Get the first key name of a JavaScript object

Question

Let's assume we have the following JavaScript object:

ahash = {"one": [1,2,3], "two": [4,5,6]}

Is there a function that returns the first key name for the given object?

From the example above I want to get one as a result of that function.

Do you want the value of the first key or the first key name of the object? I'm confused. — Diego Unanue, Jun 12 '19 at 16:32

score 439 · Accepted Answer · edited Jul 20 '17 at 09:12

439

In Javascript you can do the following:

Object.keys(ahash)[0];

edited Jul 20 '17 at 09:12

webmaster

1,658
21
25

answered Jun 25 '12 at 08:29

Pickels

30,764
23
107
174

11

Works on browsers as well – Alon Amir Jul 10 '12 at 20:08
4

This is a sick piece of code. I was expecting to have to clutter everything up with a silly for key in dict: – Jake Jul 29 '12 at 05:10
2

Much cleaner than the accepted answer, thanks! – Piotrek Okoński Nov 03 '12 at 22:37
2

this won't work on older browsers which do not support JS 1.8.5 – vsync Dec 14 '12 at 06:27
16

I'd like to point out that `for (key in dict)` is likely to execute **significantly** faster, O(1) rather than O(n). It's not silly at all. – Steven Lu Jan 08 '13 at 06:49
5

to future me... note that there is also a "value" method on Object, i.e. Object.value, if you instead want the value of the first key – engineerDave Feb 20 '18 at 23:37
8

@engineerDave, this is future you. We've changed our stackOverflow account handle and we've made a whole bunch of money by inventing time travel, but that is beside the point. I just wanted to say, "Thanks", this little note helped me out. – isaacdre Oct 11 '18 at 00:54
I get "ReferenceError: ahash is not defined (line 2, col 25)" – cheesus Oct 01 '20 at 07:44
@cheesus: ahash is the name of your javascript object – Cogicero Feb 10 '21 at 06:22

score 37 · Answer 2 · answered Jul 21 '10 at 11:03

37

There's no such thing as the "first" key in a hash (Javascript calls them objects). They are fundamentally unordered. Do you mean just choose any single key:

for (var k in ahash) {
    break
}

// k is a key in ahash.

answered Jul 21 '10 at 11:03

Ned Batchelder

323,515
67
518
625

7

should check if obj.hasOwnProperty(prop) also I would add – vsync Dec 14 '12 at 06:40
and how to get the rest of other ? – Santosh Jun 03 '17 at 10:21
I think it is not recommended solution. This uses bad feature named 'variable hoisting'. And ES6 has recommended not to use 'var' in for loop – 刘宇翔 Aug 01 '17 at 03:53
1

The EMCA script specification makes dictionaries *ordered*. They are "ordered dictionaries". The order of enumeration is the same as the order in which the dictionary items are added. This is useful. in Python, for example, OrderedDictionary is a specific class, where Dictionary will returned indeterminate ordering of keys in dictionaries on enumeration – Jay Day Zee Apr 12 '18 at 16:16
@JayDayZee ECMAScript doesn't have a thing called "dictionary", right? This answer (https://stackoverflow.com/a/5525820/14343) seems to indicate that objects are unordered, and Maps are ordered. Does that mesh with your understanding? – Ned Batchelder Apr 12 '18 at 23:28
so javascript `objects` are quite complex things, that implement: a key value store; a separate numbered value store (array); prototyping. the `key value store` part is a "dictionary". * Enumerating keys (e.g. `Object.keys(someRandomObject)` returns the keys *not* an arbitrary order, *not* a random order and *not* some other specific ordering, but in the same order as they were added. * in Python, your answer would be correct, as there is no "first" key. (i would guess that the "ordering" of keys returned in python has something to do with the key hash) – Jay Day Zee Apr 14 '18 at 11:57

Ilya Degtyarenko · Answer 3 · 2020-03-13T12:59:18.753

26

You can query the content of an object, per its array position.
For instance:

 let obj = {plainKey: 'plain value'};

 let firstKey = Object.keys(obj)[0]; // "plainKey"
 let firstValue = Object.values(obj)[0]; // "plain value"

 /* or */

 let [key, value] = Object.entries(obj)[0]; // ["plainKey", "plain value"]

 console.log(key); // "plainKey"
 console.log(value); // "plain value"

edited Mar 13 '20 at 12:59

answered Jul 10 '19 at 11:25

Ilya Degtyarenko

782
6
13

1

It does not seem to answer the question – xiawi Jul 10 '19 at 11:57
An example using an explicitly constructed object such as the question used would provide a better answer. This answer is basically correct, but it requires too subtle a knowledge of Javascript to easily decipher the meaning. – CXJ Jul 10 '19 at 21:19
Thanks for the remarks, changed to more understandable code. – Ilya Degtyarenko Jul 11 '19 at 12:22

score 18 · Answer 4 · answered Dec 08 '13 at 16:07

18

If you decide to use Underscore.js you better do

_.values(ahash)[0]

to get value, or

_.keys(ahash)[0]

to get key.

answered Dec 08 '13 at 16:07

Bunyk

6,415
4
36
68

score 15 · Answer 5 · answered Jul 21 '10 at 11:05

15

Try this:

for (var firstKey in ahash) break;

alert(firstKey);  // 'one'

answered Jul 21 '10 at 11:05

nickf

499,078
194
614
709

Note that you must use the `var` keyword; `let` or `const` will not suffice. – David Callanan Jun 01 '20 at 14:31

score 8 · Answer 6 · answered Aug 08 '13 at 17:48

8

With Underscore.js, you could do

_.find( {"one": [1,2,3], "two": [4,5,6]} )

It will return [1,2,3]

answered Aug 08 '13 at 17:48

mjlescano

759
8
13

1

Also true for lodash :) – crackmigg Jun 06 '16 at 09:31
1

there's also a `findKey` method which returns the key (instead of the value) – XoXo Jul 20 '17 at 17:04

score 3 · Answer 7 · answered Jun 25 '19 at 15:03

I use Lodash for defensive coding reasons.

In particular, there are cases where I do not know if there will or will not be any properties in the object I'm trying to get the key for.

A "fully defensive" approach with Lodash would use both keys as well as get:

const firstKey = _.get(_.keys(ahash), 0);

score 1 · Answer 8 · answered Jul 21 '10 at 13:09

you can put your elements into an array and hash at the same time.

var value = [1,2,3];
ahash = {"one": value};
array.push(value);

array can be used to get values by their order and hash could be used to get values by their key. just be be carryfull when you remove and add elements.

Get the first key name of a JavaScript object

8 Answers8

Linked

Related