Capitalize the first letter of a JavaScript string

As software engineers, we are often tasked with transforming data into presentable information. A common part of this process is capitalizing the first letter of a string.

In order to accomplish this task, we can use array destructuring in combination with String.prototype.toUpperCase() to capitalize the first letter of the string. We can then use Array.prototype.join() to combine the capitalized first with the ...rest of the characters. Additionally, we might want to transform the rest of the string into lowercase, using String.prototype.toLowerCase() and a boolean argument.

const capitalize = ([first, ...rest], lowerRest = false) =>
  first.toUpperCase() +
  (lowerRest ? rest.join('').toLowerCase() : rest.join(''));

capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'

Conversely, we can also decapitalize the first letter of a string using the same technique, but using String.prototype.toLowerCase() instead.

const decapitalize = ([first, ...rest], lowerRest = false) =>
  first.toLowerCase() +
  (lowerRest ? rest.join('').toLowerCase() : rest.join(''));

decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'foobar'