Spread operator

spread.js

// Can be used instead of Array functions

const a = [1,2,4];
const b = [3,7,6];
const c = 1;

// a.concat(b);
let arr = [...a,...b];

// Unshift, push
// The difference is spread operator creates a new reference unlike the array functions.

arr = [...a,1]; // arr = a.push(1);
arr = [1,...a]; // arr = a.unshift(1);

// Accessing properties from a complex object

let seinfeldRocksBigBangSux = 
{
  'name': 'Jerry',
  'surname': 'Seinfeld'
  'bestFriends': ['George Costanza','Elaine Benes','Cosmo Kramer'],
  'location': 'NYC'
}

 // Clean syntax

const {name, surname, ...remainingProperties} = seinfeldRocksBigBangSux;

// Here's relatively complicated example.
const params = 
{
 ...prevData,
  name: externalData.name,
  ...(hobbies && {hobbies})
}

In my case the hobbies variable has never been initialized, therefore the ReferenceError. When you wrap that in a function the reference automatically gets created for you. I guess it's not what I was expecting, but thanks anyway