Converting String to Lowercase in JavaScript


If you've ever found yourself needing to standardize user input or prepare text data for analysis, then you may have needed to lowercase strings. While there is one widely used method, there are also other string manipulation methods that you should consider as well, which is what we'll cover in this Byte.

The Need for Lowercase Conversion

Ever wondered why you'd need to convert strings to lowercase? Well, let's consider a scenario. Suppose you're building a search feature for a website. To ensure that the search is case insensitive, you would need to convert both the search input and the data being searched into a uniform case. Lowercase is often the go-to choice.

Another common use case is data preprocessing in Natural Language Processing (NLP). Before analyzing text data, it's often converted to lowercase for more uniformity.

JavaScript Methods for Lowercase Conversion

JavaScript provides two handy methods for converting strings to lowercase: toLowerCase() and toLocaleLowerCase(). These methods make converting strings to lowercase easy to do.

The toLowerCase() Method

The toLowerCase() method in JavaScript converts all the uppercase characters in a string to lowercase characters and returns the result. The original string remains unchanged as this method returns a new string.

Here's a simple example:

let str = "Hello, World!";
let lowerCaseStr = str.toLowerCase();

console.log(lowerCaseStr);  // "hello, world!"

In this example, toLowerCase() is called on the string str, converting all uppercase characters to lowercase. The result is then logged to the console.

The toLocaleLowerCase() Method

The toLocaleLowerCase() method, on the other hand, considers locale-specific rules when converting strings to lowercase. This method is particularly useful when dealing with languages that have locale-specific casing rules.

For instance, in Turkish, the uppercase of 'i' is 'İ' (dotted i), not 'I'. Let's see this in action:

let str = "İstanbul";
let lowerCaseStr = str.toLocaleLowerCase('tr-TR');

console.log(lowerCaseStr);  // "i̇stanbul"

In this example, we're converting a Turkish word to lowercase. The toLocaleLowerCase() method correctly converts the 'İ' to 'i̇', respecting the Turkish language rules.

Note: If you're dealing with English text or languages without specific casing rules, toLowerCase() and toLocaleLowerCase() will give the same results. However, for languages with specific casing rules, it's best to use toLocaleLowerCase().

More Examples of Lowercase Conversion

Let's start by converting a simple string to lowercase using the toLowerCase() method. This method doesn't require any parameters and returns a new string where all the original characters are converted to lowercase.

let greeting = "Hello, World!";
let lowerCaseGreeting = greeting.toLowerCase();
console.log(lowerCaseGreeting);  // "hello, world!"

Now, let's try using toLocaleLowerCase(). This method is similar to toLowerCase(), but it considers the host's current locale. For most languages, it behaves the same as toLowerCase().

let greetingInTurkish = "MERHABA, DÜNYA!";
let lowerCaseGreetingInTurkish = greetingInTurkish.toLocaleLowerCase('tr-TR');
console.log(lowerCaseGreetingInTurkish);  // "merhaba, dünya!"

As you can see, even the Turkish-specific characters are correctly converted to lowercase.

Note: The toLocaleLowerCase() method takes a locale argument. If no locale is provided, it uses the host's current locale.

Potential Issues and Solutions

Dealing with Null or Undefined

While working with JavaScript, you might encounter a situation where the string you're trying to convert to lowercase is null or undefined. Let's see what would happen in such a case.

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let nullString = null;
let lowerCaseNullString = nullString.toLowerCase();
// Uncaught TypeError: Cannot read property 'toLowerCase' of null

As expected, this throws a TypeError. To handle this, we can add a check to ensure that the string is not null or undefined before we try to convert it to lowercase.

let nullString = null;
let lowerCaseNullString = nullString ? nullString.toLowerCase() : null;
console.log(lowerCaseNullString);  // null

Handling Non-String Data Types

Another possible issue is trying to convert a non-string data type to lowercase. For instance, if you try to convert a number to lowercase, you'll get a TypeError.

let number = 12345;
let lowerCaseNumber = number.toLowerCase();
// Uncaught TypeError: number.toLowerCase is not a function

To handle this, you could first convert the non-string data type to a string using the toString() method, and then convert it to lowercase.

let number = 12345;
let lowerCaseNumber = number.toString().toLowerCase();
console.log(lowerCaseNumber);  // "12345"

Other Case Conversions

Converting all letters to lowercase is not the only use-case. Another possibility is to convert some, but not all, letters to lowercase while either leaving others alone or converting them to uppercase. Both toTitleCase() and toProperCase() do this.

While methods like these are not inherently supported by JavaScript, we can either write these methods ourselves or use utility libraries like Lodash that do provide them.

The toTitleCase() Method

So, how do we go about creating and using a method like this? This method will convert the first character of each word in a string to uppercase, while the rest of the characters remain in lowercase.

Here's a simple implementation:

String.prototype.toTitleCase = function() {
    return this.replace(/\w\S*/g, function(txt) {
        return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();

console.log("hello world".toTitleCase()); // Output: "Hello World"

In this code snippet, we use the replace() method with a regular expression to match each word in the string. For each matched word, we convert the first character to uppercase and the rest to lowercase.

The toProperCase() Method

Another method that JavaScript does not support natively is toProperCase(). This method is similar to toTitleCase(), but it only capitalizes the first letter of the string, leaving the rest of the string in lowercase.

Here's how you could implement it:

String.prototype.toProperCase = function() {
    return this.charAt(0).toUpperCase() + this.slice(1).toLowerCase();

console.log("hello world".toProperCase()); // Output: "Hello world"

In this code, we're using charAt(0).toUpperCase() to convert the first character of the string to uppercase. Then, we use slice(1).toLowerCase() to convert the rest of the string to lowercase.


In this Byte, we've explored how to convert strings to lowercase in JavaScript using the toLowerCase() and toLocaleLowerCase() methods. We also learned how to create custom methods like toTitleCase() and toProperCase(). We discussed potential issues you might encounter and how to handle them.

Last Updated: September 23rd, 2023
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