43

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;
        }
    }
})();
FoobarisMaximus
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  • I think you have the right idea, just wrap the logic in a utility function, then it will be easier to use. ;) – Peter Jul 20 '11 at 18:23
  • It's worth mentioning that the ES5 spec does not define the order of properties for for-in or Object.keys. It simply says that if there is an order, both for-in and Object.keys must use the same one. Although many browsers use the order the properties were defined, but I would not depend on this. – Ben Mar 21 '14 at 21:32

7 Answers7

53

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

KooiInc
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  • @Kooilnc yourObject[Objects.keys[0]] should be yourObject[Object.keys[0]] , you have an extra s there – German Sep 17 '19 at 10:32
18

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'
Isaac Han
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4
  var foo = {bar: 1};
  console.log(Object.keys(foo).toString());

which will print the string

"bar"
mewc
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2

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.

shabunc
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1

Unfortunately, there is no, "list properties" function built in, and there certainly isn't a "getFirstProperty" (especially since there is no guarantee that any property will consistently be "first").

I think you're better off writing a function like this one:

/**
 * A means to get all of the keys of a JSON-style object.
 * @param obj The object to iterate
 * @param count maximum length of returned list (defaults to Infinity).
 */
function getProperties( obj, count )
{
    if( isNaN( count ) ) count = Infinity
    var keys = []
    for( var it in obj )
    {
        if( keys.length > count ) break;
        keys.push( it );
    }
    return keys;
}

Then, you could access the name though:

instance = {"foo":"bar"}
// String() on an array of < 2 length returns the first value as a string
// or "" if there are no values.
var prop = String(getProperties(instance, 1));
cwallenpoole
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0

This is an old post, but I ended up writing the following helper function based on Object.keys().

It returns the key and value of the first property.

getFirstPropertyKeyAndValue(sourceObject) {
    var result = null;
    var ownProperties = Object.keys(sourceObject);
    if (ownProperties.length > 0) {
      if (ownProperties.length > 1) {
        console.warn('Getting first property of an object containing more than 1 own property may result in unexpected results. Ordering is not ensured.', sourceObject);
      }
      var firstPropertyName = ownProperties[0];
      result = {key: firstPropertyName, value: sourceObject[firstPropertyName]};
    }
    return result;
  }
Andrew
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0

Answers in here all good, and with the caveat that the order may be unreliable (although in practice it seems the order the properties are set tends to stay that way), this quick and dirty method also works:

var obj = {foo: 1, bar: 2};
for(var key in obj) {
  //you could use key here if you like
  break;
}
//key now contains your first key

or a shorter version should also do it:

for(var key in obj) break;
//key now contains your first key
james-geldart
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