Swapcase a JavaScript string

A fun little exercise I've seen in many beginner coding challenges is to create a string with uppercase characters converted to lowercase and vice versa. This is often referred to as swapcasing a string. While this is a fairly easy exercise, it can be a good place to practice various JavaScript methods and techniques.

Starting with a string, we first need to convert it into an array of characters, using the spread operator (...).

We can then use String.prototype.toLowerCase() and the ternary operator (?) to determine the case of each character. Depending on the case, we convert it to the opposite using String.prototype.toUpperCase() or String.prototype.toLowerCase().

Finally, we use Array.prototype.join() to combine the characters back into a string.

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

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