February 24, 2001 - Keeping Prototyping Information

Yehuda Shiran February 24, 2001
Keeping the Prototyping Information
Tips: February 2001

Yehuda Shiran, Ph.D.
Doc JavaScript

JavaScript stores the prototyping relationship in an internal property of the object, __proto__ (two underscore characters at the front and two at the end.) Let's take an example:

<SCRIPT LANGUAGE="JavaScript">
function Shape() {
  this.borderWidth = 5;
}
function Square() {
  this.edge = 12;
}
Square.prototype = new Shape;
myPicture = new Square;
alert(myPicture.__proto__);
alert(myPicture.borderWidth);
</SCRIPT>

The object myPicture has an internal property, __proto__, which is set to Shape when the statement:

Square.prototype = new Shape;
is executed. The value of __proto__ instructs JavaScript where to look for properties, when local definitions are not available. In the example above, the property borderWidth is not defined within the object Square. The browser looks for the value of __proto__, which is Shape, and then fetches the value of its borderWidth property. Being an internal property, most browsers will return undefined when asked to alert __proto__. Netscape 6, though, returns <object object>.