[javascript] How to access the first property of a Javascript object?

Is there an elegant way to access the first property of an object...

1. where you don't know the name of your properties
2. without using a loop like for .. in or jQuery's $.each

For example, I need to access foo1 object without knowing the name of foo1:

var example = {
    foo1: { /* stuff1 */},
    foo2: { /* stuff2 */},
    foo3: { /* stuff3 */}
};

You can also do Object.values(example)[0].

if someone prefers array destructuring

const [firstKey] = Object.keys(object);

No. An object literal, as defined by MDC is:

a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}).

Therefore an object literal is not an array, and you can only access the properties using their explicit name or a for loop using the in keyword.

we can also do with this approch.

var example = {
  foo1: { /* stuff1 */},
  foo2: { /* stuff2 */},
  foo3: { /* stuff3 */}
}; 
Object.entries(example)[0][1];

To get the first key name in the object you can use:

var obj = { first: 'someVal' };
Object.keys(obj)[0]; //returns 'first'

Returns a string, so you cant access nested objects if there were, like:

var obj = { first: { someVal : { id : 1} }; Here with that solution you can't access id.

The best solution if you want to get the actual object is using lodash like:

obj[_.first(_.keys(obj))].id

To return the value of the first key, (if you don't know exactly the first key name):

var obj = { first: 'someVal' };
obj[Object.keys(obj)[0]]; //returns 'someVal'

if you know the key name just use:

obj.first

or

obj['first']

If you need to access "the first property of an object", it might mean that there is something wrong with your logic. The order of an object's properties should not matter.

There isn't a "first" property. Object keys are unordered.

If you loop over them with for (var foo in bar) you will get them in some order, but it may change in future (especially if you add or remove other keys).

You can use Object.prototype.keys which returns all the keys of an object in the same order. So if you want the first object just get that array and use the first element as desired key.

const o = { "key1": "value1", "key2": "value2"};
const idx = 0; // add the index for which you want value
var key = Object.keys(o)[idx];
value = o[key]
console.log(key,value); // key2 value2

Use Object.keys to get an array of the properties on an object. Example:

var example = {
    foo1: { /* stuff1 */},
    foo2: { /* stuff2 */},
    foo3: { /* stuff3 */}
};

var keys = Object.keys(example); // => ["foo1", "foo2", "foo3"] (Note: the order here is not reliable)

Documentation and cross-browser shim provided here. An example of its use can be found in another one of my answers here.

Edit: for clarity, I just want to echo what was correctly stated in other answers: the key order in javascript objects is undefined.

Use an array instead of an object (square brackets).

var example = [ {/* stuff1 */}, { /* stuff2 */}, { /* stuff3 */}];
var fist = example[0];

Note that you lose the 'foo' identifiers. But you could add a name property to the contained objects:

var example = [ 
  {name: 'foo1', /* stuff1 */},
  {name: 'foo2', /* stuff2 */},
  {name: 'foo3', /* stuff3 */}
];
var whatWasFirst = example[0].name;

Try the for … in loop and break after the first iteration:

for (var prop in object) {
    // object[prop]
    break;
}

Solution with lodash library:

_.find(example) // => {name: "foo1"}

but there is no guarantee of the object properties internal storage order because it depends on javascript VM implementation.

This has been covered here before.

The concept of first does not apply to object properties, and the order of a for...in loop is not guaranteed by the specs, however in practice it is reliably FIFO except critically for chrome (bug report). Make your decisions accordingly.

A one-rule version:

var val = example[function() { for (var k in example) return k }()];

The top answer could generate the whole array and then capture from the list. Here is an another effective shortcut

var obj = { first: 'someVal' };
Object.entries(obj)[0][1] // someVal

I don't recommend you to use Object.keys since its not supported in old IE versions. But if you really need that, you could use the code above to guarantee the back compatibility:

if (!Object.keys) {
Object.keys = (function () {
var hasOwnProperty = Object.prototype.hasOwnProperty,
    hasDontEnumBug = !({toString: null}).propertyIsEnumerable('toString'),
    dontEnums = [
      'toString',
      'toLocaleString',
      'valueOf',
      'hasOwnProperty',
      'isPrototypeOf',
      'propertyIsEnumerable',
      'constructor'
    ],
    dontEnumsLength = dontEnums.length;

return function (obj) {
  if (typeof obj !== 'object' && typeof obj !== 'function' || obj === null) throw new TypeError('Object.keys called on non-object');

  var result = [];

  for (var prop in obj) {
    if (hasOwnProperty.call(obj, prop)) result.push(prop);
  }

  if (hasDontEnumBug) {
    for (var i=0; i < dontEnumsLength; i++) {
      if (hasOwnProperty.call(obj, dontEnums[i])) result.push(dontEnums[i]);
    }
  }
  return result;
}})()};

Feature Firefox (Gecko)4 (2.0) Chrome 5 Internet Explorer 9 Opera 12 Safari 5

More info: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Object/keys

But if you only need the first one, we could arrange a shorter solution like:

var data = {"key1":"123","key2":"456"};
var first = {};
for(key in data){
    if(data.hasOwnProperty(key)){
        first.key = key;
        first.content =  data[key];
        break;
    }
}
console.log(first); // {key:"key",content:"123"}

Any reason not to do this?

> example.map(x => x.name);

(3) ["foo1", "foo2", "foo3"]

Here is a cleaner way of getting the first key:

var object = {
    foo1: 'value of the first property "foo1"',
    foo2: { /* stuff2 */},
    foo3: { /* stuff3 */}
};

let [firstKey] = Object.keys(object)

console.log(firstKey)
console.log(object[firstKey])