Introduction to the URL Object in JavaScript

Introduction to the URL Object in JavaScript

Introduction to JavaScript URL Object, You'll learn using the URL Object in JavaScript and learn everything there is to know about the URL object

Manipulating and extracting parts from a URL is a pain if we have to write all the code ourselves. Fortunately, most browsers have the URL object built into their standard library. Now, we can pass in the URL, as a string, to the URL constructor and create an instance of it. Then, we can use the handy value properties and methods to manipulate and get the parts of a URL that we want.

To create a URL object with the URL constructor, we use the new keyword like in the following code:

new URL('http://medium.com');

In above code, we create an instance of the URL object with an absolute URL. We can also create a URL object by passing-in a relative URL as the first argument and the base URL as the second argument. For example, we can write:

new URL('/@hohanga', 'http://medium.com');

Note that the second argument must be a base URL. This means that it must be a valid URL and not just part of it. It must start with the protocol like http:// or https://. We can also define them in a chain-like in the following code:

const mozillaUrl = 'https://developer.mozilla.org';
const mozillaDevUrl = new URL("/", mozillaUrl);
const mozillaUrl2 = new URL(mozillaUrl);
const docsUrl = new URL('/en-US/docs', mozillaUrl2);
const urlUrl = new URL('Web/API/URL', docsUrl);

If we call toString() on the URL objects above, we should get something like:

https://developer.mozilla.org
https://developer.mozilla.org/
https://developer.mozilla.org/
https://developer.mozilla.org/en-US/docs
https://developer.mozilla.org/en-US/Web/API/URL

The second argument is an optional argument and it should only be passed in when the first argument is a relative URL. The strings or URL objects that we pass in are converted to USVString objects which correspond to the set of all possible sequences of Unicode scalar values. In our code, we can treat them the same as regular strings. A TypeError will be thrown if both argument are relative URLs or if the base and the relative URL together aren’t valid. We can pass in the URL object directly to the second argument because the toString method of the URL object will convert a URL object into a full URL string before being manipulated in the constructor.

A URL object can have the following properties. This article will have the first half of the list of properties and Part 2 of this article will have the second half.

Hash Property

The hash property has the part of the URL that comes after the pound sign (# ). The string isn’t percent decoded, so special symbols like the following are still encoded. They’re encoding with the following mapping. The characters on the left side are converted to the ones on the right side during encoding:

  • ‘:’%3A
  • ‘/’%2F
  • ‘?’%3F
  • ‘#’%23
  • ‘[‘%5B
  • ‘]’%5D
  • ‘@’%40
  • ‘!’%21
  • ‘$’%24
  • “‘“%27
  • ‘(‘%28
  • ‘)’%29
  • ‘*’%2A
  • ‘+’%2B
  • ‘,’%2C
  • ‘;’%3B
  • ‘=’%3D
  • ‘%’%25
  • ‘ ‘%20 or +

For example, if we have the URL [https://example.com#hash](https://example.com#hash') . Then we can extract the hash with the hash property as follows:

const exampleUrl = new URL(‘https://example.com#hash');
console.log(exampleUrl.hash);

Then we get '#hash' in the console.log statement. The property is a USVString , but it’s converted to a string when we retrieve it like we did above. This is not a read-only property, so we can also assign to it like in the following code:

exampleUrl.hash = '#newHash';

For example, if we have:

const exampleUrl = new URL('https://example.com#hash');
exampleUrl.hash = '#newHash';
console.log(exampleUrl.hash);

Then we get [https://example.com/#newHash](https://example.com/#newHash) as the new URL when we get it with the href property.

Host Property

The host property of the URL object is a USVString that contains the host name. If a port is included after the : , then we also get the port number along with the host. For example, if we have:

const exampleUrl = new URL(‘https://example.com:8888');
console.log(exampleUrl.host);

Then we get example.com:8888 . Like other USVString properties, it’s converted to a string when we retrieve it. This is not a read only property, so we can also assign to it like in the following code:

exampleUrl.host = 'example.com';

We can also set the host property to change the URL structure. For example, if we have:

const exampleUrl = new URL('https://example.com:8888/en-US/docs/Web/API/URL/href/');

exampleUrl.host = 'newExample.com';
console.log(exampleUrl);

Then we get [https://newexample.com:8888/en-US/docs/Web/API/URL/href/](https://newexample.com:8888/en-US/docs/Web/API/URL/href/) as the new URL when we get it with the href property.

