Proxies are Awesome!

Proxies are Awesome! Brendan Eich (w/ Mark Miller & Tom Van Cutsem) Tuesday, September 28, 2010

ECMAScript 5 (ES5) Tuesday, September 28, 2010

ES5 Review • Array extras • JSON (based on json2.js) • Strict Mode (“use strict”) • Object meta-programming API • accessor properties (getters & setters) • mutability and enumerability controls • “javascript.lang.reﬂect” • In Firefox 4, IE9 (w/o strict?), WebKit nightlies Tuesday, September 28, 2010

ES5 Property Descriptors • Data vs. accessor properties • Property attributes var point = { x: 5, y: 8, get r() { return Math.sqrt(this.x*this.x + this.y*this.y); } }; Object.getOwnPropertyDescriptor(point, ‘x’); { value: 5, writable: true, enumerable: true, configurable: true } Object.getOwnPropertyDescriptor(point, ‘r’); { get: function () { return Math.sqrt(this.x*this.x + ...); }, set: undefined, enumerable: true, configurable: true } Tuesday, September 28, 2010

Property Descriptor Maps var point = { x: 5, y: 8, get r() { return Math.sqrt(this.x*this.x + this.y*this.y); } }; Object.defineProperty(point, ‘z’, { value: 0, enumerable: true, writable: false, configurable: false }); var point = Object.create( Object.prototype, { x: { value: 5, ... }, y: { value: 8, ... }, r: { get: function() {...}, enumerable: true, ... }, z: { value: 0, enumerable: true, ... } }); Tuesday, September 28, 2010

Property Descriptor Maps var point = { x: 5, y: 8, get r() { return Math.sqrt(this.x*this.x + this.y*this.y); } }; Object.defineProperty(point, ‘z’, { value: 0, enumerable: true, writable: false, name pd configurable: false }); x {...} y {...} r {...} var point = Object.create( z {...} Object.prototype, { x: { value: 5, ... }, y: { value: 8, ... }, r: { get: function() {...}, enumerable: true, ... }, z: { value: 0, enumerable: true, ... } }); Tuesday, September 28, 2010

Tamper-prooﬁng Objects var point = { x: 5, y: 8, get r() { return Math.sqrt(this.x*this.x + this.y*this.y); } }; Object.preventExtensions(point); point.z = 0; // can’t add new properties Object.seal(point); delete point.x; // can’t delete properties Object.freeze(point); point.x = 7; // can’t assign properties Tuesday, September 28, 2010

ES-Harmony Proxies Tuesday, September 28, 2010

Dynamic Proxies • Generic handling of property access: • Generic wrappers: security, aspects, logging, proﬁling, ... • Stratiﬁed form of SpiderMonkey’s __noSuchMethod__ js> o = {__noSuchMethod__: function (id, args) { print(id, args); }} ({__noSuchMethod__:(function (id, args) {print(id, args);})}) js> o.m(1,2,3) m 1,2,3 • Generic handling of other operations applicable to objects: • Virtual objects: persistent objects, remote objects, ... • Emulate the dreaded “host objects” Tuesday, September 28, 2010

Example w/ just ES5: logging function makePoint(x, y) { return { x: x, y: y, ... }; } Tuesday, September 28, 2010

Example w/ just ES5: logging function makePoint(x, y) { return { x: x, y: y, ... }; } function makeLoggedPoint(p) { return { get x() { log(‘get’,‘x’,p); return p.x; }, set x(v) { log(‘set’,‘x’,p,v); p.x = v; }, // get y, set y, ... }; } var lp = makeLoggedPoint(makePoint(1,2)); Tuesday, September 28, 2010

Example w/ just ES5: logging function makePoint(x, y) { return { x: x, y: y, ... }; } function makeLoggedPoint(p) { return { Too ad hoc. What about: get x() { • logging other data types }, log(‘get’,‘x’,p); return p.x; • proﬁling, persistence, set x(v) { access control, ... log(‘set’,‘x’,p,v); p.x = v; }, // get y, set y, ... }; } var lp = makeLoggedPoint(makePoint(1,2)); Tuesday, September 28, 2010

