Apply a mapping function to each character in a JavaScript string

Ever wished Array.prototype.map() worked on strings? Well, it doesn't, but you can easily replicate its behavior. As strings can easily be turned into arrays and back, you only need to take care of the mapping function and the arguments it receives.

When working with arrays, the mapping function expects three arguments - the current element, the index of the current element and the array itself. When working with strings, the mapping function should expect the current character, the index of the current character and the string itself. This, in fact, is the only part that requires special attention.

For the rest of the process, you can simply use String.prototype.split() to turn the string into an array. Then, use Array.prototype.map() to apply the mapping function to each character, taking extra care to pass the correct arguments. Finally, use Array.prototype.join() to turn the array back into a string.

const mapString = (str, fn) =>
  str
    .split('')
    .map((c, i) => fn(c, i, str))
    .join('');

mapString('lorem ipsum', c => c === ' ' ?  ' ' : c + c.toUpperCase() + c);
// 'lLloOorRreEemMm iIipPpsSsuUumMm'