no_validation.js

$.get('/my_data_service', function(data) {
    // Am I guaranteed that data.values will always be there and always will be an array? 
    for (var i in data.values) {
        do_things(data.values[i])
    }
})

validation.js

$.get('/my_data_service', function(data) {
    // Write these kinds of checks enough times and your code starts to look like crap
    if (data.values && data.values instanceof Array) {
        for (var i in data.values) {
            do_things(data.values[i])
        }
    } else {
        throw new Exception("Uh, wtf dude")
    }
})