In ES6, you can define the default values of a function's parameters. This is quite a useful improvement because the equivalent implementation in ES5 is not only tedious but also decreases the readability of the code.

Let's see an example here:

const shoppingCart = [];
function addToCart(item, size = 1) {
  shoppingCart.push({item: item, count: size});
}
addToCart('Apple');     // size is 1
addToCart('Orange', 2); // size is 2

In this example, we give the parameter size a default value, 1. And let's see how we can archive the same thing in ES5. Here is an equivalent of the addToCart function in ES5:

function addToCart(item, size) {
  size = (typeof size !== 'undefined') ? size : 1;
  shoppingCart.push({item: item, count: size});
}

As you can see, using the ES6 default parameter can improve the readability of the code and make the code easier to maintain.