What is returned from a constructor?

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Short Answer

The constructor returns the this object.

function Car() {
   this.num_wheels = 4;
}

// car = { num_wheels:4 };
var car = new Car();

Long Answer

By the Javascript spec, when a function is invoked with new, Javascript creates a new object, then sets the “constructor” property of that object to the function invoked, and finally assigns that object to the name this. You then have access to the this object from the body of the function.

Once the function body is executed, Javascript will return:

ANY object if the type of the returned value is object:

function Car(){
  this.num_wheels = 4;
  return { num_wheels:37 };
}

var car = new Car();
alert(car.num_wheels); // 37

The this object if the function has no return statement OR if the function returns a value of a type other than object:

function Car() {
  this.num_wheels = 4;
  return 'VROOM';
}

var car = new Car();
alert(car.num_wheels); // 4
alert(Car()); // No 'new', so the alert will show 'VROOM'

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