Александр Скачков "JavaScript metaprogramming with ES2015 Proxy"

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Starting with ECMAScript 2015, JavaScript gains support for the Proxy object allowing you to intercept and define custom behavior for fundamental language operations. ECMAScript 2015 Proxy is rarely used because cannot be transpiled by Babel, due to the limitations of ES5. As for now, all evergreen browsers support it and it is time to learn more about ES2015 Proxy. We will overview ES2015 Proxy features, how to use them, and look to the cases where the proxy can be used.

Technology

Oleksandr Skachkov
Itera
JavaScript Metaprogramming with ES2015 Proxy

ABOUT ME
• Itera’s FrontEnd Tech Lead
• WebKit contributor & JS hacker:
• Arrow function
• Async iterator & generators
• Promise.finally
• Twitter: @alSkachkov

ES2015 PROXY
JAVASCRIPT METAPROGRAMMING WITH
• Metaprogramming before ES2015
• Metaprogramming with ES2015 Proxy
• Use cases

BACKBONE
Backbone my first MV* framework

THEORY
Metaprogramming refers to a variety of ways a program
has knowledge of itself or can manipulate itself
stackoverflow

METAPROGRAMMING. HOW?
eval – the best strategy to do metaprogramming in JS :-)

METAPROGRAMMING. HOW?
Reflection – is key strategy to do metaprogramming:
• Introspection - read the structure of a program
• Self-modification - change program structure
• Intercession - change the semantics of some language operations

REFLECTION. INTROSPECTION
Object.keys()
Object.getOwnPropertyNames()

REFLECTION. SELF-MODIFICATION
Object.seal()
Object.freeze()
Object.preventExtension()

REFLECTION. INTERCESSION
•Is not supported by ES5
• Some browsers do __noSuchMethod__ (deprecated)

PROXY API
• Mirror-based architecture – Tom
Van Cutsem 2007
• Proxy API : Tom Van Cutsem and
Mark S. Miller 2010

PROXY
target - can be any sorts of objects, or another proxy

PROXY
handler - An object whose properties are functions which define the behavior
of the proxy when an operation is performed on it.

PROXY
traps: The methods that provide property access. This is analogous to the
concept of traps in operating systems

PROXY. INVARIANTS
Semantics that remain unchanged when implementing custom operations are
called invariants.

CAN WE USE ES2015 PROXY?
from 6.5
Not fully supported in

SYNC OBSERVABLE
Use set, deleteProperty traps

VALIDATION OR ACCESS CONTROL
Use set trap with check conditions

PROFILING OR LOGGING
Use get, set, deleteProperty traps

VIRTUAL OBJECT
Virtual objects are proxies that emulate other objects
without those objects needing to be in the same address
space.
Use set, get, defineProperty, deleteProperty traps

MORE CASES
Firefox uses ES2015 Proxy to protect Host object

SUMMARY
• Powerful abstraction
• Adding functionality transparently
• Magic
• Slow in most of the browsers

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