JavaScript toLowerCase is Not a Function

Like any programming language, it comes with its own set of challenges and errors. One of these errors is the “JavaScript toLowerCase is not a function“.

In this article, we will discuss the causes of this error, provide step-by-step solutions, and example codes to help you overcome it.

So, let’s start!

What is JavaScript toLowerCase?

The “JavaScript toLowerCase is not a function” error usually occurs when attempting to use the toLowerCase() function on a value that isn’t a string.

The “toLowerCase()” function is used to convert a string to lowercase, making it useful for string manipulations.

However, if you apply it to a non-string value, you will encounter this error.

Let’s explore some common cases where this error might occur.

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Common Scenarios and Causes

Here are the following common scenarios and cases:

Incorrect Data Type

In JavaScript, every data type has its own set of methods and properties. If you try to use toLowerCase() on a data type other than a string, such as a number or Boolean, the error will occur.

JavaScript is a dynamically typed language, which means it does not restrict variable types. Thus, it is important to ensure you are applying methods to the correct data type.

Null or Undefined Values

Another case that triggers the “JavaScript toLowerCase is not a function” error is trying to use toLowerCase() on a variable that holds a value of null or undefined.

These values don’t have the “toLowerCase()” method, leading to the error. Always validate your variables before applying string methods to prevent this issue.

let inputValueSample = "Hello";
let numericValueSample = 23;

let resultValue1 = inputValueSample.toLowerCase(); 
let resultValue2 = numericValueSample.toLowerCase(); 

Solutions to Solve the JavaScript toLowerCase is Not a Function

Now that we understand the causes, let’s explore solutions to fix the “tolowercase is not a function JavaScript” error.

Also read or visit this article: How to Read Text File in JavaScript Line by Line

Solution 1: Using Type Checking

To avoid this error, use type checking to assure you are applying the toLowerCase() function only to string values.

You can accomplish this using the typeof operator to check the data type before applying the method.

Here’s an example code of using Type Checking:

let textValueExpression = "WELCOME! ITSOURCECODE";
let numericValueExpression = 25;

function convertToLowerCaseValue(value) {
    if (typeof value === "string") {
        return value.toLowerCase();
    return value;

let resultValue1 = convertToLowerCaseValue(textValueExpression);
let resultValue2 = convertToLowerCaseValue(numericValueExpression);

Solution 2: Checking for Null or Undefined

Additionally, always check for null and undefined values before applying the toLowerCase() method.

This is to assures that the function is only executed on valid string values.

Example of Handling Null or Undefined Values:

function convertToLowerCaseValues(value) {
    if (value === null || value === undefined) {
        return value;
    return value.toLowerCase();

let resultValue1 = convertToLowerCaseValues("WELCOME! ITSOURCECODE");   
let resultValue2 = convertToLowerCaseValues(null);     
let resultValue3 = convertToLowerCaseValues(undefined);



Does this error impact all browsers?

Yes, this error is not specific to any browser; it’s an essential issue in JavaScript.

Are there alternatives to toLowerCase()?

Yes, JavaScript provides alternative methods like toUpperCase() and toLocaleLowerCase() for scenarios conversion.

Is type checking the only way to prevent this error?

While type checking is a common method, assuring valid data and proper variable handling is equally important.

Are there any automated tools to catch such errors?

Yes, linters and static code analysis tools can help catch possible issues in your code, including this error.


In JavaScript, encountering errors like “JavaScript toLowerCase is not a function” is part of the learning process.

By understanding the root causes and implementing the provided solutions, you can confidently implement this error and improve the reliability of your code.

Remember to always validate input data, perform type checks, and handle edge cases effectively to create powerful and error-free JavaScript applications.

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