Logging: static ES5 “proxies” function makeLogger(obj) { var proxy = Object.create(Object.getProtoypeOf(obj), {}); Object.getOwnPropertyNames(obj).forEach(function(name) { var pd = Object.getOwnPropertyDescriptor(obj, name); Object.defineProperty(proxy, name, { get: function() { log(‘get’, name, obj); return obj[name]; }, set: function(v) { log(‘set’, name, obj, v); obj[name] = v; }, // copy attributes from pd }); }); return proxy; } Tuesday, September 28, 2010

Logging: static ES5 “proxies” function makeLogger(obj) { var proxy = Object.create(Object.getProtoypeOf(obj), {}); Object.getOwnPropertyNames(obj).forEach(function(name) { var pd = Object.getOwnPropertyDescriptor(obj, name); Object.defineProperty(proxy, name, { get: function() { log(‘get’, name, obj); }, return obj[name]; • proxy doesn’t reﬂect set: function(v) { structural changes made to ‘obj’ log(‘set’, name, obj, v); obj[name] = v; • structural changes made to }, proxy are not reﬂected in ‘obj’ // copy attributes from pd • structural changes: }); }); • add/delete properties return proxy; • change property attributes } Tuesday, September 28, 2010

Logging: dynamic (harmony) proxies function makeLogger(obj) { var proxy = Proxy.create({ get: function(rcvr, name) { log(‘get’, name, obj); return obj[name]; }, set: function(rcvr, name, val) { log(‘set’, name, obj, val); obj[name] = val; return true; }, ... }, Object.getPrototypeOf(obj)); return proxy; } Tuesday, September 28, 2010

Logging: dynamic (harmony) proxies function makeLogger(obj) { var proxy = Proxy.create({ get: function(rcvr, name) { log(‘get’, name, obj); handler return obj[name]; }, set: function(rcvr, name, val) { meta log(‘set’, name, obj, val); base obj[name] = val; return true; }, ... proxy }, Object.getPrototypeOf(obj)); return proxy; } Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler meta base proxy Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler.get(proxy, ‘foo’) handler meta base proxy.foo proxy Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler.get(proxy, ‘foo’) handler handler.set(proxy, ‘foo’, 42) meta base proxy.foo proxy.foo = 42 proxy Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler.get(proxy, ‘foo’) handler handler.set(proxy, ‘foo’, 42) handler.get(proxy, ‘foo’).apply(proxy,[1,2,3]) meta base proxy.foo proxy.foo = 42 proxy.foo(1,2,3) proxy Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler.get(proxy, ‘foo’) handler handler.set(proxy, ‘foo’, 42) handler.get(proxy, ‘foo’).apply(proxy,[1,2,3]) handler.get(proxy, ‘get’) meta base proxy.foo proxy.foo = 42 proxy.foo(1,2,3) proxy proxy.get Tuesday, September 28, 2010

Stratiﬁed API var proxy = Proxy.create(handler, proto); handler.get(proxy, ‘foo’) handler handler.set(proxy, ‘foo’, 42) handler.get(proxy, ‘foo’).apply(proxy,[1,2,3]) handler.get(proxy, ‘get’) meta base proxy.foo proxy.foo = 42 proxy.foo(1,2,3) proxy proto proxy.get Tuesday, September 28, 2010

Not just property accesses var proxy = Proxy.create(handler, proto); handler meta base proxy Tuesday, September 28, 2010

Not just property accesses var proxy = Proxy.create(handler, proto); handler.has(‘foo’) handler meta base ‘foo’ in proxy proxy Tuesday, September 28, 2010

Not just property accesses var proxy = Proxy.create(handler, proto); handler.has(‘foo’) handler.delete(‘foo’) handler meta base ‘foo’ in proxy delete proxy.foo proxy Tuesday, September 28, 2010

