Prototype methods, objects without proto

Setting or reading the prototype with obj.__proto__ is considered outdated and somewhat deprecated (moved to the so-called “Annex B” of the JavaScript standard, meant for browsers only).

The modern methods to get/set a prototype are:

Object.getPrototypeOf(obj) – returns the [[Prototype]] of obj.
Object.setPrototypeOf(obj, proto) – sets the [[Prototype]] of obj to proto.

The only usage of __proto__, that’s not frowned upon, is as a property when creating a new object: { __proto__: ... }.

Although, there’s a special method for this too:

Object.create(proto[, descriptors]) – creates an empty object with given proto as [[Prototype]] and optional property descriptors.

let animal = {
  eats: true
};

// create a new object with animal as a prototype
let rabbit = Object.create(animal); // same as {__proto__: animal}

alert(rabbit.eats); // true

alert(Object.getPrototypeOf(rabbit) === animal); // true

Object.setPrototypeOf(rabbit, {}); // change the prototype of rabbit to {}

The Object.create method is a bit more powerful, as it has an optional second argument: property descriptors.

We can provide additional properties to the new object there, like this:

let animal = {
  eats: true
};

let rabbit = Object.create(animal, {
  jumps: {
    value: true
  }
});

alert(rabbit.jumps); // true

The descriptors are in the same format as described in the chapter Property flags and descriptors.

We can use Object.create to perform an object cloning more powerful than copying properties in for..in:

let clone = Object.create(
  Object.getPrototypeOf(obj), Object.getOwnPropertyDescriptors(obj)
);

This call makes a truly exact copy of obj, including all properties: enumerable and non-enumerable, data properties and setters/getters – everything, and with the right [[Prototype]].

"Very plain" objects

As we know, objects can be used as associative arrays to store key/value pairs.

…But if we try to store user-provided keys in it (for instance, a user-entered dictionary), we can see an interesting glitch: all keys work fine except "__proto__".

Check out the example:

let obj = {};

let key = prompt("What's the key?", "__proto__");
obj[key] = "some value";

alert(obj[key]); // [object Object], not "some value"!

Here, if the user types in __proto__, the assignment in line 4 is ignored!

That could surely be surprising for a non-developer, but pretty understandable for us. The __proto__ property is special: it must be either an object or null. A string can not become a prototype. That’s why assigning a string to __proto__ is ignored.

But we didn’t intend to implement such behavior, right? We want to store key/value pairs, and the key named "__proto__" was not properly saved. So that’s a bug!

Here the consequences are not terrible. But in other cases we may be storing objects instead of strings in obj, and then the prototype will indeed be changed. As a result, the execution will go wrong in totally unexpected ways.

What’s worse – usually developers do not think about such possibility at all. That makes such bugs hard to notice and even turn them into vulnerabilities, especially when JavaScript is used on server-side.

Unexpected things also may happen when assigning to obj.toString, as it’s a built-in object method.

How can we avoid this problem?

First, we can just switch to using Map for storage instead of plain objects, then everything’s fine:

let map = new Map();

let key = prompt("What's the key?", "__proto__");
map.set(key, "some value");

alert(map.get(key)); // "some value" (as intended)

…But Object syntax is often more appealing, as it’s more concise.

Fortunately, we can use objects, because language creators gave thought to that problem long ago.

As we know, __proto__ is not a property of an object, but an accessor property of Object.prototype:

So, if obj.__proto__ is read or set, the corresponding getter/setter is called from its prototype, and it gets/sets [[Prototype]].

As it was said in the beginning of this tutorial section: __proto__ is a way to access [[Prototype]], it is not [[Prototype]] itself.

Now, if we intend to use an object as an associative array and be free of such problems, we can do it with a little trick:

let obj = Object.create(null);
// or: obj = { __proto__: null }

let key = prompt("What's the key?", "__proto__");
obj[key] = "some value";

alert(obj[key]); // "some value"

Object.create(null) creates an empty object without a prototype ([[Prototype]] is null):

So, there is no inherited getter/setter for __proto__. Now it is processed as a regular data property, so the example above works right.

We can call such objects “very plain” or “pure dictionary” objects, because they are even simpler than the regular plain object {...}.

A downside is that such objects lack any built-in object methods, e.g. toString:

let obj = Object.create(null);

alert(obj); // Error (no toString)

…But that’s usually fine for associative arrays.

Note that most object-related methods are Object.something(...), like Object.keys(obj) – they are not in the prototype, so they will keep working on such objects:

let chineseDictionary = Object.create(null);
chineseDictionary.hello = "你好";
chineseDictionary.bye = "再见";

alert(Object.keys(chineseDictionary)); // hello,bye

"Very plain" objects​

"Very plain" objects