Javascript - Using an array as a parameter to a function and changing values when creating new prototype object

Arrays can be declared inline using square brackets:

var a = [1, 2, 3];

In your case, you can use this directly in your call to new animal:

var myPet = new animal("Olly", ["dog", 3, "brown"]);

An alternative approach, which if you want to store all the details together in this way is the one I'd take, is to pass in an object:

var myPet = new animal("Olly", {type: "dog", age: 3, colour: "brown"});

This means you can then access the details by name:

console.log(myPet.animal_details.type); //dog

You can declare an array like this:

var myPet = new animal("Olly", ['cat','white','big']);
//myPet.animal_details[0] === 'cat';

but I think an object will fit better with your solution

 var myPet = new animal("Olly", {type: 'cat',color: 'white', size: 'big'});
 //myPet.animal_details.type === 'cat';
 //myPet.animal_details.color === 'white';
 //myPet.animal_details.size === 'big';

You may find it easier to use object instead of an array because it would be easier to define default properties if they are not present. An object's structure is a little better for handling this type of data too, imo:

function Animal(details) {
  this.name = details.name || 'Dave';
  this.age = details.age || 10;
  this.location = details.location || 'Dorset';
}

var cat = new Animal({ name: 'Simon', location: 'London' });

// Object { name: "Simon", age: 10, location: "London" }

PS. Always captitalise your constructor names.

Of course, if you're not worried about default values and just want the items specified in your object to be added to the instance, this is a simple method to use:

function Animal(details) {
  for (var p in details) {
    this[p] = details[p];
  }
}

var cat = new Animal({ name: 'Simon', location: 'London' });

// Object { name: "Simon", location: "London" }

You can use this approach, to dynamically add details for every instance:

//Base Animal Class
function Animal(name,options) {

    //Localise Class Variable
    var me = this;

    //Assign name to class variable
    me.name = name;

     //Assign details dynamically, depends on every instance
     for(var index in options) {
        me[index] = options[index];
     }
}

  //Create  Animal Instance
  var PetInstance = new Animal("Max",{
      age:11,
      kind:"Pudel"
  });
  console.log(PetInstance);

  //Create Another Animal With Different Details
  var PetInstance2 = new Animal("Foo",{
      age:22,
      kind:"Doberman"
  });
  console.log(PetInstance2);

See this JSFiddle Example: http://jsfiddle.net/shako92/bretpu5c/

You could use instant-array notation: ['Furry', 'Sharp nails', 'Black']. Although you might wanna go with an object so you aren't depending on "which index did I use to put the skin-type?" So something like:

{
  'skin':'Furry',
  'paw':'Sharp nails',
  'color':'Black'
}

(If this looks like JSON to you, it's because it is, JSON = JavaScript Object Notation)

Alternatively, you could leverage the built-in arguments array to extract (part of) the arguments given as an array and assign it:

function animal(name) {
  this.animal_name = name;
  this.animal_details = arguments.slice(1); // get a "slice" of the arguments array starting at index 1 (skipping the name argument)
}
var myPet = new animal("Olly", 'Furry', 'Sharp nails', 'Black');

you should try use the 'arguments' var inside your function, like this:

function a (){
  for(arg in arguments){
    console.log(arg);
  }
}
a(1,2,3,4,5,6,7);

The result will be:

1
2
3 [...]

This way you let your code simple and useful.