Not just property accesses var proxy = Proxy.create(handler, proto); handler.has(‘foo’) handler.delete(‘foo’) var props = handler.enumerate(); handler for (var i=0;i<props.length;i++) { var prop = props[i]; ... } meta base ‘foo’ in proxy delete proxy.foo proxy for (var prop in proxy) { ... } Tuesday, September 28, 2010

Not just property accesses var proxy = Proxy.create(handler, proto); handler.has(‘foo’) handler.delete(‘foo’) var props = handler.enumerate(); handler for (var i=0;i<props.length;i++) { var prop = props[i]; ... } handler.defineProperty(‘foo’, pd) meta base ‘foo’ in proxy delete proxy.foo proxy for (var prop in proxy) { ... } Object.defineProperty(proxy,‘foo’, pd) Tuesday, September 28, 2010

But not quite everything, either var proxy = Proxy.create(handler, proto); handler meta base proxy Tuesday, September 28, 2010

But not quite everything, either var proxy = Proxy.create(handler, proto); handler meta base proxy === obj proxy Tuesday, September 28, 2010

But not quite everything, either var proxy = Proxy.create(handler, proto); handler meta base proxy === obj Object.getPrototypeOf(proxy) => proto proxy proto Tuesday, September 28, 2010

But not quite everything, either var proxy = Proxy.create(handler, proto); handler meta base proxy === obj Object.getPrototypeOf(proxy) => proto proxy proto proxy instanceof SomeFunction Tuesday, September 28, 2010

But not quite everything, either var proxy = Proxy.create(handler, proto); handler meta base proxy === obj Object.getPrototypeOf(proxy) => proto proxy proto proxy instanceof SomeFunction typeof proxy => “object” Tuesday, September 28, 2010

Full Handler API handler proxy Object.getOwnPropertyDescriptor(proxy) handler.getOwnPropertyDescriptor(name) Object.getPropertyDescriptor(proxy) handler.getPropertyDescriptor(name) Object.defineProperty(proxy,name,pd) handler.defineProperty(name, pd) Object.getOwnPropertyNames(proxy) handler.getOwnPropertyNames() delete proxy.name handler.delete(name) for (name in proxy) { ... } handler.enumerate() Object.{freeze|seal|...}(proxy) handler.fix() name in proxy handler.has(name) ({}).hasOwnProperty.call(proxy, name) handler.hasOwn(name) receiver.name handler.get(receiver, name) receiver.name = val handler.set(receiver, name, val) Object.keys(proxy) handler.keys() for (name in proxy) { ... } handler.iterate() Tuesday, September 28, 2010

Fundamental vs Derived Traps handler proxy Fundamental traps Object.getOwnPropertyDescriptor(proxy) handler.getOwnPropertyDescriptor(name) Object.getPropertyDescriptor(proxy) handler.getPropertyDescriptor(name) Object.defineProperty(proxy,name,pd) handler.defineProperty(name, pd) Object.getOwnPropertyNames(proxy) handler.getOwnPropertyNames() delete proxy.name handler.delete(name) for (name in proxy) { ... } handler.enumerate() -> [string] Object.{freeze|seal|...}(proxy) handler.fix() Derived traps name in proxy handler.has(name) ({}).hasOwnProperty.call(proxy, name) handler.hasOwn(name) receiver.name handler.get(receiver, name) receiver.name = val handler.set(receiver, name, val) Object.keys(proxy) handler.keys() for (name in proxy) { ... } handler.iterate() -> iterator Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); handler call construct funproxy Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); call(1,2,3) handler call construct funproxy(1,2,3) funproxy Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); call(1,2,3) handler call construct construct(1,2,3) funproxy(1,2,3) new funproxy(1,2,3) funproxy Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); call(1,2,3) handler call construct construct(1,2,3) handler.get(funproxy,‘prototype’) funproxy(1,2,3) new funproxy(1,2,3) funproxy.prototype funproxy Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); call(1,2,3) handler call construct construct(1,2,3) handler.get(funproxy,‘prototype’) funproxy(1,2,3) new funproxy(1,2,3) funproxy.prototype funproxy typeof funproxy => “function” Tuesday, September 28, 2010

