What Year Is It – Understanding Iterators, Constructors, and Arrays The similarities between JavaScript and Python continue to grow over time, and I couldn’t be more excited. Today we talk about Yehuda Katz’s next Python proposal for ECMAScript: decorators.
Updated 10/02/18: Here’s a command line utility to update your script from the old decorator hints to the new decorator hints:
What Year Is It
What is a decorator anyway? In Python, decorators provide a very simple syntax for calling higher-level functions. A Python decorator is a function that uses another function to extend the behavior of the second function without explicitly changing it. The simplest decorator in Python is:
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The above (`@mydecorator`) is a decorator, not much different in ES2016 (ES7), so be careful! :).
Refer to the function by its name. Our decorator takes an argument (the function to decorate) and returns the same function with additional functionality.
Decorators are useful for anything you want to transparently wrap with additional functionality. This includes note-taking, access control and authentication, tools and timing functions, logs, rate limiting, and more.
Implementing imperative decorators (or pure functions) in ES5 is very simple. In ES2015 (formerly ES6), classes support extensions, but if you have multiple classes that need to share the same functionality, you need better functionality. There is a better way to distribute it.
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Yehuda’s decoration proposal is to allow JavaScript classes, properties, and object literals to be interpreted and modified at design time while maintaining a declarative syntax.
So remember what we learned in Python. An ES2016 decorator is an expression that returns a function and can take a target, a name, and a property ID as arguments. Apply a decorator by prefixing it with the `@` symbol and place it on top of the element you want to decorate. Decorators can be defined by classes or properties.
Imagine you want to invalidate a property or method name. Decorators come to make a statement that defines a property. So we can define the “@readonly” decorator like this:
A decorator is simply an expression that must be evaluated and return a function. That’s why `@readonly` and `@something(parameter)` can work.
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It’s your knee, right? We’ll look at decorative classes (instead of properties) and talk a little about libraries in a moment. Despite its young age, decorator libraries started appearing in 2016, including Jay Phelps’s https://github.com/jayphelps/core-decorators.js .
Similar to the read-only prop above, I’ve included my own implementation for “@readonly”.
It also includes other decorator helpers, such as “@deprecate”, for cases where an API method needs to be indicated as mutable.
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Next, let’s look at the decoration class. In this case, according to the recommended specification, the decorator uses the target constructor. For our hypothetical MySuperHero class, we can define a simple decorator like this using the @superhero decorator:
This can be further extended to provide arguments for specifying a decorator function as a factory.
ES2016 decorators work on property descriptions and classes. If we stop short, we automatically get the name of the passed property and the target object. Accessing a descriptor allows a decorator to do things like change a property to use a getter, and it can enable awkward behavior like automatically binding a method for the current instance when the property is accessed for the first time.
I really enjoyed reading Reg Braithwaite’s recent article on ES2016 decorators as a precursor to mixins and Functional mixins. Reg offered a helper to mix behavior for all purposes (class prototype or standalone) and went on to describe class-specific versions. A functional mixin that mixes instance behavior with a class prototype:
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Excessive. Now we can define some combinations and use them to decorate our class. Let’s imagine we have a simple “ComicBookCharacter” class:
This might be the most boring character in the world, but you can assign him a few combos that give him SuperPowers and moves that give him the UtilityBelt. Let’s do it with Reg’s mixin assistant.
Now we can use the “@” syntax with the mixin function name to decorate the “ComicBookCharacter” with the desired behavior. Notice how we add a few decorators to the class.
These class decorators are relatively compact and can be used as alternatives to function calls or as helpers for higher-level components.
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Note: @WebReflection has several alternatives to the mixin pattern used in this section, which can be found in the comments.
Decorators are (at the time of writing) still suggestions only. Not confirmed yet. Fortunately, Babel supports syntax conversion in experimental mode, so you can try most of the examples in this post yourself.
Fortunately, Paul Lewis, sitting next to me, has been experimenting with decorators as a means of modifying code that reads and writes to the DOM. It takes ideas from Wilson Page’s FastDOM, but provides a small API surface. Paul’s read/write decorator can alert you via the console if you call a method or property while invoking a layout during @write (or modifying the DOM during @read).
Here’s an example from Paul’s experiment that shows a warning in the console when you try to change the DOM to @read.
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In the short term, ES2016 decorators are useful for solving problems with declarative decoration and annotations, type checking, and applying decorators to ES2015 classes. In the long run, it can be very useful for static analysis (it can provide compile-time checking or auto-completion tools).
Patterns are not much different from decorators in classic OOP, which allow you to statically or dynamically decorate the behavior of an object without affecting objects of the same class. I think they are a beautiful addition. The meaning of decorators for class properties is still fluid, but stay tuned to the Yehuda repository for updates.
The library’s authors are currently discussing where decorators can replace mixins and ways to use them for higher-order components in React.
Personally, I am very happy that the experiences of using them are increasing. I hope you’ll give it a try with Babel, identify reusable decorators, and even share your work like Paul 🙂
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Thanks to Jay Phelps, Sebastian McKenzie, Paul Lewis and Surma for reviewing this article and providing detailed feedback ❤
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The Gregorian calendar is currently the most widely used civil calendar and is the de facto global standard. 168 countries of the world use the Gregorian calendar as a single calendar.
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The Gregorian calendar is associated with the Catholic Church, but has been adopted for convenience in many religious and non-Christian countries.
Four countries still use different civil calendars: Afghanistan and Iran (Hijri solar calendar), Ethiopia (Ethiopian calendar) and Nepal (Bikram Sambat and Nepal Sambat).
Four countries use a modified version of the Gregorian calendar (dates different from Anno Domini): Japan (Japanese calendar), North Korea (North Korean calendar), Taiwan (Republican calendar), and Thailand (Thai solar calendar). The Anno Domini period is also used in the former two countries. Korea used the Korean calendar from 1945 to 1961.
Eighteen countries use other calendars in addition to the Gregorian calendar. Algeria (Hijri) ). ). ) Hijri calendar), Myanmar (Burmese calendar), Oman, Pakistan, Saudi Arabia, Somalia, Tunisia, United Arab Emirates, Yemen (Hijri monthly calendar).
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The Japanese calendar type includes various formal and informal systems. Today, Japan uses the Gregorian calendar to explain the signs of the year:
