TypeError Fix: How to Resolve Cannot Read Property toLowerCase of Undefined in JavaScript

title: TypeError Fix: How to Resolve 'Cannot Read Property toLowerCase of Undefined' in JavaScript
author: Your Name
date: 2021-10-11


In JavaScript, a common error that developers encounter is `TypeError: Cannot read property 'toLowerCase' of undefined`. This error occurs when trying to call the `toLowerCase()` method on a variable that is `undefined`.

In this guide, we will discuss the cause of this error and provide a step-by-step solution to fix it.

## Table of Contents

1. [Understanding the Error](#understanding-the-error)
2. [Step-by-Step Solution](#step-by-step-solution)
3. [FAQs](#faqs)
4. [Related Links](#related-links)

## Understanding the Error

The `toLowerCase()` method is used to convert the characters of a string to lowercase. When calling this method, if the variable is not a string or is `undefined`, JavaScript will throw the `TypeError: Cannot read property 'toLowerCase' of undefined` error.

Here is an example of code that would trigger this error:

var userInput;

function processInput() {
  var processedInput = userInput.toLowerCase();


In this example, the userInput variable is not assigned a value and is undefined. When we try to call toLowerCase() on it, the error occurs.

Step-by-Step Solution

To fix this error, follow these steps:

Identify the problematic variable: Look for the variable that is throwing the error. In our example, it is the userInput variable.

Check if the variable is defined: Ensure that the variable has been assigned a value before calling the toLowerCase() method.

Check the variable's type: The toLowerCase() method can only be called on strings. Use the typeof operator to check if the variable is a string before calling the method.

Here's the corrected code for our example:

var userInput = "Hello World!";

function processInput() {
  if (typeof userInput === "string") {
    var processedInput = userInput.toLowerCase();
  } else {
    console.log("Invalid input");


In this revised code, we check if userInput is a string before calling the toLowerCase() method. Now the code will run without throwing the error.


1. What is the toLowerCase() method in JavaScript?

The toLowerCase() method is a built-in JavaScript function that converts all the characters in a string to lowercase. Learn more about the toLowerCase() method.

2. Can I use the toLowerCase() method on numbers and other data types?

No, the toLowerCase() method is only applicable to string data types. If you need to convert a number or another data type to a string, you can use the toString() method before calling toLowerCase().

3. How can I convert a string to uppercase in JavaScript?

You can use the toUpperCase() method to convert a string to uppercase in JavaScript. Learn more about the toUpperCase() method.

4. Are there any alternative methods to convert a string to lowercase in JavaScript?

While toLowerCase() is the most commonly used method, you can also use the toLocaleLowerCase() method, which takes into account the current locale when converting the string. Learn more about the toLocaleLowerCase() method.

5. How do I check if a variable is undefined in JavaScript?

You can use the typeof operator or strict equality (===) to check if a variable is undefined. For example:

if (typeof variable === "undefined") {
  // variable is undefined

if (variable === undefined) {
  // variable is undefined

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