Hostname Property

With the hostname property, we can get the host name from the URL as a USVString. It will not include the port even if it’s included in the URL string that we used to construct the URL object. For example, if we have:

const exampleUrl = new URL(‘https://example.com:8888');
console.log(exampleUrl.hostname);

Then we get example.com from the console.log statement. Like other USVString properties, it’s converted to a string when we retrieve it. This is not a read-only property, so we can also assign to it like in the following code:

exampleUrl.hostname = ‘newExample.com’;

Then we get that the new URL is [https://newexample.com:8888/](https://newexample.com:8888/en-US/docs/Web/API/URL/href/) when we get it with the href property.

Href Property

The href property of the URL object is USVString that has the whole URL. For example, if we have:

const exampleUrl = new URL(‘https://example.com:8888/en-US/docs/Web/API/URL/href/');
console.log(exampleUrl.href);

Then when we run the code above, we get [https://example.com:8888/](https://example.com#hash')en-US/docs/Web/API/URL/href/ which is what we passed into the URL constructor. We can also set the href property by assigning our own value to it. For example, we can write:

const exampleUrl = new URL(‘https://example.com:8888/en-US/docs/Web/API/URL/href/');
exampleUrl.href = 'https://newExample.com';
console.log(exampleUrl.href);
Origin Property

The origin property is a read-only property that returns a USVString with the Unicode serialization of the origin of the URL. The structure of the origin will depend on the type of URL that’s used to construct the URL object. For http or https URLs, the origin will be the protocol followed by :// , then followed by the domain, then followed by the colon (: ), then followed by the port. The default port will only be omitted, which is 80 for http and 443 for https , if it’s explicitly specified. For file protocol URLs, the value will be browser dependent. For blob: URLs, the origin will be the part following blob: . For example, "blob:http://example.com" will be returned as "http://example.com" . For example, if we have:

const url = new URL("https://example.org:443")
console.log(url.origin);

Then we get:

https://example.org

If we have an http URL like the following:

const url = new URL("http://example.org:80")
console.log(url.origin);

Then we get:

http://example.org

If we have a non-default port for http or https URLs, like in the following:

const url = new URL("http://example.org:8888")
console.log(url.origin);

const url2 = new URL("http://example.org:8888")
console.log(url.origin);

Then we get [http://example.org:8888](http://example.org:8888)
and [http://example.org:8888](http://example.org:8888) respectively.

If we have a blob URL, like in the following:

const url = new URL("blob:http://example.org:8888")
console.log(url.origin);

Then we get [http://example.org:8888](http://example.org:8888) . And for file URLs like the following:

const url = new URL("file://c:/documents/document.docx")
console.log(url.origin);

Then we get file:// in Chrome.

Password Property

The password property is a writable property that extracts the password portion of the URL before the domain name. Like the other properties, the string is a USVString . If it’s set without the username property being set first, then assigning to this property will silently fail. For example, if we have:

const url = new URL('https://username:[email protected]');
console.log(url.password);

Then we get password in the console.log statement above. We can also set the password property to the password of our choice. For example, if we have:

const url = new URL('https://username:[email protected]');
url.password = 'newPassword'
console.log(url);

Then we get [https://username:[email protected]](https://username:[email protected]) as the new URL when we get it with the href property.

Pathname Property

The pathname property is a USVString property which has the part following the first slash (/). It’s writable property. For example, we can get the pathname value from the URL in the following code:

const url = new URL('https://example.com/path1/path2');
console.log(url.pathname);

Then we get /path1/path2 when we run the console.log statement above. We can also set the pathname property to the relative URL of your choice by setting it to the part after the domain. For example, we can write:

const url = new URL('https://example.com/path1/path2');
url.pathname = '/newpath1/newpath2';
console.log(url.href);

Then we get [https://example.com/newpath1/newpath2](https://example.com/newpath1/newpath2) when we run the console.log statement above.

