Getting a list of associative array keys

I have an associative array in Javascript:

var dictionary = {
    "cats": [1,2,3,4,5], 
    "dogs": [6,7,8,9,10]

How do I get this dictionary's keys? i.e. I want

var keys = ["cats", "dogs"];

Edit 7 years later: Just to get the terminology correct - there is no such thing as an 'associative array' in Javascript - this is technically just an object and it is the object keys we want.



You can use: Object.keys(obj)


var dictionary = {
  "cats": [1, 2, 37, 38, 40, 32, 33, 35, 39, 36],
  "dogs": [4, 5, 6, 3, 2]

// Get the keys
var keys = Object.keys(dictionary);


See reference below for browser support. It is supported in Firefox 4.20, Chrome 5, IE9. The link below contains a code snippet that you can add if Object.keys() is not supported in your browser.

for (var key in dictionary) {
  // do something with key

It's the statement.


Just a quick note, be wary of using if you use a library (jQuery, prototype, etc.), as most of them add methods to created Objects (including dictionaries).

This will mean that when you loop over them, method names will appear as keys. If you are using a library, look at the documentation and look for an enumerable section, where you will find the right methods for iteration of your objects.


Simple JQUERY way.

This is what I use
DictionaryObj being the javascript dictionary object you want to go through. value, key ofcourse being the names of them in the dictionary.

 $.each(DictionaryObj, function (key, value) {
                .attr("value", key)

I am currently using Rob de la Cruz's reply


and in a file loaded early on I have some lines of code borrowed from elsewhere on the internet which cover the case of old versions of script interpreters that do not have Object.keys built in.

if (!Object.keys) {
    Object.keys = function(object) {
        var keys = [];
        for (var o in object) {
            if (object.hasOwnProperty(o)) {
        return keys;

I think this is the best of both worlds for large projects: simple modern code and backwards compatible support for old versions of browsers etc.

Effectively it puts JW's solution into the function when Rob de la Cruz's Object.keys(obj) is not natively available.


Try this:

var keys = [];
for (var key in dictionary) {
  if (dictionary.hasOwnProperty(key)) {

hasOwnProperty is needed because it's possible to insert keys into the prototype object of dictionary. But you typically don't want those keys included in your list.

For example, if you do this:

Object.prototype.c = 3;
var dictionary = {a: 1, b: 2};

and then do a loop over dictionary, you'll get a and b, but you'll also get c.


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