Function Proxies JavaScript functions are objects. Additionally, they are also callable and constructible var call = function() { ... }; var construct = function() { ... }; var funproxy = Proxy.createFunction(handler, call, construct); call(1,2,3) handler call construct construct(1,2,3) handler.get(funproxy,‘prototype’) funproxy(1,2,3) new funproxy(1,2,3) funproxy.prototype funproxy typeof funproxy => “function” Object.getPrototypeOf(funproxy) => Function.prototype Tuesday, September 28, 2010

Dilemma: Invoke vs Get+Call • Fundamental vs derived traps = tradeoff in performance (method allocation or caching) vs. consistency var p = Proxy.create({ get: function(receiver, name) { ... }, invoke: function(receiver, name, args) { ... }, ... }); p.x; // get(p,'x') p.m(a); // invoke(p, 'm', [a]) • invoke can intercept arguments • but notably, JS methods can be extracted as functions and called later (functional FTW!) • breaks invariant o.m.call(o) <=> o.m() Tuesday, September 28, 2010

Selective Interception meta base object Tuesday, September 28, 2010

Selective Interception VM territory (C++) meta base object JavaScript territory Tuesday, September 28, 2010

Selective Interception VM territory (C++) VM handler meta base object JavaScript territory Tuesday, September 28, 2010

Selective Interception VM territory (C++) VM handler VM handler meta base host object object JavaScript territory Tuesday, September 28, 2010

Selective Interception VM territory (C++) VM handler VM handler handler meta base host object object proxy JavaScript territory Tuesday, September 28, 2010

Selective Interception VM territory (C++) Self-hosted VM handler VM handler handler meta base host object object proxy JavaScript territory Tuesday, September 28, 2010

Example: no-op forwarding proxy handler function ForwardingHandler(obj) { this.target = obj; proxy target } ForwardingHandler.prototype = { has: function(name) { return name in this.target; }, get: function(rcvr,name) { return this.target[name]; }, set: function(rcvr,name,val) { this.target[name]=val;return true; }, delete: function(name) { return delete this.target[name]; } enumerate: function() { var props = []; for (name in this.target) { props.push(name); }; return props; }, ... } var proxy = Proxy.create(new ForwardingHandler(o), Object.getPrototypeOf(o)); Tuesday, September 28, 2010

Example: counting property access function makeSimpleProfiler(target) { var forwarder = new ForwardingHandler(target); var count = Object.create(null); forwarder.get = function(rcvr, name) { count[name] = (count[name] || 0) + 1; return this.target[name]; }; return { proxy: Proxy.create(forwarder, Object.getPrototypeOf(target)), get stats() { return count; } } } Tuesday, September 28, 2010

Example: counting property access function makeSimpleProfiler(target) { var forwarder = new ForwardingHandler(target); var count = Object.create(null); forwarder.get = function(rcvr, name) { count[name] = (count[name] || 0) + 1; return this.target[name]; }; return { proxy: Proxy.create(forwarder, Object.getPrototypeOf(target)), get stats() { return count; } } } var subject = { ... }; var profiler = makeSimpleProfiler(subject); runApp(profiler.proxy); display(profiler.stats); Tuesday, September 28, 2010

Fixing a Proxy var proxy = Proxy.create(handler, proto); handler meta base proxy Tuesday, September 28, 2010

Fixing a Proxy var proxy = Proxy.create(handler, proto); handler meta base Object.freeze(proxy) Object.seal(proxy) proxy Object.preventExtensions(proxy) Tuesday, September 28, 2010

Fixing a Proxy var proxy = Proxy.create(handler, proto); var pdmap = handler.fix(); if (pdmap === undefined) throw TypeError(); handler become(proxy, Object.freeze( Object.create(proto, pdmap))); meta base Object.freeze(proxy) Object.seal(proxy) proxy Object.preventExtensions(proxy) Tuesday, September 28, 2010

Fixing a Proxy var proxy = Proxy.create(handler, proto); var pdmap = handler.fix(); if (pdmap === undefined) throw TypeError(); handler become(proxy, Object.freeze( Object.create(proto, pdmap))); meta base Object.freeze(proxy) Object.seal(proxy) proxy Object.preventExtensions(proxy) ﬁx Trapping Fixed Tuesday, September 28, 2010

Fixing a Proxy var proxy = Proxy.create(handler, proto); var pdmap = handler.fix(); if (pdmap === undefined) throw TypeError(); become(proxy, Object.freeze( Object.create(proto, pdmap))); meta base Object.freeze(proxy) Object.seal(proxy) proxy Object.preventExtensions(proxy) ﬁx Trapping Fixed Tuesday, September 28, 2010

Meta-level Shifting handler proxy Object.getOwnPropertyDescriptor(proxy) handler.getOwnPropertyDescriptor(name) Object.getPropertyDescriptor(proxy) handler.getPropertyDescriptor(name) Object.defineProperty(proxy,name,pd) handler.defineProperty(name, pd) Object.getOwnPropertyNames(proxy) handler.getOwnPropertyNames() delete proxy.name handler.delete(name) for (name in proxy) { ... } handler.enumerate() Object.{freeze|seal|...}(proxy) handler.fix() name in proxy handler.has(name) ({}).hasOwnProperty.call(proxy, name) handler.hasOwn(name) receiver.name handler.get(receiver, name) receiver.name = val handler.set(receiver, name, val) Object.keys(proxy) handler.keys() for (name in proxy) { ... } handler.iterate() base-level: many operations meta-level: all operations reiﬁed as on objects invocations of traps Tuesday, September 28, 2010

Meta-level Shifting a proxy whose μhandler handler is a proxy handler handler.getOwnPropertyDescriptor(name) μhandler.get(handler, ‘getOwnP..’)(name) handler.getPropertyDescriptor(name) μhandler.get(handler, ‘getProp..’)(name) handler.defineOwnProperty(name, pd) μhandler.get(handler, ‘define...’)(name,pd) handler.delete(name) μhandler.get(handler, ‘delete’)(name) handler.getOwnPropertyNames() μhandler.get(handler, ‘getOwnP..’)() handler.enumerate() μhandler.get(handler, ‘enumerate’)() handler.fix() μhandler.get(handler, ‘fix’)() handler.has(name) μhandler.get(handler, ‘has’)(name) handler.hasOwn(name) μhandler.get(handler, ‘hasOwn’)(name) handler.get(receiver, name) μhandler.get(handler, ‘get’)(receiver,name) handler.set(receiver, name, val) μhandler.get(handler, ‘set’)(receiver,name,val) handler.keys() μhandler.get(handler, ‘keys’)() handler.iterate() μhandler.get(handler, ‘iterate’)() meta-level: all operations reiﬁed as meta-meta-level: all operations invocations of traps reiﬁed as invocations of ‘get’ trap Tuesday, September 28, 2010

Example: Membranes • Firefox security wrappers (anti-XSS, XUL, etc.) • Google Caja: capability-secure subset • Object-capability model: an object is powerless unless given a reference to other objects • References can be made revocable through a membrane Tuesday, September 28, 2010

Example: Membranes function makeSimpleMembrane(initTarget) { var enabled = true; } Tuesday, September 28, 2010

Example: Membranes function makeSimpleMembrane(initTarget) { var enabled = true; return { wrapper: wrap(initTarget), revoke: function() { enabled = false; } }; } Tuesday, September 28, 2010

Example: Membranes function makeSimpleMembrane(initTarget) { var enabled = true; function wrap(target) { if (Object.isPrimitive(target)) { return target; } var baseHandler = new ForwardingHandler(target); var revokeHandler = Proxy.create({ get: function(rcvr, name) { return wrapFunction(baseHandler[name]); } }); } return { wrapper: wrap(initTarget), revoke: function() { enabled = false; } }; } Tuesday, September 28, 2010

