Use Object.keys:

var foo = {
  'alpha': 'puffin',
  'beta': 'beagle'
};

var keys = Object.keys(foo);
console.log(keys) // ['alpha', 'beta'] 
// (or maybe some other order, keys are unordered).

This is an ES5 feature. This means it works in all modern browsers but will not work in legacy browsers.

The ES5-shim has a implementation of Object.keys you can steal

Use map() and return the first key the object. You can get keys using Object.keys()

let data = [{"ja":"大阪市"},{"en":"Osaka"}]
let res = data.map(x => Object.keys(x)[0]);
console.log(res)

If you don't want to use [0] use flatMap()

let data = [{"ja":"大阪市"},{"en":"Osaka"}]
let res = data.flatMap(x => Object.keys(x));
console.log(res)

Note: The second method will also get the other properties other than first. For example

[{"ja":"大阪市","other":"value"},{"en":"Osaka"}] //["ja","other","en"];

Merge to a single object by spreading into Object.assign(), and then get the keys:

var obj = [{"a":1},{"b":2},{"c":3}];

const result = Object.keys(Object.assign({}, ...obj));
console.log(result);

Or use Array.flatMap() with Object.keys():

var obj = [{"a":1},{"b":2},{"c":3}];

const result = obj.flatMap(Object.keys);
console.log(result);

If each object will definitely have only one enumerable property, then you can use Object.keys(someArray[i])[0] to get that property's name in your loop. Object.keys returns an array of the object's own, enumerable property names, and [0] gets the first entry from it. (And of course, someArray[i][theName] will give you the value of that property.)

Example:

var objX = {
  x: "ecks"
};
var objY = {
  y: "why"
};
var objZ = {
  z: "zee"
};
var someArray = [];
someArray.push(objX, objY, objZ);

for (var i = 0; i < someArray.length; i++) {
  var arrayEntry = someArray[i];
  var name = Object.keys(arrayEntry)[0];
  console.log(name + " is " + arrayEntry[name]);
}

Here is a version where you could have many sub key for each of your 3 object.

var arr = [
  {
    "a": {
      "property1": "false"
    }
  },
  {
    "b": {
      "property1": "false",
      "property2": "truthy"
    }
  },
  {
    "c": {
      "propertycheck": "required"
    }
  }
]

var arr = arr.filter(function(elem){   //Filter on each object in the array
  var checkKey = true;                 //checkKey will be the return value of filter
  Object.keys(elem).forEach(function(key){  //For each key of elem
    if("property1" in elem[key]){           //If value of key (object) own "property1"
      checkKey = false                      //You want to remove the object from table so return false to filter
    }
  })
  return checkKey
})

–

Reference to Array.filter https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/Array/filter

You can take Array.map(). This method returns an array with the elements from the callback returned. It expect that all elements return something. If not set, undefined will be returned.

var students = [{
    name: 'Nick',
    achievements: 158,
    points: 14730
}, {
    name: 'Jordan',
    achievements: '175',
    points: '16375'
}, {
    name: 'Ramon',
    achievements: '55',
    points: '2025'
}];
var nameArray = students.map(function (el) { return el.name; });
document.getElementById('out').innerHTML = JSON.stringify(nameArray, null, 4);

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