Randomly select enumerable property of object in Javascript

Question

Given a dictionary-like object in Javascript such as {a:1, b:-2, c:42}, is there a simple way to randomly choose a property?

In the above example, I would like to have a function that would return a, b or c randomly.

The solution I've come up with is like the following:

    var proplist = []
    forEach(property in foo) {
        if(propertyIsEnumerable(foo[property]) {
            proplist.push(property);
        }
    }
    var n = proplist.length;
    // randomly choose property (randInt(n) returns a random integer in [0,n))
    proplist[randInt(n)];

Is there a more idiomatic way to do this?

possible duplicate of [Pick random property from a Javascript object](http://stackoverflow.com/questions/2532218/pick-random-property-from-a-javascript-object) — Gazler, Jan 24 '12 at 22:58

score 7 · Accepted Answer · answered Jan 24 '12 at 23:00

7

Use Object.keys (or even Object.getOwnPropertyNames) to get a list of all properties. Then, select a random property by multiplying Math.random() with the length of the list, floored.

var propList = {}; //...
var tmpList = Object.keys(propList);
var randomPropertyName = tmpList[ Math.floor(Math.random()*tmpList.length) ];
var propertyValue = propList[randomPropertyName];

answered Jan 24 '12 at 23:00

Rob W

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The `Object.keys` method is exactly what I was looking for, thank you! – tlehman Jan 24 '12 at 23:14

georg · Answer 2 · 2016-04-10T09:26:55.190

5

This can be quite idiomatic with underscore.js:

 randomProp = _.shuffle(_.keys(obj))[0]

Edit: actually, one should use _.sample for that.

edited Apr 10 '16 at 09:26

answered Jan 24 '12 at 23:06

georg

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Of course this will involve shuffling the entire list of keys, with probably `n - 1` calls to `random`, where `n` is the length of `keys`. As shown in another answer and the question, this is doable with only a single `random` call. – Scott Sauyet Apr 10 '16 at 01:18
@ScottSauyet: of course, this is a wrong answer, edited. – georg Apr 10 '16 at 09:27
@georg your link for `_.sample` points to lodash, not underscore, although the function is available in both – krodmannix May 02 '16 at 14:28

score 2 · Answer 3 · answered Apr 10 '16 at 01:36

Or if you want to write a reusable function for this, you could do

const randomFrom = list => list[Math.floor(list.length * Math.random())];
const randomProp = obj => randomFrom(Object.keys(obj));

randomProp(propList); //=> one of the keys of propList

This will return undefined if your object has no properties, but that's probably the best we could do in any case.

Randomly select enumerable property of object in Javascript

3 Answers3