... allows an array to be expanded.
Here is one way you can use it
let arr1 = ["one", "two"];
let arr2 = ["three", ...arr1,"four", "five"];
/*Please note that let arr2 =["three", arr1, "four", "five"] is legal but will yield something ifferent. The second set means that you are adding arr as ONE element whereas the ... means you are speading out the array.