Categorising a number in JavaScript

I've written a function that takes a number and returns a value depending on where that number lies on a pre-defined scale. That's probably a little confusing so I've put the code below and added a JSFiddle (although you might be better off in the dev console).

var getSize = function(number){
        var sizes = {
            100 : 'url_t',
            240 : 'url_s',
            320 : 'url_n',
            500 : 'url_m',
            640 : 'url_z',
            800 : 'url_c',
            1024 : 'url_l',
        },
        result = sizes[100];

    for (var size in sizes){
        if(number < size){
            result = sizes[size];
            break;
        }
        result = sizes[1024];
    }
    return result;
};
getSize(10) // returns url_t
getSize(130) // returns url_s
getSize(2000) // returns url_l
getSize(600) // returns url_z

It works and it's fairly performant, it's just a bit icky. I've tried to come up with a better solution but it just feels ickier. Is there anything I'm missing that would make this a bit nicer? I was playing about with putting the sizes in an array so they're a little more JSON-ish, but then I got lost.

You might not need to loop through an object, and a simple switch statement would suffice.

function getSize(number) {
    switch (true) {
        case number < 100: return "url_t";
        case number < 240: return "url_s";
        case number < 320: return "url_n";
        case number < 500: return "url_m";
        case number < 640: return "url_z";
        case number < 800: return "url_c";
        default:           return "url_l";
    }
}

As a side note, I've seem people use switch (true) before when they want to use some logic in their case's instead of just hard coded values. Kind of a nice alternative to an if-else construct, and would probably perform better than looping over an object using a for-in loop.

You could still keep the sizes in an object:

var sizes = {
    100 : 'url_t',
    240 : 'url_s',
    320 : 'url_n',
    500 : 'url_m',
    640 : 'url_z',
    800 : 'url_c',
    1024 : 'url_l'
};

function getSize(number) {
    switch (true) {
        case number < 100: return sizes[100];
        case number < 240: return sizes[240];
        case number < 320: return sizes[320];
        case number < 500: return sizes[500];
        case number < 640: return sizes[640];
        case number < 800: return sizes[800];
        default:           return sizes[1024];
    }
}

Edit: A problem arises if you need to change the sizes for your application. That's why I'm leaning towards the first function, which just a switch statement and hard coded values.

StackExchange Code Review Q#57689, answer score: 13