Retrieve an object from an array with a specific parameter

I was wondering whether there is an easy way to select a random object from an array where one of the objects attributes matches a variable.

Something like this:

var ninjas = [
    { name: "Sanji Wu", affiliation: "good" },
    { name: "Chian Xi", affiliation: "good" },
    { name: "Chansi Xian", affiliation: "bad" },
    { name: "Chin Chu", affiliation: "bad" },
    { name: "Shinobi San", affiliation: "neutral" },
    { name: "Guisan Hui", affiliation: "neutral" }
];

function getRandom(attr) {
    var r = Math.floor(Math.random() * ninjas.length);

    //pseudo code below  
    if (this affiliation is "attr") {
        return a random one that matches
    }
    // end pseudo code
};

var randomItem = getRandom("good");