Spread operator or Rest parameter or Ellipsis in JavaScript (ES6)

Spread/Rest/Ellipsis operator in JavaScript is very useful in many context. First important use of this operator is to pass indefinite number of argument in function

Operator syntax: ...name

Three dot and then a variable name

 function sum(a,b,...n){
  return a+b+n[0]+n[1];
 }
 sum(1,2,3,4); // output: 10 

This is also called rest parameter. inside function it can be used directly as array.

As we already know that arguments is already available in function to get many parameters as shown below: but actually that is not array

function fun(){
 return arguments[0] + arguments[1];
}

fun(1,2); // 3


function fun2(...n){
 console.log(arguments);
 console.log(n);
}

fun2(1,2,3); // it will print arguments object and array n.

There are other advantages of spread operator. Suppose we have 2 arrays and need to merge both

var a = [1,2,3], b = [4,5,6];
a.push(...b);
console.log(a); // [1,2,3,4,5,6]