JavaScript toLowerCase()
Summary: in this tutorial, you’ll learn how to use the JavaScript String.prototype.toLowerCase() method to return a string with all the characters converted to lowercase.
Introduction to the JavaScript toLowerCase() method
The toLowerCase() method returns a new string with all characters converted to lowercase. The following shows the syntax of the toLowerCase() method:
str.toLowerCase()Code language: CSS (css)
For example:
const message = 'Hi';
const newMessage = message.toLowerCase();console.log(newMessage);
Code language: JavaScript (javascript)
Output:
hi
Because a string is immutable, the toLowerCase() method doesn’t change the original string. Instead, it returns a new string with all characters converted to lowercase.
Calling JavaScript toLowerCase() method on null or undefined
If you call the toLowerCase() method on null or undefined, the method will throw a TypeError exception.
The following findUserById function returns a string if the id is greater than zero or null otherwise:
const findUserById = (id) => {
if (id > 0) {
// look up the user from the database
// ...
//
return 'admin';
}
return null;
};Code language: JavaScript (javascript)
If you call the toLowerCase() method on the result of the findUserById() function, you’ll get the TypeError when the id is zero or negative:
console.log(findUserById(-1).toLowerCase());Code language: CSS (css)
Error:
TypeError: Cannot read properties of null (reading 'toLowerCase')Code language: JavaScript (javascript)
To make it safe, you can use the optional chaining operator ?. as follows:
console.log(findUserById(-1)?.toLowerCase());Code language: CSS (css)
Output:
undefinedCode language: JavaScript (javascript)
Converting a non-string to a string
The toLowerCase() method will convert a non-string value to a string if you set its this value to a non-string value. For example:
const user = {
username: 'JOE',
toString() {
return this.username;
},
};const username = String.prototype.toLowerCase.call(user);
console.log(username);
Code language: JavaScript (javascript)
Output:
joe
In this example, we invoke the toLowerCase() method with the this sets to the user object by using the call() method. The toLowerCase() method converts the user object to a string by calling its toString() method.
Summary
- Use the
toLowerCase()method to return a string with all characters converted to lowercase.