Methods of primitives

JavaScript allows to work with primitives (strings, numbers etc) as if they were objects. They also have methods and such. Of course, primitives are not objects (and here we plan to make it even more clear), but can be used like them.

[cut]

Let's formulate the key distinction between primitives and objects.

A primitive: Is a value of a primitive type. There are 6 primitive types: string, number, boolean, symbol, null and undefined.
An object: Is capable of storing multiple values as properties. Can be created with {}, for instance: {name: "John", age: 30}. There are other kinds of objects in JavaScript, e.g. functions are objects.

One of the best thing about objects is that we can store a function as one of properties:

let john = { 
  name: "John",
  sayHi: function() {
    alert("Hi buddy!");
  }
};

john.sayHi(); // Hi buddy!

So, here we've made an object john with the method sayHi.

There exist many built-in objects, including those that work with dates, errors, HTML elements etc. They have different properties and methods.

But features come at a price!

Objects are "heavier" than primitives. They require additional resources to support the internal machinery. But properties and methods are useful in programming, Javascript engines try to optimize them, so the price is usually fair.

A primitive as an object

Here's the paradox faced by the creator of JavaScript:

There are many things one would want to do with a primitive like a string or a number. Could be great to access them as methods.
Primitives must be as fast and lightweight as possible.

The solution looks a little bit awkward, but here it is.

Primitives are still primitive. A single value, as desired.
The language allows to access methods and properties of strings, numbers, booleans and symbols.
When it happens, a special "object wrapper" is created that provides the functionality and then is destroyed.

The "object wrappers" are different for each primitive type and are named specifically: String, Number, Boolean and Symbol. Thus they provide different sets of methods.

For instance, there exists a method str.toUpperCase() that returns the capitalized string.

Here's how it works:

let str = "Hello";

alert( str.toUpperCase() ); // HELLO

Simple, right? And here's what actually happens in str.toUpperCase():

The string str is a primitive. So in the moment of accessing its property a special object is created that both knows the value of the string and has useful methods, like toUpperCase().
That method runs and returns a new string (shown by alert).
The special object is destroyed, leaving the primitive str alone.

So, primitives can provide methods, but they still remain lightweight.

Of course, a JavaScript engine highly optimizes that process. Internally it may skip the creation of the extra object at all. But it must adhere to the specification and behave as if it creates one.

A number has methods of it's own, for instance, toFixed(n) rounds the number to the given precision:

let n = 1.23456;

alert( n.toFixed(2) ); // 1.23

We'll see more specific methods in chapters info:number and info:string.

Special primitives `null` and `undefined` are exceptions. They have no corresponding "wrapper objects" and provide no methods. In a sense, they are "the most primitive".

An attempt to access a property of such value would give an error:

```js run
alert(null.test); // error

Summary

Primitives except null and undefined provide many helpful methods. We plan to study those in the next chapters.
Formally, these methods work via temporary objects, but JavaScript engines are very well tuned to optimize that internally, so they are not expensive to call.

4 KiB Raw Blame History

Methods of primitives

A primitive as an object

Summary

4 KiB

Raw Blame History