Compared to some (primarily functional) languages, function definition in JavaScript looks rather cumbersome:

const square = (x) => {
  return x ** 2;
};

It uses a lot of extra characters and the word return. Since version ES6, an alternative, shorter syntax has appeared in the language, making it much easier to understand and shorten code in some cases.

// It takes a little time to get used to it,
// but after a while it'll save your life
const double = (x) => x ** 2;

There are two differences from the full definition, it omits the curly brackets and the return statement. The shortened version carries out the return automatically. It means that there is exactly one computable expression inside such a function, and its result will be immediately returned.

Note that the differences are purely syntactic, there are no differences in terms of usage. An example with two arguments:

The full version

const sum = (a, b) => {
  return a + b;
};

The shortened version

const sum = (a, b) => a + b;

Note the absence of curly brackets. Developers unaccustomed to this syntax sometimes write const sum = (a, b) => { a + b }; and then have a hard time understanding why it doesn't work. The answer is simple: if there are curly brackets, then this isn't the shortened form, which means if you want the function to return a value, you will have to use return.

Write the capitalize() function that takes a non-empty string and converts the first letter of the first word to uppercase:

const name = 'arya';
console.log(capitalize(name)); // => "Arya"

To get a substring (or character) from a string, use the slice() method:

'welcome'.slice(2, 5); // "lco"

To convert the string to uppercase, use the toUpperCase() method:

'welcome'.toUpperCase(); // "WELCOME"