How to access the first property of a Javascript object?

Question

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

Answer 1

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

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

Answer 2

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.

Answer 3

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).

Answer 4

A one-rule version:

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

Answer 5

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"}

Answer 6

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])