Spread operator

The spread operator allows us to pull apart arrays, iterable collections such as sets or maps, and even objects in the latest standard. This gives us a nicer syntax for performing some common operations such as the ones here:

// working with a variable amount of arguments
const oldVarArgs = function() {
    console.log('old variable amount of arguments', arguments);
}
const varArgs = function(...args) {
    console.log('variable amount of arguments', args);
}
// transform HTML list into a basic array so we have access to array
// operations
const domArr = [...document.getElementsByTagName('li')];
// clone array
const oldArr = [1, 2, 3, 4, 5];
const clonedArr = [...oldArr];
// clone object
const oldObj = {item1 : 'that', item2 : 'this'};
const cloneObj = {...oldObj};

What used to be for loops and other versions of iteration is now a simple one-line piece of code. Also, the first item is nice since it shows the reader of this code that we are utilizing the function as a variable argument function. Instead of needing documentation to lay this out, we can see this with the code.

When dealing with arguments, if we are going to mutate them at all in the function, create a copy and then mutate. Certain de-optimizations happen if we decide to mutate the arguments directly.