Definition and Usage

The toUpperCase() function returns the given string converted to uppercase.

Features

  • The return value is a new string with all characters of the given string converted to uppercase.
  • Since it only returns the converted string, the original string remains unchanged.

Basic Example

JavaScript 
const originalString = "Hello, World!";
const uppercaseStr = originalString.toUpperCase();

console.log(uppercaseStr); // Output: "HELLO, WORLD!"

/* 👇 The original string remains unchanged. */
console.log(originalString); // Output: "Hello, World!"

To convert all characters of the given string to lowercase, use the toLowerCase() function.

Syntax

JavaScript 
str.toUpperCase();

str is the string to which the toUpperCase() function is applied.
This function is intended for strings only. Calling it on a value of any other data type will result in a TypeError.

Parameters

None.

Return value

The return value is a new string with all characters of the given string (str) converted to uppercase. During this process, the original string (str) remains unchanged.

Things to Keep in Mind

The toUpperCase() function is a method that can be used only on strings. If you call this method on a value of any data type other than a string, a TypeError will occur. Such a call is not a valid operation.

JavaScript 
const number = 17; // A number, not a string

try {
    let result = number.toUpperCase(); // A TypeError occurs here
    console.log(result);
} catch (error) {
    console.error(error); // TypeError: number.toUpperCase is not a function
}

In the code above, toUpperCase() is used on a number value, so a TypeError is raised. To prevent this, you should check the data type before calling toUpperCase() or convert the value to a string when necessary.

Practical Examples

The toUpperCase() function is one of the most widely used functions in JavaScript. Let's explore this function through various practical examples.

String Comparison

You can compare strings without case sensitivity by converting them to uppercase. The example below uses the toUpperCase() function to convert both strings to uppercase and then checks whether they match.

JavaScript 
const str1 = "apple";
const str2 = "APPLE";

if (str1.toUpperCase() === str2.toUpperCase()) {
    console.log("The two strings are the same.");
} else {
    console.log("The two strings are different.");
}

// Output: "The two strings are the same."

Output Formatting With Uppercase

English text often distinguishes meaning through case usage. Titles, headings, safety notices, and warning messages are frequently written in uppercase, making the toUpperCase() function especially useful for standardizing output.

JavaScript Converting a title or heading to uppercase 
const title = "web development";
console.log(title.toUpperCase()); // Output: "WEB DEVELOPMENT"
JavaScript Converting safety notices and warning messages to uppercase 
const warningMessage = "keep out of reach of children";
console.log(warningMessage.toUpperCase());
// Output: "KEEP OUT OF REACH OF CHILDREN"

Specifications

Specification for JavaScript toUpperCase() Function
Specification
toUpperCase() ECMAScript® 2026 Language Specification
#sec-string.prototype.touppercase

Compatibility

Compatibility for JavaScript toUpperCase() Function
Method
Desktop Chrome
Chrome
DesktopDesktop Edge
Edge
Desktop Firefox
Firefox
Safari
Safari
Node.js
Node.js
toUpperCase() 1 12 1 1 0.10

References

See also