Trouble with basic Javascript

A few things:

You forgot var:

var params = new Array();

But an array takes numeric indices, so params['return'] is not really a member of the array but of the object that represents the array, so it doesn't affect the length.

You could use an object but objects have no length:

var params = {};
params['return'] = 'json';

To get the length though you can count the keys in that object and get the length of the resulting array:

var len = Object.keys(params).length;

Javascript arrays don't hold key value pairs like objects do, so the length isn't incremented when you assign a value. They are however objects themselves, so you can assign values to its properties (in this case the return property).
You probably want params to be a plain object: params = {}

You need to count the properties, like this for example:

function dictionarySize(dict){
    var size = 0;
    for (key in dict){
        // In practice you'd probably want to filter using hasOwnProperty
        size++;
    }
    return size
}
alert(dictionarySize(params));

Or using a library like underscore.js:

_.size(params);

Object.keys is also an option, although it won't work in IE8 and older.

If you don't need an key value pairs use params.push:

params=new Array();
params.push('json')
alert(params.length); // 1

You can create an array, push stuff on it, and assign properties to values of it like so:

var params=[];
params.push('firstly');
params[0]="jonson";
params['return']="fredy"
params.newone="json";
params.push('Carl');

NOW, if you do:

console.log(params.length,params);

the result of that is:

2 ["jonson", "Carl", return: "fredy", newone: "json"]

SO, you see "firstly" was replaced by "jonson" in the [0] - so the "pushed" value is addresed by the numerical [0]