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The "prototype" property is widely used by the core of JavaScript itself. All built-in constructor functions use it.

We’ll see how it is for plain objects first, and then for more complex ones.


Let’s say we output an empty object:

let obj = {};
alert( obj ); // "[object Object]" ?

Where’s the code that generates the string "[object Object]"? That’s a built-in toString method, but where is it? The obj is empty!

…But the short notation obj = {} is the same as obj = new Object(), where Object – is a built-in object constructor function. And that function has Object.prototype that references a huge object with toString and other functions.

Like this (all that is built-in):

When new Object() is called (or a literal object {...} is created), the [[Prototype]] of it is set to Object.prototype by the rule that we’ve discussed in the previous chapter:

Afterwards when obj.toString() is called – the method is taken from Object.prototype.

We can check it like this:

let obj = {};

alert(obj.__proto__ === Object.prototype); // true
// obj.toString === obj.__proto__.toString == Object.prototype.toString

Please note that there is no additional [[Prototype]] in the chain above Object.prototype:

alert(Object.prototype.__proto__); // null

Other built-in prototypes

Other built-in objects such as Array, Date, Function and others also keep methods in prototypes.

For instance, when we create an array [1, 2, 3], the default new Array() constructor is used internally. So the array data is written into the new object, and Array.prototype becomes its prototype and provides methods. That’s very memory-efficient.

By specification, all built-in prototypes have Object.prototype on the top. Sometimes people say that “everything inherits from objects”.

Here’s the overall picture (for 3 built-ins to fit):

Let’s check the prototypes manually:

let arr = [1, 2, 3];

// it inherits from Array.prototype?
alert( arr.__proto__ === Array.prototype ); // true

// then from Object.prototype?
alert( arr.__proto__.__proto__ === Object.prototype ); // true

// and null on the top.
alert( arr.__proto__.__proto__.__proto__ ); // null

Some methods in prototypes may overlap, for instance, Array.prototype has its own toString that lists comma-delimited elements:

let arr = [1, 2, 3]
alert(arr); // 1,2,3 <-- the result of Array.prototype.toString

As we’ve seen before, Object.prototype has toString as well, but Array.prototype is closer in the chain, so the array variant is used.

In-browser tools like Chrome developer console also show inheritance (may need to use console.dir for built-in objects):

Other built-in objects also work the same way. Even functions. They are objects of a built-in Function constructor, and their methods: call/apply and others are taken from Function.prototype. Functions have their own toString too.

function f() {}

alert(f.__proto__ == Function.prototype); // true
alert(f.__proto__.__proto__ == Object.prototype); // true, inherit from objects


The most intricate thing happens with strings, numbers and booleans.

As we remember, they are not objects. But if we try to access their properties, then temporary wrapper objects are created using built-in constructors String, Number, Boolean, they provide the methods and disappear.

These objects are created invisibly to us and most engines optimize them out, but the specification describes it exactly this way. Methods of these objects also reside in prototypes, available as String.prototype, Number.prototype and Boolean.prototype.

Values null and undefined have no object wrappers

Special values null and undefined stand apart. They have no object wrappers, so methods and properties are not available for them. And there are no corresponding prototypes too.

Changing native prototypes

Native prototypes can be modified. For instance, if we add a method to String.prototype, it becomes available to all strings:

String.prototype.show = function() {

"BOOM!".show(); // BOOM!

During the process of development we may have ideas which new built-in methods we’d like to have. And there may be a slight temptation to add them to native prototypes. But that is generally a bad idea.

Prototypes are global, so it’s easy to get a conflict. If two libraries add a method String.prototype.show, then one of them overwrites the other one.

In modern programming, there is only one case when modifying native prototypes is approved. That’s polyfills. In other words, if there’s a method in JavaScript specification that is not yet supported by our JavaScript engine (or any of those that we want to support), then may implement it manually and populate the built-in prototype with it.

For instance:

if (!String.prototype.repeat) { // if there's no such method
  // add it to the prototype

  String.prototype.repeat = function(n) {
    // repeat the string n times

    // actually, the code should be more complex than that,
    // throw errors for negative values of "n"
    // the full algorithm is in the specification
    return new Array(n + 1).join(this);

alert( "La".repeat(3) ); // LaLaLa

Borrowing from prototypes

In the chapter Decorators and forwarding, call/apply we talked about method borrowing:

function showArgs() {
  // borrow join from array and call in the context of arguments
  alert( [].join.call(arguments, " - ") );

showArgs("John", "Pete", "Alice"); // John - Pete - Alice

Because join resides in Array.prototype, we can call it from there directly and rewrite it as:

function showArgs() {
  alert( Array.prototype.join.call(arguments, " - ") );

That’s more efficient, because it avoids the creation of an extra array object []. On the other hand, it is longer to write.


  • All built-in objects follow the same pattern:
    • The methods are stored in the prototype (Array.prototype, Object.prototype, Date.prototype etc).
    • The object itself stores only the data (array items, object properties, the date).
  • Primitives also store methods in prototypes of wrapper objects: Number.prototype, String.prototype, Boolean.prototype. There are no wrapper objects only for undefined and null.
  • Built-in prototypes can be modified or populated with new methods. But it’s not recommended to change them. Probably the only allowable cause is when we add-in a new standard, but not yet supported by the engine JavaScript method.


importance: 5

Add to the prototype of all functions the method defer(ms), that runs the function after ms milliseconds.

After you do it, such code should work:

function f() {

f.defer(1000); // shows "Hello!" after 1 second
Function.prototype.defer = function(ms) {
  setTimeout(this, ms);

function f() {

f.defer(1000); // shows "Hello!" after 1 sec
importance: 4

Add to the prototype of all functions the method defer(ms), that returns a wrapper, delaying the call by ms milliseconds.

Here’s an example of how it should work:

function f(a, b) {
  alert( a + b );

f.defer(1000)(1, 2); // shows 3 after 1 second

Please note that the arguments should be passed to the original function.

Function.prototype.defer = function(ms) {
  let f = this;
  return function(...args) {
    setTimeout(() => f.apply(this, args), ms);

// check it
function f(a, b) {
  alert( a + b );

f.defer(1000)(1, 2); // shows 3 after 1 sec
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