String .toUpperCase() / toLowerCase()

// Converts all alphabetic characters to uppercase.
string.toUpperCase()

// Converts all alphabetic characters to lowercase.
string.toLowerCase()

sample_toUpperCase.js

var str = "Hello World";

// Converts to uppercase.
console.log(str.toUpperCase()); // Outputs "HELLO WORLD".

// Converts to lowercase.
console.log(str.toLowerCase()); // Outputs "hello world".

// The original string is not modified.
console.log(str); // Outputs "Hello World" unchanged.

// Useful for case-insensitive comparisons.
var input = "JavaScript";
var target = "javascript";
console.log(input.toLowerCase() === target.toLowerCase()); // Outputs "true".

// Non-alphabetic characters such as digits and spaces are left as-is.
var mixed = "Hello world 123";
console.log(mixed.toUpperCase()); // Outputs "HELLO WORLD 123".

// Case-insensitive search filtering.
var members = ["Okabe Rintaro", "Makise Kurisu", "Shiina Mayuri", "Hashida Itaru"];
var query = "kurisu";
var i;
for (i = 0; i < members.length; i++) {
	if (members[i].toLowerCase().indexOf(query.toLowerCase()) !== -1) {
		console.log(members[i] + " matched"); // Detects a match regardless of letter case.
	}
}

// URL slug generation: Convert to lowercase and replace spaces with hyphens.
var title = "Future Gadget Lab";
var slug = title.toLowerCase().replaceAll(" ", "-");
console.log(slug); // Outputs "future-gadget-lab".

HELLO WORLD
hello world
Hello World
true
HELLO WORLD 123
Makise Kurisu matched
future-gadget-lab

string.toUpperCase() and string.toLowerCase() are methods that convert the alphabetic characters in a string to uppercase or lowercase. Non-alphabetic characters such as digits and symbols are unaffected and remain unchanged.

The most common use case is case-insensitive string comparison. When searching or matching user input, convert both strings to lowercase before comparing. Because string.indexOf() and string.startsWith() are case-sensitive, these methods are often used together with toLowerCase().

Both methods leave the original string unchanged and return a new string. To use the result, assign it to a variable or use it directly.