Using Javascript array objects

Add common properties for Array object instances.

The prototype object.

  • The Array object have a prototype property which is a reference to an object that contains common properties for all object instances you creates out of the Array object.
  • All created array object instances has a hidden reference to this prototype object, but you don't need to use that. You can access the common properties directly through the array object instance variable.
  • You must use the keyword 'this', which is a reference to the object you have created out of an Array object, to get access to all the common properties.

Create common properties for all array instances.

  • There are two main ways to add new common properties for all array instances:
    1. Add a property to the prototype property using the Array constructor.


      Example:
      <script type="text/javascript">
      var arr =[3,4,5,62];
      // summary is a property that has a value of a function
        Array.prototype.summary=function(initial) {
          var sum=initial;
          for (var s=0 ; s<this.length; s++) {
            sum+=this[s];
          }
          return sum;
        }
        // summary is a new method for all instances of arrays
        document.write("array sum: "+arr.summary(0)+"<br>");
        arr =["I"," love"," my"," baby!"];
        document.write("array sum: "+arr.summary("")+"<br>");
      </script>
    2. Add a property to the prototype property using a created array object instance.

      A created array object has a property, constructor, that refer back to the constructor name of the object.

      To do it this way we use the property, prototype, on that constructor:

      Example:
      <script type="text/javascript">
      var arr =[3,4,5,62];
      // summary is a property that has a value of a function
        arr.constructor.prototype.summary=function(initial) {
          var sum=initial;
          for (var s=0 ; s<this.length; s++) {
            sum+=this[s];
          }
          return sum;
        }
        // summary is a new method for all instances of arrays
        document.write("array sum: "+arr.summary(0)+"<br>");
        arr =["I"," love"," my"," baby!"];
        document.write("array sum: "+arr.summary("")+"<br>");
      </script>

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