Manipulating and extract parts from a URL is a pain if we have to write all the code to do it ourselves. Fortunately, most browsers have the URL object built in to their standard library. Now we can pass in the URL as a string to the URL constructor and to create an instance of a URL. Then we can use the handy value properties and methods to manipulate and get the parts of a URL that we want.

To create an URL object with the URL constructor, we use the new keyword like in the following code:

new URL('http://medium.com');

To see more ways to create an URL object, see Part 1 for more details.

A URL object can have the following properties. This article will have the second half of the list of properties and Part 1 of this article will have the first half. All string values are stored as USVString. USVString are objects which correspond to the set of all possible sequences of Unicode scalar values. All USVString values are converted to strings before we get them, so we can treat them all like regular strings.

Value Properties

port Property

The port property is a USVString property that has the port number of the URL. For example, if we have:

const url = new URL('https://example.com:8888');
console.log(url.port);

Then we get 8888 from the console.log statement. We can also set the port to get a new URL with by assigning the port property with a new value. For example, if we have the following code:

const url = new URL('https://example.com:8888');
url.port = '3333';
console.log(url.href);

Then we get [https://example.com:3333/](https://example.com:3333/) from the console.log statement when we run the code above.

protocol Property

With the protocol property, we can get the the protocol portion of the URL as a USVString , which is the very first part of the URL. Example protocols include http: , https: , ftp: , file: , etc. For instance, if we have the URL [https://example.com](https://example.com:8888'), then https is the protocol portion of the URL. If we have the following code:

const url = new URL('https://example.com:8888');
console.log(url.protocol);

Then we get https: from the console.log statement in the code above.

For other protocols like ftp , it also works:

const url = new URL('ftp://example.com:8888');
console.log(url.protocol);

If we run the code above, then we get ftp: from the console.log statement in the code above.

We can also set the protocol property to get a new URL. For example, if we have:

const url = new URL('ftp://example.com:8888');
url.protocol = 'http://'
console.log(url.href);

Then we get [http://example.com:8888/](http://example.com:8888/) from the console.log above. It also works if we omit the slashes or omit both the colon and the slashes. For example, if we have:

const url = new URL('ftp://example.com:8888');
url.protocol = 'http:'
console.log(url.href);

Then we also get [http://example.com:8888/](http://example.com:8888/) from the console.log above. Likewise, if we have:

const url = new URL('ftp://example.com:8888');
url.protocol = 'http'
console.log(url.href);

We get the same thing.

search Property

To get the search string or query string from a URL, we can use the search property. The query string is anything after the ? in a URL. Like all the other properties, this one is also USVString property. Note that this property get the whole query string. If you want the individual keys and values from the query string, then we can use the searchParams property. We can use the search property like the following code:

const url = new URL('http://example.com:8888?key1=value1&key2=value2');
console.log(url.search);

Then we get back ?key1=value1&key2=value2 from the console.log statement when we run the code above. We can also change the query string of the URL by assigning it a new value. For example, if we have the following code:

const url = new URL('http://example.com:8888?key1=value1&key2=value2');
url.search = 'newKey1=newValue1&newKey2=newValue2';
console.log(url.href);

Then we get [http://example.com:8888/?newKey1=newValue1&newKey2=newValue2](http://example.com:8888/?newKey1=newValue1&newKey2=newValue2) from the console.log statement when we run the code above.

searchParams Property

The search property only gets us the whole query string. To parse the query string into keys and values, we can use the searchParams property, which will get us a URLSearchParams object which has the list of keys and values listed on the query string. The keys and values will be decoded so that special characters are all intact. This is a read only property so we can’t set the query string directly by assigning a new value to this property. For example, to get the query string as a list of keys and values, we can write the following code:

const url = new URL('http://example.com:8888/?key1=value1&key2=value2');
console.log(url.searchParams.get('key1'));
console.log(url.searchParams.get('key2'));

Then we get value1 from the first console.log statement and value2 from the second console.log statement. The URLSearchParams object has a get method to get the value of the given query string key by the key name.

Username Property

We can get and set the username part of the URL with the username property. The username is a URL comes before the password and domain name. For example, if we have the following code:

const url = new URL('http://username:[email protected]');
console.log(url.username);

Then when we run the code above, we get username from the console.log statement above. Also, we can use it to set the username in a URL. For instance, if we have the following code:

const url = new URL('http://username:[email protected]');
url.username = 'newUserName';
console.log(url.href);

Then we get [http://newUserName:[email protected]/](http://newUserName:[email protected]/) from the console.log statement in the code above.

Instance Methods

URL object instances also have 2 methods. It has a toString() and a toJSON() method. The toString() method returns a USVString containing the whole URL. It’s the same as the href property for getting the whole URL, but we can’t use the toString() method to set the whole URL like we do with the href property. For example, if we have:

const url = new URL('http://username:[email protected]');
console.log(url.toString());

Then we get [http://username:[email protected]/](http://username:[email protected]/) from the console.log statement above.

The toJSON() method returns a JSON serialized version of the URL object. However, in most cases, it’s actually the same as what toString() will return. For example, if we have:

const url = new URL('http://username:[email protected]');
console.log(url.toJSON());
console.log(url.toString());
console.log(url.toJSON() === url.toString());

Then we get:

http://username:[email protected]/
http://username:[email protected]/
true

from the console.log statements when the code above is run. Then url.toJSON() and url.toString() calls returned the same output, and it’s confirmed by the comparison statement url.toJSON() === url.toString() , which evaluated to true .

Static Methods

There are 2 static methods in the URL constructor. It has the createObjectURL() method and the revokeObjectURL() method. The createObjectURL() static method creates a DOMString containing a URL representing the object given in the parameter. A DOMString is a UTF-16 encoded string. Since JavaScript uses UTF-16 strings, DOMStrings are mapped directly to strings. Passing null to a method that accepts a DOMString will convert null to 'null' . The createObbjectURL() method accepts an object, which can be a File, Blob, or MediaSource object to create a URL for.For example, we can call the createObjectURL() method like in the following code:

const objUrl = URL.createObjectURL(new File([""], "filename"));
console.log(objUrl);

Then we get something like:

blob:https://fiddle.jshell.net/4cfe2ef3-74ea-4145-87a7-0483b117b429

Each time we create an object URL with the createObjectURL() method, we have to release it by calling the revokeObjectURL() method to clear it from memory. However, browser will release object URLs automatically when the document is unloaded, but for the sake for improving performance, we should release it manually. The revokeObjectURL() method takes in an object URL as its argument. For example, if we have the object URL above created, then we can use the revokeObjectURL() method to release the object URL from memory like in the following code:

const objUrl = URL.createObjectURL(new File([""], "filename"));
console.log(objUrl);
URL.revokeObjectURL(objUrl);

The revokeObjectURL() method returns undefined .

When any part of the URL has special characters, they’re encoded by the rules found in RFC-3896, which has a long list of rules on how to encode different types of characters. It also includes non-English characters, and the percent encoding standard for encoding URLs. The full list of rules are at https://tools.ietf.org/html/rfc3986.

With the URL object, manipulating and extract parts from a URL is no longer a pain since we don’t have to write all the code to do it ourselves. Most browsers have the URL object built in to their standard library. Now we can pass in the URL as a string to the URL constructor and to create an instance of a URL. Then we can use the handy value properties and methods to manipulate and get the parts of a URL that we want. It can manipulate all parts of the URL except the query string portion, which can be manipulated by parsing it into a URLSeachParams object and then call its methods and set its properties to modify the query string. To toString and toJSON methods both convert a URL object into a full URL string.

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JavaScript Tutorial: if-else Statement in JavaScript

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

Decision Making in programming is similar to decision making in real life. In programming also we face some situations where we want a certain block of code to be executed when some condition is fulfilled.
A programming language uses control statements to control the flow of execution of the program based on certain conditions. These are used to cause the flow of execution to advance and branch based on changes to the state of a program.

JavaScript’s conditional statements:

  • if
  • if-else
  • nested-if
  • if-else-if

These statements allow you to control the flow of your program’s execution based upon conditions known only during run time.

  • if: if statement is the most simple decision making statement. It is used to decide whether a certain statement or block of statements will be executed or not i.e if a certain condition is true then a block of statement is executed otherwise not.
    Syntax:
if(condition) 
{
   // Statements to execute if
   // condition is true
}

Here, condition after evaluation will be either true or false. if statement accepts boolean values – if the value is true then it will execute the block of statements under it.
If we do not provide the curly braces ‘{‘ and ‘}’ after if( condition ) then by default if statement will consider the immediate one statement to be inside its block. For example,

if(condition)
   statement1;
   statement2;

// Here if the condition is true, if block 
// will consider only statement1 to be inside 
// its block.

Flow chart:

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If statement 

var i = 10; 

if (i > 15) 
document.write("10 is less than 15"); 

// This statement will be executed 
// as if considers one statement by default 
document.write("I am Not in if"); 

< /script> 

Output:

I am Not in if
  • if-else: The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false. Here comes the else statement. We can use the else statement with if statement to execute a block of code when the condition is false.
    Syntax:
if (condition)
{
    // Executes this block if
    // condition is true
}
else
{
    // Executes this block if
    // condition is false
}


Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If-else statement 

var i = 10; 

if (i < 15) 
document.write("10 is less than 15"); 
else
document.write("I am Not in if"); 

< /script> 

Output:

i is smaller than 15
  • nested-if A nested if is an if statement that is the target of another if or else. Nested if statements means an if statement inside an if statement. Yes, JavaScript allows us to nest if statements within if statements. i.e, we can place an if statement inside another if statement.
    Syntax:
if (condition1) 
{
   // Executes when condition1 is true
   if (condition2) 
   {
      // Executes when condition2 is true
   }
}

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate nested-if statement 

var i = 10; 

if (i == 10) { 

// First if statement 
if (i < 15) 
	document.write("i is smaller than 15"); 

// Nested - if statement 
// Will only be executed if statement above 
// it is true 
if (i < 12) 
	document.write("i is smaller than 12 too"); 
else
	document.write("i is greater than 15"); 
} 
< /script> 

Output:

i is smaller than 15
i is smaller than 12 too
  • if-else-if ladder Here, a user can decide among multiple options.The if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
if (condition)
    statement;
else if (condition)
    statement;
.
.
else
    statement;


Example:

<script type = "text/javaScript"> 
// JavaScript program to illustrate nested-if statement 

var i = 20; 

if (i == 10) 
document.wrte("i is 10"); 
else if (i == 15) 
document.wrte("i is 15"); 
else if (i == 20) 
document.wrte("i is 20"); 
else
document.wrte("i is not present"); 
< /script> 

Output:

i is 20

Building a Powerful Virtual Machine in JavaScript

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This JavaScript tutorial explains how to build a powerful Virtual Machine in JavaScript. A flexible, extensible, register-based virtual machine. Support for signed, unsigned and floating point operations. A call stack. Interrupt capabilities. Ability to do memory mapping for IO. An assembly language with macro and module support. A higher level, C like language. We'll use and expand the library from the parser combinators from scratch series. And finally, to be able to take the whole thing into the browser and exend it to create a sort of fantasy console - an emulator for a machine that never existed

16-Bit Virtual Machine in JavaScript 001

In this episode we begin implementing a 16-bit virtual machine from scratch in JavaScript. The concepts of computation are introduced, along with the basics of assembly language and machine code.

Memory Access and Branching (16-Bit VM in JavaScript 002)

In this video we establish the core instruction set of the VM, give the VM the capabilities to read and write to main memory, and also to make decisions about how the program should proceed with branching instructions.

What is the Stack and why do we need it? (16-Bit VM in JavaScript 003)

In this episode we understand what a stack is, how it can be implemented on the lowest level, and how it can then be harnessed to allow the Virtual Machine to run subroutines without losing state.

Implementing Stack Mechanics (16-Bit VM in JavaScript 004)

In this episode, we create an implementation for the stack mechanisms that were described in the last episode.

What is Memory Mapped I/O? (16-Bit VM in JavaScript 005)

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Understanding JavaScript Decorators

Understanding JavaScript Decorators

Decorators are actually nothing more than functions that return another function, and that are called with the appropriate details of the item being decorated. Using decorators in your projects today requires some transpiler configuration. What is a Decorator? JavaScript Decorators and Property Descriptors. How to Write a Decorator. Handling API Errors. Decorating Classes. A Babel Workaround

Decorators aren’t a core feature of JavaScript yet; they’re working their way through ECMA TC39’s standardization process. That doesn’t mean we can’t get familiar with them, though. It looks like they’ll be supported natively by Node and browsers at some point in the near future, and in the meantime we’ve got Babel.

What is a Decorator?

Decorator is shorthand for “decorator function” (or method). It’s a function that modifies the behavior of the function or method passed to it by returning a new function. I said “function” a lot there. That’s an occupational hazard when you’re discussing higher-order functions.

You can implement decorators in any language that supports functions as first-class citizens, e.g. JavaScript, where you can bind a function to a variable or pass it as an argument to another function. A couple of those languages have special syntactic sugar for defining and using decorators; one of them is Python:


def cashify(fn):
    def wrap():
        print("$$$$")
        fn()
        print("$$$$")
    return wrap

@cashify
def sayHello():
    print("hello!")

sayHello()

# $$$$
# hello!
# $$$$

Let’s take a look at what’s going on there. Our cashify function is a decorator: it receives a function as an argument, and its return value is also a function. We use Python’s “pie” syntax to apply the decorator to our sayHello function, which is essentially the same thing as if we’d done this below the definition of sayHello:


def sayHello():
    print("hello!")

sayHello = cashify(sayHello)

The end result is that we print dollar signs before and after whatever we’re printing from the function we decorate.

Why am I introducing ECMAScript decorators using an example in Python? I’m glad you asked!

  • Python is a great way to explain the basics because its concept of decorators is a bit more straightforward than the way they work in JS.
  • JS and TypeScript both use Python’s “pie syntax” to apply decorators to methods and properties of classes, so it’s visually and syntactically similar.

Okay, what’s different about JS decorators?

JS Decorators and Property Descriptors

While Python decorators are passed whatever function they’re decorating as an argument, JS decorators receive quite a bit more information due to the way objects work in that language.

Objects in JS have properties, and those properties have values:


const oatmeal = {
  viscosity: 20,
  flavor: 'Brown Sugar Cinnamon',
};

But in addition to its value, each property has a bunch of other behind-the-scenes information that defines different aspects of how it works, called a property descriptor:


console.log(Object.getOwnPropertyDescriptor(oatmeal, 'viscosity'));

/*
{
  configurable: true,
  enumerable: true,
  value: 20,
  writable: true
}
*/

JS is tracking quite a few things related to that property:

  • configurable determines whether or not the type of the property can be changed, and whether it can be deleted from the object.
  • enumerable controls whether the property shows up when you enumerate the object’s properties (like when you call Object.keys(oatmeal) or use it in a for loop).
  • writable controls whether or not you can change the property’s value via the assignment operator =.
  • value is the static value of the property that you see when you access it. It’s usually the only part of the property descriptor that you see or are concerned with. It can be any JS value, including a function, which would make the property a method of the object it belongs to.

Property descriptors can also have two other properties that cause JS to treat them as “accessor descriptors” (more commonly known as getters and setters):

  • get is a function that returns the property’s value instead of using the static value property.
  • set is a special function that gets passed whatever you put on the right side of the equals sign as an argument when you assign a value to the property.

Decorating Without the Frills

JS has actually had an API for working with property descriptors since ES5, in the form of the Object.getOwnPropertyDescriptor and Object.defineProperty functions. For example, If I like the thickness of my oatmeal just the way it is, I can make it read-only using that API like so:


Object.defineProperty(oatmeal, 'viscosity', {
  writable: false,
  value: 20,
});

// When I try to set oatmeal.viscosity to a different value, it'll just silently fail.
oatmeal.viscosity = 30;
console.log(oatmeal.viscosity);
// => 20

I can even write a generic decorate function that lets me mess with the descriptor for any property of any object:


function decorate(obj, property, callback) {
  var descriptor = Object.getOwnPropertyDescriptor(obj, property);
  Object.defineProperty(obj, property, callback(descriptor));
}

decorate(oatmeal, 'viscosity', function(desc) {
  desc.configurable = false;
  desc.writable = false;
  desc.value = 20;
  return desc;
});

Adding the Shiplap and Crown Molding

The first major difference with the Decorators proposal is that it only concerns itself with ECMAScript classes, not regular objects. We’re going to need to over-engineer our breakfast in order to really demonstrate what we can accomplish, so let’s make some classes to represent our bowl of porridge:


class Porridge {
  constructor(viscosity = 10) {
    this.viscosity = viscosity;
  }

  stir() {
    if (this.viscosity > 15) {
      console.log('This is pretty thick stuff.');
    } else {
      console.log('Spoon goes round and round.');
    }
  }
}

class Oatmeal extends Porridge {
  viscosity = 20;

  constructor(flavor) {
    super();
    this.flavor = flavor;
  }
}

We’re representing our bowl of oatmeal using a class that inherits from the more generic Porridge class. Oatmeal sets the default viscosity higher than Porridge‘s default, and it adds a new flavor property. We’re also using another ECMAScript proposal, class fields, to override the viscosity value.

We can re-create our original bowl of oatmeal like so:


const oatmeal = new Oatmeal('Brown Sugar Cinnamon');

/*
Oatmeal {
  flavor: 'Brown Sugar Cinnamon',
  viscosity: 20
}
*/

Great, we’ve got our ES6 oatmeal, and we’re ready to write a decorator!

How to Write a Decorator

JS decorator functions are passed three arguments:

  1. target is the class that our object is an instance of.
  2. key is the property name, as a string, that we’re applying the decorator to.
  3. descriptor is that property’s descriptor object.

What we do inside of the decorator function depends on the purpose of our decorator. In order to decorate a method or property of an object, we need to return a new property descriptor. Here’s how we can write a decorator that makes a property read-only:


function readOnly(target, key, descriptor) {
  return {
    ...descriptor,
    writable: false,
  };
}

We’d use it by modifying our Oatmeal class like this:


class Oatmeal extends Porridge {
  @readOnly viscosity = 20;
  // (you can also put @readOnly on the line above the property)

  constructor(flavor) {
    super();
    this.flavor = flavor;
  }
}

Now our oatmeal’s glue-like consistency is immune to tampering. Thank goodness.

What if we want to do something that’s actually useful? I ran into a situation while working on a recent project where a decorator saved me a lot of typing and maintenance overhead:

Handling API Errors

In the MobX/React app I mentioned in the beginning, I have a couple of different classes that act as data stores. They each represent collections of different things that the user interacts with, and they each talk to different API endpoints for data from the server. In order to handle API errors, I made each of the stores follow a protocol when communicating over the network:

  1. Set the UI store’s networkStatus property to “loading.”
  2. Send a request to the API
  3. Handle the result
    • If successful, update local state with the response
    • If something goes wrong, set the UI store’s apiError property to the error we received
  4. Set the UI store’s networkStatus property to “idle.”

