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prototype Property 

Returns a reference to the prototype for a class of objects.



Required. The name of an object.

Use the prototype property to provide a base set of functionality to a class of objects. New instances of an object "inherit" the behavior of the prototype assigned to that object.

All intrinsic JScript objects have a prototype property that is read-only. Functionality may be added to the prototype, as in the example, but the object may not be assigned a different prototype. However, user-defined objects may be assigned a new prototype.

The method and property lists for each intrinsic object in this language reference indicate which ones are parts of the object's prototype, and which are not.


The prototype property of a built-in object cannot be modified when running in fast mode, the default for JScript. To compile a program from the command line that uses the prototype property, you must turn off the fast option by using /fast-. It is not safe to turn off the fast option in ASP.NET because of threading issues.

Say you want to add a method to the Array object that returns the value of the largest element of the array. To do this, declare the function, add it to Array.prototype, and then use it.

function array_max() {
   var i, max = this[0];
   for (i = 1; i < this.length; i++) {
      if (max < this[i])
         max = this[i];
   return max;
Array.prototype.max = array_max;
var x = new Array(1, 2, 3, 4, 5, 6);

After compiling with the /fast- option, the output of the programs is: