"..." operator in Javascript

Question

I found the following case in javascript. I don't understand '...' operator mean here. I search it on the Google, but I did not get anything about it. Is there any other usage for this operator? Can someone help me out?

var x= [1,2,3];
var y = [4,5,6];

var z = [...x, ...y]; // z will be [1,2,3,4,5,6];

Thanks.

Answer 1

I have a way of thinking which makes it very easy to understand and remember how '...' works.

var arr = [1,2,3] // this is arr which is the array

on the other hand

...arr            // this is whatever inside arr, which is 1,2,3

So you can also think of it as taking what is inside of an array.

---

Note that by its own, ...arr is not a valid syntax. You can use it in many ways , two of them coming to my mind are :

1 - Pass what is inside an array to a function

var arr = [ 1,2,3 ]
var myFunc = function(a,b,c) {
  console.log(a,b,c)
}
myFunc(..arr)   // logs 1 2 3

myFunc(1,2,3)   // logs 1 2 3

2 - Take what is inside of an array and use them in another array.

var arr = [ 1,2,3 ]
var foo = [ ...arr, 4,5,6 ] // now foo is [ 1,2,3,4,5,6 ]