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JavaScript - Characterrs to Opposite Case in a String

Last Updated : 23 Jul, 2025
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Here are the various approaches to convert each character of a string to its opposite case (uppercase to lowercase and vice versa).

Using for Loop and if-else Condition - Most Common

In this approach, we iterate through each character of the string. Using an if-else statement, it checks if each character is uppercase or lowercase and converts it accordingly.

JavaScript 
const s1 = "Hello World";

let s2 = "";
for (let i = 0; i < s1.length; i++) {
    const char = s1[i];
    s2 += char === char.toUpperCase() ? char.toLowerCase() : char.toUpperCase();
}
console.log(s2);

Output
hELLO wORLD

Using ASCII Values

Using ASCII values, you can determine whether a character is uppercase or lowercase, then convert it by adjusting its ASCII code. This method is useful for complex strings.

JavaScript 
const s1 = "Hello World";

let s2 = "";
for (let i = 0; i < s1.length; i++) {
    const charCode = s1.charCodeAt(i);
    if (charCode >= 65 && charCode <= 90) {
        s2 += String.fromCharCode(charCode + 32);
    } else if (charCode >= 97 && charCode <= 122) {
        s2 += String.fromCharCode(charCode - 32);
    } else {
        s2 += s1[i];
    }
}
console.log(s2);

Output
hELLO wORLD

Using replace() Method with Regular Expression

The replace() method, along with a regular expression, can convert uppercase letters to lowercase and vice versa in a single line.

JavaScript 
const s1 = "Hello World";

// Use replace with regex to swap case
const s2 = s1.replace(/[a-zA-Z]/g, char =>
    char === char.toUpperCase() ? 
        char.toLowerCase() : char.toUpperCase()
);
console.log(s2);

Output
hELLO wORLD

Using split() and map() Method

The split() method splits the string into an array of characters, uses map() to change the case of each character, and then joins the array back into a string.

JavaScript 
const s1 = "Hello World";

const s2 = s1
    .split("")
    .map(char => (char === char.toUpperCase() ? 
        char.toLowerCase() : char.toUpperCase()))
    .join("");
console.log(s2);

Output
hELLO wORLD

Using the reduce Method

The reduce() method goes through each character in the string, turning it into an array. For each character, it adds the opposite case (uppercase becomes lowercase and vice versa) to the result. Finally, it joins everything back into a single string.

JavaScript 
function swapCase(str) {
    return str.split('').reduce((acc, char) => {
        return acc + (char === char.toUpperCase() ?
            char.toLowerCase() : char.toUpperCase());
    }, '');
}

console.log(swapCase("Hello World!"));

Output
hELLO wORLD!

Using Array.from() Method

Array.from() converts the string into an array, allowing you to change each character’s case and join them back together.

JavaScript 
const s1 = "Hello World";

const s2 = Array.from(s1, char =>
    char === char.toUpperCase() ? 
        char.toLowerCase() : char.toUpperCase()
).join("");
console.log(s2);

Output
hELLO wORLD

Using for…of Loop and Ternary Operator

We can combine for...of loop with a ternary operator to check and convert each character’s case.

JavaScript 
const s1 = "Hello World";

let s2 = "";
for (const char of s1) {
    s2 += char === char.toUpperCase() ?
        char.toLowerCase() : char.toUpperCase();
}
console.log(s2);

Output
hELLO wORLD

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