ranking arbitrary array of objects

Question

If I have an array like this

var cars=
[
      { 'title':'brand', 'value':'honda'}
      { 'title':'brand', 'value':'toyota'}
      { 'title':'color', 'value':'red'}
      { 'title':'color', 'value':'white'}
      { 'title':'year', 'value':'1995'}
      { 'title':'year', 'value':'2006'}
      { 'title':'year', 'value':'2007'}     
 ]

How can I write a single function that would return element's rank. So for this array, for elements with title "brand" it should return 0, for elements with title "color" should return 1 and so on. It should not cache or use any mapping table but should determine the rank on flight, so any time you call

getRank(cars[6]) == 2 //true for the last element 
getRank(cars[0]) == 0 //true for the first element 
getRank(cars[1]) == 0 //true for the second element 
getRank(cars[3]) == 1 //true for the fourth element 

Answer 1

Updated answer based on your further explanations

Javascript

var cars = [
      { 'title':'brand', 'value':'honda'},
      { 'title':'brand', 'value':'toyota'},
      { 'title':'color', 'value':'red'},
      { 'title':'color', 'value':'white'},
      { 'title':'year', 'value':'1995'},
      { 'title':'year', 'value':'2006'},
      { 'title':'year', 'value':'2007'}     
 ];

function getRank(data, number) {
    var ranks = {},
        rank = 0;

    data.forEach(function (entry) {
        if (typeof ranks[entry.title] !== "number") {
            ranks[entry.title] = rank;
            rank += 1;
        }
    });

    return ranks[data[number].title];
}


console.log(getRank(cars, 6));
console.log(getRank(cars, 0));
console.log(getRank(cars, 1));
console.log(getRank(cars, 3));

On jsfiddle

Output

2
0
0
1

Answer 2

If it's already ordered (it must be cause otherwise I would really not understand your problem) you could do something like the following

var current_index = -1;
var current_title = "";

for (var i in arr) {
    if (arr[i].title!=current_title) {
        current_title = arr[i].title;
        current_index++;
    }
    arr[i].rank = current_index;

}