I found myself repeating this pattern a few times before I noticed the smell:


class WidgetStore {
  async getWidget(id) {
    this.setNetworkStatus('loading');

    try {
      const { widget } = await api.getWidget(id);
      // Do something with the response to update local state:
      this.addWidget(widget);
    } catch (err) {
      this.setApiError(err);
    } finally {
      this.setNetworkStatus('idle');
    }
  }
}

That’s a lot of error handling boilerplate. I decided that since I was already using MobX’s @action decorators on all the methods that updated observable properties (not shown here for the sake of simplicity), I might as well just tack on an additional decorator that allowed me to recycle my error handling code. I came up with this:


function apiRequest(target, key, descriptor) {
  const apiAction = async function(...args) {
    // More about this line shortly:
    const original = descriptor.value || descriptor.initializer.call(this);

    this.setNetworkStatus('loading');

    try {
      const result = await original(...args);
      return result;
    } catch (e) {
      this.setApiError(e);
    } finally {
      this.setNetworkStatus('idle');
    }
  };

  return {
    ...descriptor,
    value: apiAction,
    initializer: undefined,
  };
}

I could then replace the boilerplate that I was writing in each API action method with something like this:


class WidgetStore {
  @apiRequest
  async getWidget(id) {
    const { widget } = await api.getWidget(id);
    this.addWidget(widget);
    return widget;
  }
}

My error handling code is still there, but now I only need to write it once and ensure that each class that uses it has a setNetworkStatus and setApiError method.

A Babel Workaround

So what’s up with that line where I’m choosing between descriptor.value and calling descriptor.initializer? That’s a Babel thing. My hunch is that it won’t work that way when JS supports decorators natively, but it’s necessary right now because of how Babel handles arrow functions defined as class properties.

When you define a class property and assign an arrow function as its value, Babel does a little trick to bind that function to the correct instance of the class and give you the right this value. It does this by setting descriptor.initializer to a function that returns the function you wrote, with the correct this value in its scope.

An example should make things less muddy:


class Example {
  @myDecorator
  someMethod() {
    // In this case, our method would be referred to by descriptor.value
  }

  @myDecorator
  boundMethod = () => {
    // Here, descriptor.initializer would be a function that, when called, would return our `boundMethod` function, properly scoped so that `this` refers to the current instance of Example.
  };
}

Decorating Classes

In addition to properties and methods, you can also decorate an entire class. In order to do that, you really only need the first argument passed to your decorator function, target. For example, I can write a decorator that automatically registers the class it’s wrapping as a custom HTML element. I’m using a closure here to enable the decorator to receive whatever name we want to give the element as an argument:


function customElement(name) {
  return function(target) {
    customElements.define(name, target);
  };
}

We’d use it like this:


@customElement('intro-message');
class IntroMessage extends HTMLElement {
  constructor() {
    super();

    const shadow = this.attachShadow({ mode: 'open' });
    this.wrapper = this.createElement('div', 'intro-message');
    this.header = this.createElement('h1', 'intro-message__title');
    this.content = this.createElement('div', 'intro-message__text');
    this.header.textContent = this.getAttribute('header');
    this.content.innerHTML = this.innerHTML;

    shadow.appendChild(this.wrapper);
    this.wrapper.appendChild(this.header);
    this.wrapper.appendChild(this.content);
  }

  createElement(tag, className) {
    const elem = document.createElement(tag);
    elem.classList.add(className);
    return elem;
  }
}

Load that into our HTML, and we can use it like this:


<intro-message header="Welcome to Decorators">
  <p>Something something content...</p>
</intro-message>

Which gives us this in the browser:

Wrapping Up

Using decorators in your projects today requires some transpiler configuration. The most straightforward guide that I’ve seen is located in the MobX docs. It has info for TypeScript and two major versions of Babel.

Keep in mind that decorators are an evolving proposal at this point, so if you use them in production code now, you’ll probably either need to make some updates or keep using Babel’s decorators plugin in legacy mode once they become an official part of the ECMAScript specification. While it’s not even well-supported by Babel yet, the latest version of the decorators proposal already contains big changes that are not backward compatible with the previous version.

Decorators, like many bleeding edge JS features, are a useful tool to have in your kit. They can greatly simplify the sharing of behavior across different, unrelated classes. However, there’s always a cost associated with early adoption. Use decorators, but do so with a clear idea of the implications for your codebase.