Enforce required parameters

In ES6, the default value of a parameter is evaluated if the parameter is missing. It allows us to enfore that a given parameter is required.

const required = () => {
    throw new Error('Missing parameter');
};

const getAges = (yearOfBirth = required()) => new Date().getFullYear() - yearOfBirth;

Calling `getAges()` without parameter will throw the exception `Missing parameter`.

Early return

Escape CSS class names