Javascript: Getting the first index of an object

Question

Consider:

var object = {
    foo:{},
    bar:{},
    baz:{}
}

How would I ...

var first=object[0];
console.log(first);

Obviously that doesn't work because the first index is named "foo", not 0.

console.log(object['foo']);

Works, but I don't know it's named foo. It could be named anything. I just want the first :)

Thanks!

Answer 1

There is no way to get the first element, seeing as "hashes" (objects) in JavaScript have unordered properties. Your best bet is to store the keys in an array:

var keys = ["foo", "bar", "baz"];

Then use that to get the proper value:

object[keys[0]]

Answer 2

I had the same problem yesterday. I solved it like this:

var obj = {
        foo:{},
        bar:{},
        baz:{}
    },
   first = null,
   key = null;
for (var key in obj) {
    first = obj[key];
    if(typeof(first) !== 'function') {
        break;
    }
}
// first is the first enumerated property, and key it's corresponding key.

Not the most elegant solution, and I am pretty sure that it may yield different results in different browsers (i.e. the specs says that enumeration is not required to enumerate the properties in the same order as they were defined). However, I only had a single property in my object so that was a non-issue. I just needed the first key.

Answer 3

You could do something like this:

var object = {
    foo:{a:'first'},
    bar:{},
    baz:{}
}


function getAttributeByIndex(obj, index){
  var i = 0;
  for (var attr in obj){
    if (index === i){
      return obj[attr];
    }
    i++;
  }
  return null;
}


var first = getAttributeByIndex(object, 0); // returns the value of the
                                            // first (0 index) attribute
                                            // of the object ( {a:'first'} )

Answer 4

Thanks for the replies.

I've done this:

var first=false;
for(i in obj){
 if(!first) first=i;
}

I'll consider some of the other solutions if this bites me later :)

Thanks again.

Answer 5

If the order of the objects is significant, you should revise your JSON schema to store the objects in an array:

[
    {"name":"foo", ...},
    {"name":"bar", ...},
    {"name":"baz", ...}
]

or maybe:

[
    ["foo", {}],
    ["bar", {}],
    ["baz", {}]
]

As Ben Alpert points out, properties of Javascript objects are unordered, and your code is broken if you expect them to enumerate in the same order that they are specified in the object literal—there is no "first" property.

Answer 6

they're not really ordered, but you can do:

var first;
for (var i in obj) {
    if (obj.hasOwnProperty(i) && typeof(first) !== 'function') {
        first = obj[i];
        break;
    }
}

the .hasOwnProperty() is important to ignore prototyped objects.

Answer 7

If you want something concise try:

for (first in obj) break;

alert(first);