How does a `return` statement work in a JavaScript constructor function?

Usually return simply exits the constructor. However, if the returned value is an Object, it is used as the new expression's value.

Consider:

function f() {
   this.x = 1;
   return;
}
alert((new f()).x);

displays 1, but

function f() {
   this.x = 1;
   return { x: 2};
}
alert((new f()).x);

displays 2.

The reason to use the new operator is to ensure that this inside the constructor refers to a new context, which supports:

this.functionName = function(){...};

, and to allow the use of the instanceof operator:

function foo() {...}
var bar = new foo();
alert(bar instanceof foo);

Using return {...} inside such a constructor negates both of these effects as this will not be needed with such a pattern, and as instanceof will return false.