JavaScript: Get first and only property name of object

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

Though my answer is downvoted, it's still worth to know that there is no such thing as order of keys in javascript object. Therefore, in theory, any code build on iterating values can be inconsistent. One approach could be creating an object and to define setter which actually provides counting, ordering and so on, and provide some methods to access this fields. This could be done in modern browsers.

So, to answer you question, in general you approach is still most closs-browser. You can iterate using lodash or any other modern framework wich will hide "hasOwnProperty" complexity from you. As of August'15 Object.keys can be accepted as cross-browser and universal. After all IE8 happened years ago. Still there are some cases when you just don't wont store all set of keys in array. But I'd go with Object.keys - it's more flexible compared to iteration.

  var foo = {bar: 1};
  console.log(Object.keys(foo).toString());

which will print the string

"bar"

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'