Spread, Rest

spread operator

The spread operator, represented by consecutive 3 periods ... takes an iterable and expands it into the individual elements. There are a variety of useful things you can do with this.

concat arrays

Concating has never been easier. Try adding the 3rd array here below!

const first = ['nicolas', 'julia'];
const second = ['catherine', 'whitney'];
const third = ['rob', 'stacey'];

console.log('merging! '+ [...first, ...second])

spread into key/value pairs

A useful feature is using the spread operator to turn an array into key value pairs. To output the elements 1 by 1, change the const spread value to brackets [] instead of curly braces {}.

const books = ["Don Quixote", "The Hobbit", "Old Yeller", "Black Beauty"];

const spread = {...books};
console.log(spread);

rest parameter

The rest parameter looks exactly the same as the spread, but gathers remaining elements and compacts them rather then expanding.

represent arguments

The rest parameter syntax allows us to represent an indefinite number of arguments as an array.

function sum(...args) {
  console.log('arguments are '+args);
};

sum('name', 'class', 'score');

create subarray

Another interesting thing you an do is take part of an array and turn it into a subarray. Look at an example from the page about destructuring. Please Note It MUST be placed at the end of the array! You would not be able to grab another named variable once you've put all remaining elements into the players subarray.

    const kids = ['nicolas', 'julia', 'catherine', 'sophia', 'william']
    let captain, cocaptain, players;
    const team = [ captain, cocaptain, ...players ] = kids;
    
    console.log('we created a subarray called players: ' +players);
    

arguments

Functions have a special array-like object named arguments that contains all arguments by their index (weird as it's an object, not an array!) This can be very confusing if you are looking for a variable called arguments, as it is under the hood, and not something you'll find in the code that calls your function! Please Note arrow functions do not contain the arguments object.

function gradeStudent(){
  console.log('argument 1 is: '+arguments[0]);
  console.log('argument 2 is: '+arguments[1]);
  console.log('argument 3 is: '+arguments[2]);
};
gradeStudent('Carlos', 'History', 'B+');