Object.keys()

var arr = ["a", "b", "c"];
console.log(Object.keys(arr)); // : ['0', '1', '2']

//  
var obj = { 0: "a", 1: "b", 2: "c" };
console.log(Object.keys(obj)); // : ['0', '1', '2']

//      
var an_obj = { 100: "a", 2: "b", 7: "c" };
console.log(Object.keys(an_obj)); // : ['2', '7', '100']

//  getFoo    
var my_obj = Object.create(
  {},
  {
    getFoo: {
      value: function () {
        return this.foo;
      },
    },
  },
);
my_obj.foo = 1;

console.log(Object.keys(my_obj)); // : ['foo']

> Object.keys('foo')
TypeError: 'foo' is not an object //  ES5

> Object.keys('foo')
['0', '1', '2']                   //  ES2015

// From https://developer.mozilla.org/ru/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
if (!Object.keys) {
  Object.keys = (function () {
    "use strict";
    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 = [],
        prop,
        i;

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

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