String .toUpperCase() / toLowerCase()

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

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

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 Japanese and spaces are left as-is.
var mixed = "Hello こんにちは 123";
console.log(mixed.toUpperCase()); // Outputs "HELLO こんにちは 123".

string.toUpperCase() and string.toLowerCase() are methods that convert the alphabetic characters in a string to uppercase or lowercase. Non-alphabetic characters such as Japanese, 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.