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By not overwriting the prototype property, we are also avoiding its
constructor property being enumerable. The object provided for us has the
DontEnum attribute set, which is impossible to recreate when we assign a cus-
tom object to the prototype property and manually restore the constructor
7.2.4 The Problem with Constructors
There is a potential problem with constructors. Because there is nothing that sep-
arates a constructor from a function, there is no guarantee that someone won't use
your constructor as a function. Listing 7.23 shows how a missing new keyword can
have grave effects.
Listing 7.23 Constructor misuse
"test calling prototype without 'new' returns undefined":
function () {
var circle = Circle(6);
assertEquals("undefined", typeof circle);
// Oops! Defined property on global object
assertEquals(6, radius);
This example shows two rather severe consequences of calling the constructor
as a function. Because the constructor does not have a return statement, the type of
the circle ends up being undefined . Even worse, because we did not call the
function with the new operator, JavaScript did not create a new object and set it as
the function's this value for us. Thus, the function executes on the global object,
causing this.radius = radius to set a radius property on the global object,
as shown by the second assertion in Listing 7.23.
In Listing 7.24 the problem is mitigated by use of the instanceof operator.
Listing 7.24 Detecting constructor misuse
function Circle(radius) {
if (!(this instanceof Circle)) {
return new Circle(radius);
this.radius = radius;
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