Question

Consider following JavaScript object

var planet = {
    id: 1001,
    Name: "Mars",
    faction: {
        factionId: 2,
        notification: function () {
            document.write("Mars notified");
        }
    },
    cities: [
        { locationId: 15, name: "Gladius" },
        { city: "MyPlanet", geo: "universal" }
    ]
}

When trying to read planet.cities in Chrome Debugger windows I see these additional items.

Are these part of JavaScript Runtime engine, and do different browsers handle them differently ?

Answer 1

Every JavaScript has a prototype object from which it inherits (EcmaScript §4.2.1). This is usually referred to as the “internal [[prototype]] property” (EcmaScript §8.6.2). That FF and Chrome make it public as the non-standard (and deprecated) __proto__ property is implementation-specific.

These additional properties you see are on Object.prototype (EcmaScript §15.2.4, MDN), from which all plain objects – and so your object literals – inherit. Again, the double-underscore-properties are implementation-specific and now deprecated by property descriptors (see Object.defineProperty at MDN).