JavaScript: Get first and only property name of object

Question

If I want to enumerate the properties of an object and want to ignore prototypes, I would use:

var instance = { ... };  for (var prop in instance) {     if (instance.hasOwnProperty(prop)) {         ...      } } 

What if instance only has one property, and I want to get that property name? Is there an easier way than doing this:

var instance = { id: "foobar" };  var singleMember = (function() {     for (var prop in instance) {         if (instance.hasOwnProperty(prop)) {             return prop;         }     } })(); 

Answer 1

Maybe Object.keys can work for you. If its length returns 1, you can use yourObject[Object.keys[0]] to get the only property of the object. The MDN-link also shows a custom function for use in environments without the keys method¹. Code like this:

var obj = {foo:'bar'},      kyz = Object.keys(obj); if (kyz.length === 1){    alert(obj[kyz[0]]); //=> 'bar' } else {   /* loop through obj */ } 

¹ Some older browsers don't support Object.keys. The MDN link supplies code to to make it work in these browsers too. See header Compatibility in the aforementioned MDN page

Answer 2

Shortest form:

instance[Object.keys(instance)[0]]; 

ES6+ function:

let first = v => v[Object.keys(v)[0]]; 

Use the function:

first({a:'first', b:'second'}) // return 'first'