Pick random property from a Javascript object

Question

Suppose you have a Javascript object like {'cat':'meow','dog':'woof' ...} Is there a more concise way to pick a random property from the object than this long winded way I came up with:

function pickRandomProperty(obj) {
    var prop, len = 0, randomPos, pos = 0;
    for (prop in obj) {
        if (obj.hasOwnProperty(prop)) {
            len += 1;
        }
    }
    randomPos = Math.floor(Math.random() * len);
    for (prop in obj) {
        if (obj.hasOwnProperty(prop)) {
            if (pos === randomPos) {
                return prop;
            }
            pos += 1;
        }
    }       
}

Answer 1

You can just build an array of key while walking through the object.

var keys = [];
for (var prop in obj) {
    if (obj.hasOwnProperty(prop)) {
        keys.push(prop);
    }
}

then randomly pick an element from the keys.

return keys[Math.floor(keys.length * Math.random())];

Answer 2

Picking a random element from a stream

function pickRandomProperty(obj) {
    var result;
    var count = 0;
    for (var prop in obj)
        if (Math.random() < 1/++count)
           result = prop;
    return result;
}