Metaprogramming with javascript

Feb 10, 20172 likes701 views

Metaprogramming involves writing computer programs that manipulate or generate other programs. It allows for more elegant code, less boilerplate code, and hiding implementation details. The document discusses different ways metaprogramming is implemented in JavaScript, including dynamic features like reflection, getters/setters, proxies, decorators, and generators, as well as code generation approaches. Real-world examples of metaprogramming include frameworks, domain-specific languages, and adding functionality without cluttering main logic.

Jakarta
Metaprogramming with
Javascript
Ahmad Rizqi Meydiarso

Jakarta
What is Metaprogramming
is the writing of computer programs that write
or manipulate other program (or themselves) as 
their data, or that do part of the work at compile
time that otherwise be done at runtime
“
writing code that writes code

Jakarta
The Purpose
To have super power!
• More elegant code
• Less boilerplate code
• Hide the how, focus on what
• Essential in framework building

Jakarta
Background
• My first encounter: Prototype.js, jQuery
• Experimenting with my own parsing (zparse) and angular
like framework foo.js (2007)
• Then created chain.js batik etc. (2008-2010)
• Now maintaining Merapi and its plugins (for YesBoss /
Kata.ai)
I love Metaprogramming

Jakarta
Uses in Real World
• Almost every framework uses Meta
Programming in some way
• Domain Specific Languages
• Add functionalities without littering
main logic (logging, measurement,
etc.)

Jakarta
Two Flavours
Dynamic Language Features
Program can inspect itself using built-in features of the language.
Code Generation
Translate meta features into generated code in target language.
examples: jquery
spring (java)
rails (ruby)
merapi ;)
examples: QT (C++)
CoﬀeeScript
React
Babel / Typescript

Jakarta
Dynamic Language Features
• Reflection
• Getter Setter
• Proxy
• Decorator
• Generator
• DOM Manipulation
• Eval

Jakarta
Reflection
• for (let i in object);
• .prototype, getPrototypeOf()
• getOwnPropertyNames(),
defineProperties()
• typeof etc…

Jakarta
Getter Setter
• {set prop() {}, get prop() {}}
• Object.defineProperty()
• Object.defineProperties()

Jakarta
Proxy
• it’s a trap!
• replace object access by function invocation
• create truly dynamic object
• warning: slow performance
• handlers:
• get(), set(), has(), ownKeys(), …

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Generator
• Function that stops on “yield”
• Returns an Iterator

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Decorator
• @Decorator
• Annotating method / property
• manipulate / give context

Jakarta
Performance
• Code Generation (compile/transpile):
• No impact performance (however depends on
implementation)
• Runtime (processed during initialization only)
• Could lead to slower initialization (still ok in most cases)
• Runtime (heavy use)
• Be careful!

Jakarta
Conclusion
• It is like doing black magic:
• know what you are doing
• can have side effects if not used properly
• Performance Trade-Off
• Unclear Program Flow
• Devs not familiar with the core can get confused

JavaScript allows for metaprogramming through manipulating programs as data. It relies on few but powerful constructs including working with functions, arguments, built-in types, and mechanisms like inheritance at runtime. The key building blocks are objects, numbers, strings, booleans, and undefined, with everything else being objects. Functions are objects that can return other functions, taking advantage of closures to refer to outer variables from inner functions. This flexibility allows for patching implementations, self-optimizing code, custom APIs, and domain-specific languages.

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Metaprogramming with javascript

1. Jakarta Metaprogramming with Javascript Ahmad Rizqi Meydiarso

2. Jakarta What is Metaprogramming is the writing of computer programs that write or manipulate other program (or themselves) as  their data, or that do part of the work at compile time that otherwise be done at runtime “ writing code that writes code

3. Jakarta The Purpose To have super power! • More elegant code • Less boilerplate code • Hide the how, focus on what • Essential in framework building

4. Jakarta Background • My first encounter: Prototype.js, jQuery • Experimenting with my own parsing (zparse) and angular like framework foo.js (2007) • Then created chain.js batik etc. (2008-2010) • Now maintaining Merapi and its plugins (for YesBoss / Kata.ai) I love Metaprogramming

5. Jakarta Uses in Real World • Almost every framework uses Meta Programming in some way • Domain Specific Languages • Add functionalities without littering main logic (logging, measurement, etc.)

6. Jakarta Two Flavours Dynamic Language Features Program can inspect itself using built-in features of the language. Code Generation Translate meta features into generated code in target language. examples: jquery spring (java) rails (ruby) merapi ;) examples: QT (C++) CoﬀeeScript React Babel / Typescript

7. Jakarta Code Generation Examples

8. Jakarta Code Generation Examples

9. Jakarta Code Generation Examples

10. Jakarta Dynamic Language Features • Reflection • Getter Setter • Proxy • Decorator • Generator • DOM Manipulation • Eval

11. Jakarta Reflection • for (let i in object); • .prototype, getPrototypeOf() • getOwnPropertyNames(), defineProperties() • typeof etc…

12. Jakarta Reflection Example

13. Jakarta Reflection Example

14. Jakarta Getter Setter • {set prop() {}, get prop() {}} • Object.defineProperty() • Object.defineProperties()

15. Jakarta Getter Setter Example

16. Jakarta Getter Setter Example

17. Jakarta Proxy • it’s a trap! • replace object access by function invocation • create truly dynamic object • warning: slow performance • handlers: • get(), set(), has(), ownKeys(), …

18. Jakarta Proxy Example

19. Jakarta Proxy Example

20. Jakarta Generator • Function that stops on “yield” • Returns an Iterator

21. Jakarta Generator Example

22. Jakarta Generator Example

23. Jakarta Decorator • @Decorator • Annotating method / property • manipulate / give context

24. Jakarta Decorator Example

25. Jakarta Performance • Code Generation (compile/transpile): • No impact performance (however depends on implementation) • Runtime (processed during initialization only) • Could lead to slower initialization (still ok in most cases) • Runtime (heavy use) • Be careful!

26. Jakarta Conclusion • It is like doing black magic: • know what you are doing • can have side effects if not used properly • Performance Trade-Off • Unclear Program Flow • Devs not familiar with the core can get confused

27. [email protected]

Metaprogramming with javascript

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