Rest and Spread operators in ES6

const Mathlib = {

    multiply(...abc){

        return this.add(...abc)//  what argument it is passing to add function array or individual value 
    },

    add(p,q,r){
        return p+q+r;
    }
}

Mathlib.multiply(2,4,5);

above code is working fine ,so I am thinking if first time ....abc will store all value in array i.e when we are calling Mathlib.multiply(2,4,5) and second time when we are calling add(...abc) then also it should pass the array but it is passing individual value and thats why output is coming as 11.

So if above scenario is meaningful then below code should not produce error:

var a =[1,2,3];
...a  ;   // error

I am unable to understand what is going on under the hood??