Three dots (...) mean in JavaScript

This is one of the cool new feature of Javascript's ECMA6; ... is one of these new Javascript functionalities.

It can be used in two different ways; as a spread operator OR as a rest parameter

Spread Operator
This is very useful when we want to copy properties of one object in to another but with a bit of modification in value of some attributes.
eg. Let say we have a below object:

const object1 = {
  fullName: 'Rahul',
  occupation: 'Software developer',
  age: 31,
  website: 'https://rahul.xyz'
};

and we want to create different objects with only change in name.
We can do this very easily with the help of spread operator :

const object2 = {
   ...object1
   fullName: 'Tom',
}

Otherwise we need to copy object first then we have to change its property. Now its all happening in single step only

Rest Parameter
This I really found very helpful, Sometimes we have to design some Api's which can accept n number of parameters, in those scenarios this can be really useful.
Let me try to explain you with a simple example, We want to design a method to sum n numbers :

function sum(...numbers){
  return numbers.reduce((sum, val)=>{
    	return sum += val;
    });
}

Now run this for :
sum(3,5) // gives 8
sum(1,2, 3, 5) //gives 11.

Cool .. right !!!