swapCase javascript Code Snippet in 2025

Creates a string with uppercase characters converted to lowercase and vice versa.

• Use the spread operator (...) to convert str into an array of characters.
• Use String.prototype.toLowerCase() and String.prototype.toUpperCase() to convert lowercase characters to uppercase and vice versa.
• Use Array.prototype.map() to apply the transformation to each character, Array.prototype.join() to combine back into a string.
• Note that it is not necessarily true that swapCase(swapCase(str)) === str.

const swapCase = str =>
  [...str]
    .map(c => (c === c.toLowerCase() ? c.toUpperCase() : c.toLowerCase()))
    .join('');

swapCase('Hello world!'); // 'hELLO WORLD!'