Getting a list of associative array keys

Question

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"];

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.

Thanks for clarifying the terminology! Can you maybe make that a bit bigger in bright red blinking lights? — Joe Phillips
– Joe Phillips, Commented Aug 21, 2018 at 18:32
@Joe I'll make it the first result in google for you when I get a chance — Simon_Weaver
– Simon_Weaver, Commented Aug 21, 2018 at 19:46

JW. · Accepted Answer · 2014-09-08 15:35:59Z

382

Try this:

var keys = [];
for (var key in dictionary) {
  if (dictionary.hasOwnProperty(key)) {
    keys.push(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 for...in loop over dictionary, you'll get a and b, but you'll also get c.

edited Sep 8, 2014 at 15:35

answered Feb 17, 2009 at 22:26

JW.

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4 Comments

mzalazar Over a year ago

No need to declare "var keys = []", unless you need to use it after the loop.

b00t Over a year ago

@mzalazar, you better do or it'll be a global variable, which is bad practice.

mzalazar Over a year ago

@b00t i though that by declaring a variable inside a for loop... it wouln't be set in global space... now you made me doubt hehe :)

b00t Over a year ago

@mzalazar, but you're not declaring it (keys) at any point if you're just using keys.push(key);. You're just pulling (and thus declaring it) from the global namespace. :)

Peter Mortensen · Accepted Answer · 2020-07-15 00:41:41Z

184

for (var key in dictionary) {
  // Do something with key
}

It's the for..in statement.

edited Jul 15, 2020 at 0:41

Peter Mortensen

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answered Feb 17, 2009 at 22:28

wombleton

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4 Comments

wombleton Over a year ago

Just noticed that there should be a colon instead of a comma between "dogs" and the array above. Assume it's due to transcription.

Tigraine Over a year ago

Very important to check for dictionary.hasOwnProperty(key) otherwise you may end up with methods from the prototype chain..

mcmlxxxvi Over a year ago

From the same article: Iterates over the enumerable properties of an object, in arbitrary order. If key order is important, you need to do something like pushing them in an array, sorting it, and then using a for() loop to get keys from the sorted array with which to index the original object.

fixermark Over a year ago

It is, of course, fair to optimize by omitting the dictionary.hasOwnProperty check if you can be confident that the object lacks a prototype. But it's important to be aware of the possibility of prototype inheritance in this context, since JavaScript dictionaries are really objects.

Peter Mortensen · Accepted Answer · 2020-07-15 00:40:52Z

104

You can use: Object.keys(obj)

Example:

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);

console.log(keys);

See reference below for browser support. It is supported in Firefox 4.20, Chrome 5, and Internet Explorer 9. Object.keys() contains a code snippet that you can add if Object.keys() is not supported in your browser.

edited Jul 15, 2020 at 0:40

Peter Mortensen

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answered Mar 2, 2012 at 19:26

Rob de la Cruz

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1 Comment

Simon_Weaver Over a year ago

I'm switching this to the accepted answer now. Only IE8 doesn't support it anymore (for which a polyfill is available developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…)

Peter Mortensen · Accepted Answer · 2020-07-15 00:43:19Z

16

Just a quick note. Be wary of using for..in 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.

edited Jul 15, 2020 at 0:43

Peter Mortensen

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answered Jun 15, 2010 at 2:34

Jesse

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Comments

Peter Mortensen · Accepted Answer · 2020-07-15 00:45:10Z

3

Simple jQuery way:

This is what I use:

DictionaryObj being the JavaScript dictionary object you want to go through. And value, key of course being the names of them in the dictionary.

$.each(DictionaryObj, function (key, value) {
    $("#storeDuplicationList")
        .append($("<li></li>")
        .attr("value", key)
        .text(value));
});

edited Jul 15, 2020 at 0:45

Peter Mortensen

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answered Feb 5, 2015 at 21:51

Exzile

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3 Comments

Simon_Weaver Over a year ago

this requires jQuery (which is fine) but you forgot to mention it :-)

Chris Over a year ago

@Exzile The argument names in your function definition are very confusing. It says at api.jquery.com/jquery.each that the callback is Function( String propertyName, Object valueOfProperty ). Your names imply the reverse.

Exzile Over a year ago

@Chris I will update that for you. Thanks for pointing that out.

Peter Mortensen · Accepted Answer · 2020-07-15 00:46:50Z

1

I am currently using Rob de la Cruz's reply:

Object.keys(obj)

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)) {
                keys.push(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.

edited Jul 15, 2020 at 0:46

Peter Mortensen

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answered May 16, 2016 at 13:16

Ivan

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