Pick random property from a Javascript object

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;
        }
    }       
}

The chosen answer will work well. However, this answer will run faster:

var randomProperty = function (obj) {
    var keys = Object.keys(obj);
    return obj[keys[ keys.length * Math.random() << 0]];
};

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;
}

I didn't think any of the examples were confusing enough, so here's a really hard to read example doing the same thing.

Edit: You probably shouldn't do this unless you want your coworkers to hate you.

var animals = {
    'cat': 'meow',
    'dog': 'woof',
    'cow': 'moo',
    'sheep': 'baaah',
    'bird': 'tweet'
};

// Random Key
console.log(Object.keys(animals)[Math.floor(Math.random()*Object.keys(animals).length)]);

// Random Value
console.log(animals[Object.keys(animals)[Math.floor(Math.random()*Object.keys(animals).length)]]);

Explanation:

// gets an array of keys in the animals object.
Object.keys(animals) 

// This is a number between 0 and the length of the number of keys in the animals object
Math.floor(Math.random()*Object.keys(animals).length)

// Thus this will return a random key
// Object.keys(animals)[0], Object.keys(animals)[1], etc
Object.keys(animals)[Math.floor(Math.random()*Object.keys(animals).length)]

// Then of course you can use the random key to get a random value
// animals['cat'], animals['dog'], animals['cow'], etc
animals[Object.keys(animals)[Math.floor(Math.random()*Object.keys(animals).length)]]

Long hand, less confusing:

var animalArray  = Object.keys(animals);
var randomNumber = Math.random();
var animalIndex  = Math.floor(randomNumber * animalArray.length);

var randomKey    = animalArray[animalIndex];
// This will course this will return the value of the randomKey
// instead of a fresh random value
var randomValue  = animals[randomKey];

If you are capable of using libraries, you may find that Lo-Dash JS library has lots of very useful methods for such cases. In this case, go ahead and check _.sample().

(Note Lo-Dash convention is naming the library object _. Don't forget to check installation in the same page to set it up for your project.)

_.sample([1, 2, 3, 4]);
// → 2

In your case, go ahead and use:

_.sample({
    cat: 'meow',
    dog: 'woof',
    mouse: 'squeak'
});
// → "woof"

You can just build an array of keys 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[keys.length * Math.random() << 0];

Pick random property from a Javascript object

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