Get the first key name of a JavaScript object

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.

In Javascript you can do the following:

Object.keys(ahash)[0];

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"

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.

If you decide to use Underscore.js you better do

_.values(ahash)[0]

to get value, or

_.keys(ahash)[0]

to get key.

Try this:

for (var firstKey in ahash) break;

alert(firstKey);  // 'one'

With Underscore.js, you could do

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

It will return [1,2,3]

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);

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.