Say we have:

function myObj() {
  this.input = Array.prototype.slice.call(arguments);
}

We can pass a variable or an array but :

var myObj1 = new myObj(1);
myObj1.input // -> [1]
var myArr = [1, 2, 3];
var myObj2 = new myObj(myArr);
myObj2.input // -> [[1, 2, 3]]

How then can we get, while still using myArr:

myObj3.input // -> [1, 2, 3]

Here’s how, courtesy of this site:

var myObj3 = new myObj();
myObj.prototype.constructor.apply(myObj3, myArr);

Note that myObj3 needn’t be an instance of myObj, it just needs to be an existing value so that the global object isn’t used as the value of this.

This is pretty cool, though of course you can do it much easier in languages such as Scala:

val myObj4 = new myObj(myArr: _*)