Get the first key name of a JavaScript object

Fine details and edge cases

Object.keys() and key order

Object.keys() fetches keys based on the order in which they were created. Numerical keys, however, are sorted:

let obj = { 100: 'a', 2: 'b', 7: 'c' };
console.log(Object.keys(obj)); // ["2", "7", "100"], yep, numerical order


for...in, the old-fashioned way

Though a bit "retro", a for...in loop can also fetch keys, but might not keep the Object.keys() order:

for (let key in obj) {
    console.log(key); // Might not match Object.keys() result
    break; // Gotta escape this loop ASAP!
}


"Is Object.keys() even allowed here?"

Checking compatibility is a defensive programming must:

if (Object.keys) {
    console.log(Object.keys(obj)[0]);
} else {
    // Sorry, we don't support this feature?
}


This way, your code won't freak out in older JS contexts.

That "modern" library, Underscore.js

If you're already a fan of Underscore.js, it's got us covered:

console.log(_.keys(obj)[0]); // Yep, Underscore.js is cool like that


Extracting all the juicy details

More than just keys

Object.keys(obj)[0] gets the first key, Object.values(obj)[0] the first value, and Object.entries(obj)[0] gets both as a pair:

console.log(Object.values(obj)[0]); // First value, check!
console.log(Object.entries(obj)[0]); // First pair, another check!


JavaScript objects: predictably unpredictable

Objects in JavaScript are quite mysterious - they maintain no guaranteed key order. You're in the Matrix now!

When you need the order guaranteed

If the key order is of utmost importance to you, use a Map:

let orderedMap = new Map();
orderedMap.set('firstKey', 'value');
orderedMap.set('secondKey', 'value');
console.log([...orderedMap.keys()][0]); // Guaranteed to be 'firstKey', pinky promise!


One loop to rule them all

Controlling the for...in loop with break gets us the first property without extra steps:

for (let key in obj) {
    console.log(key); // firstKey, as easy as 1-2-3!
    break;
}


Minding those not-so-obvious gotchas

JavaScript engines: agreeing to disagree

The way property order is handled may differ between JS engines, like heated debates in pizza toppings:

- Chrome/V8: Maintains insertion order, like cheese on top
- Firefox/SpiderMonkey: Generally the same, but with some spicy differences
- Node.js: Mostly supportive like V8, but look out for version disparities

Obj-, obj-, objections!

Object.keys() is shy around non-enumerable parameters. Say hi to Reflect.ownKeys():

let obj = Object.create({}, {
    getFirst: { value: function() { return 'firstKey'; } }
});
console.log(Reflect.ownKeys(obj)[0]); // Hola, non-enumerable keys!


The Babel factor

Using transpilers like Babel? Better check if it doesn't jumble up your key order:

- Babel could reshuffle keys for compatibility.
- Examine transpiled code or configure Babel to keep key order sacred.

Useful resources

1. Object.keys() - JavaScript | MDN — nitty-gritty details on getting property names.
2. ECMAScript® 2024 Language Specification — level up your JS knowledge and stay trendy!
3. Iterating Over JavaScript Objects - Looping Techniques | DigitalOcean — different ways of object iterations unpacked.
4. Reflect.ownKeys() - JavaScript | MDN — how to see even the non-enumerable ones!

Sign-off

Practice, polish skills, and keep asking questions! Happy coding!👩‍💻

Oh, and if this answer helped you save the world (or your code), why not give a vote? 😉