The spread operator (...) allows you to "explode" an array into its individual elements.

 

Spreate an array:

console.log([1,2,3]);  // [1, 2, 3]     
console.log(...[1,2,3]);  // 1 2 3

 

Spread out the second array and push that in first array:

let first = [1,2,3];
let second = [4,5,6];

first.push(second);
console.log(first); //  [1, 2, 3, [4, 5, 6]]

first.push(...second);
console.log(first); // [1, 2, 3, 4, 5, 6]

 

Pushing in an array of parameters:

let var1 = [1,2,3];
let var2 = [4,5,6];

function sumUp(a,b,c){
  console.log( a+b+c );
}

sumUp(...var1); // 6
sumUp(...var2); // 15