Example: Membranes function makeSimpleMembrane(initTarget) { var enabled = true; function wrap(target) { if (Object.isPrimitive(target)) { return target; } var baseHandler = new ForwardingHandler(target); var revokeHandler = Proxy.create({ get: function(rcvr, name) { return wrapFunction(baseHandler[name]); } }); if (typeof target === “function”) { return Proxy.createFunction(revokeHandler, wrapFunction(target)); } return Proxy.create(revokeHandler, wrap(Object.getPrototypeOf(target))); } return { wrapper: wrap(initTarget), revoke: function() { enabled = false; } }; } Tuesday, September 28, 2010

Example: Membranes function makeSimpleMembrane(initTarget) { var enabled = true; function wrapFunction(f) { return function() { // variable-argument function if (!enabled) { throw new Error("revoked"); } return wrap(f.apply(wrap(this), arguments.map(wrap))); } } function wrap(target) { if (Object.isPrimitive(target)) { return target; } var baseHandler = new ForwardingHandler(target); var revokeHandler = Proxy.create({ get: function(rcvr, name) { return wrapFunction(baseHandler[name]); } }); if (typeof target === “function”) { return Proxy.createFunction(revokeHandler, wrapFunction(target)); } return Proxy.create(revokeHandler, wrap(Object.getPrototypeOf(target))); } return { wrapper: wrap(initTarget), revoke: function() { enabled = false; } }; } Tuesday, September 28, 2010

Prior Work handler meta base proxy Tuesday, September 28, 2010

Prior Work handler mirror ProxyHandler InvocationHandler meta base proxy mirage proxy java.lang.reﬂect.Proxy Tuesday, September 28, 2010

Prior Work handler mirror ProxyHandler InvocationHandler meta base proxy mirage proxy java.lang.reﬂect.Proxy # traps 13 30 3 1 Tuesday, September 28, 2010

Making JavaScript Extensible • Extending JavaScript today: “Host objects” (the IE DOM; anything implemented in C++) • Proxies are sufﬁciently powerful to emulate most of the behavior of host objects in JavaScript itself • Two possible avenues to close the gap: • Make proxies even more powerful • Make host objects only as powerful as proxies Tuesday, September 28, 2010

Status • Presented at ECMA TC-39 meetings • Approved for inclusion in ES-Harmony • http://wiki.ecmascript.org/doku.php? id=harmony:proxies • In Firefox 4 already, thanks to Andreas Gal! • The basis of all of Gecko’s security wrappers • Used by Zaphod (Narcissus as JS engine add-on, source at http://github.com/taustin/Zaphod/) Tuesday, September 28, 2010

Lessons for Web Standards • Standards need savvy academic research • Standards must evolve quickly on the Web • They can’t evolve without prototype trials • These experiments need tons of user-testing • To reach users at scale, prototypes must ship • Ecma TC39 committed to prototyping specs before ﬁnalizing standards • Committee members work together, no blind-siding, to uphold Harmony (it’s social!) Tuesday, September 28, 2010

Micro-benchmark Results Tuesday, September 28, 2010

Micro-benchmark: Overhead Tuesday, September 28, 2010

Proxies: Summary • Proxies enable: • Generic wrappers: access control, profiling, adaptors, test injection, etc. • Virtual objects: persistent objects, remote objects, emulated host objects, ... • API: • Robust: stratified, not all operations intercepted • Secure: can’t trap non-proxy or fixed objects • Performance: no overhead for non-proxy objects Tuesday, September 28, 2010

Conclusions • ES5 provides new meta-programming APIs • ES-Harmony Proxies: robust dynamic meta- programming for virtual objects, wrappers • Proxies help put developers in control of extending JavaScript, instead of Ecma TC39 • JavaScript: the Revenge of Smalltalk! Tuesday, September 28, 2010

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Proxies are Awesome!

by Brendan Eich on Sep 28, 2010

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Proxies are Awesome! — Presentation Transcript