TypeError: toLowerCase is not a function in JavaScript

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Borislav Hadzhiev

Last updated: Jul 25, 2022

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TypeError: toLowerCase is not a function in JavaScript #

The "toLowerCase is not a function" error occurs when we call the toLowerCase() method on a value that is not a string. To solve the error, convert the value to a string using the toString() method or make sure to only call the toLowerCase method on strings.

typeerror tolowercase is not a function

Here is an example of how the error occurs.

index.js
const str = {}; // ⛔️ TypeError: str.toLowerCase is not a function const result = str.toLowerCase();

We called the String.toLowerCase method on an object which caused the error.

To solve the error, make sure to only call the toLowerCase() method on strings. You can convert most values to a string by using the toString() method.

index.js
const num = 123; const result = num.toString().toLowerCase();

Alternatively, you can check if the value is a string before calling the toLowerCase() method.

index.js
const str = null; const result = typeof str === 'string' ? str.toLowerCase() : ''; console.log(result); // 👉️ ""

We used a ternary operator to check if the str variable stores a string.

If it does, the value to the left of the comma is returned, otherwise the value to the right is returned.

If the value is a string, we return the result of calling the toLowerCase method, otherwise we return an empty string to be consistent.

If you want to convert all strings in an array to lowercase, use the map() method to iterate over the array and call the toLowerCase() method on each string.

index.js
const arr = ['A', 'B', 'C']; const result = arr.map(str => str.toLowerCase()); // 👇️ ['a', 'b', 'c'] console.log(result);
The map() method takes a function and calls the function on each element in the array. On each iteration, we call the toLowerCase() method to convert the current string to lowercase.

If the error persists, console.log the value you're calling the toLowerCase method on and use the typeof operator to check its type.

If the value is an object, there's a very good chance that you are forgetting to access a specific property on which you need to call the toLowerCase() method.

Conclusion #

The "toLowerCase is not a function" error occurs when we call the toLowerCase() method on a value that is not a string. To solve the error, convert the value to a string using the toString() method or make sure to only call the toLowerCase method on strings.

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