript will throw the 'undefined is not a function' error because doSomehting() does not exist.

function doSomething() {
  console.log("Doing something!");
}

doSomehting(); // This will cause 'undefined is not a function'

2. Incorrect Scope

This error can also occur if the function is not in the correct scope. JavaScript will not recognize the function if it is out of the scope from where it is being called.

function outerFunction() {
  function innerFunction() {
    console.log("Inside inner function");
  }

  // Correct scope
  innerFunction(); // This works

  // Incorrect scope
  someOtherFunction();
}

function someOtherFunction() {
  console.log("Inside some other function");
}

outerFunction();

3. Asynchronous Issues

Sometimes, the function may not yet be initialized or defined due to asynchronous operations. This is especially common in scenarios involving API calls or event-driven architectures.

let data;

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(json => {
    data = json;
  });

console.log(data); // Undefined because the fetch operation is not complete

How to Fix It

Double-Check Spellings

The first and simplest step is to ensure all function names are correctly spelled and match their declarations.

Check Scope

Make sure the function you are trying to call is within the correct scope. This might involve moving the function declaration or ensuring it is globally accessible.

Handle Asynchronous Code

For functions involving asynchronous code, make sure the function is called only after the asynchronous operation is complete. This can be done using async and await, or by chaining .then() methods.

let data;

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  data = await response.json();
  console.log(data); // Correctly logs the data after fetch is complete
}

fetchData();

Conclusion

The ‘undefined is not a function’ error is a common but fixable issue in JavaScript. By understanding its common causes and following the outlined solutions, you can quickly debug and resolve this error. Happy coding!