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

TypeError: .toLowerCase is not a function occurs when we call toLowerCase() function on object which is not an string. toLowerCase() function can be only called on string. To resolve this issue, convert value to string using toString() ,method before calling toLowerCase() method.

Let’s see with help of simple example

You will get following error when you execute the program:

We got this error because str is not an string here.

Check type before calling toLowerCase() method

Check if the object on which you are calling toLowerCase() is String or not. Use console.log() to print it. It may be undefined sometimes.

You can check if it is type of string before calling toLowerCase() to make sure that toLowerCase() is being called on string only.

Output:

As you can see num is not a string, so result is empty string.

We used ternary operator to check if num is string or not. If it is a string, call the toLowerCase() method, otherwise return empty string.

You can also check if type variable has toLowerCase method or not.

For example:
We can check if toLowerCase method exists in Number class.

Call toLowerCase() on String object only

Call toString() method on object

Convert object to string using toString() method and it will work fine.

Output:

Use String() constructor

To convert a value to String, use String() constructor.

Output:

Use + operator

You can convert value to String using + operator and call toLowerCase() on variable.

Output:

Read also: map is not a function in JavaScript.

That’s all about how to resolve TypeError: toLowerCase is not a function in JavaScript.