Everyone who has used Java or other languages that support varargs (or ‘variable number of arguments’) would know what a big boon that is.

Consider you have a function that creates file path strings. (This is basically what path.join() in Javascript does)

function pathJoin(folder1, folder2, folder3) {
    return "/" + folder1 + "/" + folder2 + "/" + folder3 + "/";
}

var p = pathJoin("my", "favourite", "folder")

Here p will be “/my/favourite/folder/”

But we want to create the function in such a way that it works for any number of arguments and not just for 3 arguments.

Java supports, what it calls as varargs. Which allows us to send and optional number of arguments in form of an array.

public static int sumAll(Integer... numbers) {
    int sum = 0;
    if (numbers.length > 0 ) {
        for (Integer n : numbers) {
            sum += n;
        }
    }
    return sum;
}

int sumOdd = sumAll(1,3,5,7,9); // 25
int sumEven = sumAll(2,4,6,8); // 20
int sumZero = sumAll(); // 0

So basically we are sending in 3 or 4 (or even 0) seperate variables to the function sumAll but the function treats them as an array numbers
This is exactly what the varargs operator (that elipsis, or 3 dots) does.

So int... n means a variable number of integers, represented as an array int[] n

Now, with ECMAScript2015 (ES6) having been standardized, we have the spread and rest operators in Javascript, that does something similar, and even better it has support of varargs both in function definition, as well as function calls.

Spread Syntax

Take a look at how the spread syntax can spread an array

var parts = ['shoulders', 'knees'];
var lyrics = ['head', ...parts, 'and', 'toes']; // ["head", "shoulders", "knees", "and", "toes"]

And check how it can help inject arrays into functions

function myFunction(x, y, z) { }
var args = [0, 1, 2];
myFunction(...args);

Rest Parameters

Take a look at how the rest parameters syntax works. This uses the spread operator we saw above

function fun1(...theArgs) {
  console.log(theArgs.length);
}

fun1();  // 0
fun1(5); // 1
fun1(5, 6, 7); // 3

NOTE: Javascript, prior to ES6, also had the global ‘argument’ in each function object, but that was not a real array. Rest parameters have sort, map, pop etc calls, because they are real Arrays

IMPORTANT NOTE: The spread operator does only 1-level copy of array, and not a deep copy, so do not use for multi-dimensional arrays