Meta programming
Starting with ECMAScript 2015, JavaScript gains support for the Proxy
and Reflect
objects allowing you to intercept and define custom behavior for fundamental language operations (e.g. property lookup, assignment, enumeration, function invocation, etc). With the help of these two objects you are able to program at the meta level of JavaScript.
Proxies
Introduced in ECMAScript 6, Proxy
objects allow you to intercept certain operations and to implement custom behaviors.
For example, getting a property on an object:
let handler = {
get: function(target, name) {
return name in target ? target[name] : 42
}
}
let p = new Proxy({}, handler)
p.a = 1
console.log(p.a, p.b) // 1, 42
The Proxy
object defines a target
(an empty object here) and a handler
object, in which a get
trap is implemented. Here, an object that is proxied will not return undefined
when getting undefined properties, but will instead return the number 42
.
Additional examples are available on the Proxy
reference page.
Terminology
The following terms are used when talking about the functionality of proxies.
- handler
-
Placeholder object which contains traps.
- traps
-
The methods that provide property access. (This is analogous to the concept of traps in operating systems.)
- target
-
Object which the proxy virtualizes. It is often used as storage backend for the proxy. Invariants (semantics that remain unchanged) regarding object non-extensibility or non-configurable properties are verified against the target.
- invariants
-
Semantics that remain unchanged when implementing custom operations are called invariants. If you violate the invariants of a handler, a
TypeError
will be thrown.
Handlers and traps
The following table summarizes the available traps available to Proxy
objects. See the reference pages for detailed explanations and examples.
Handler / trap | Interceptions | Invariants |
---|---|---|
handler.getPrototypeOf() |
Object.getPrototypeOf() Reflect.getPrototypeOf() __proto__ Object.prototype.isPrototypeOf() instanceof |
|
handler.setPrototypeOf() |
Object.setPrototypeOf() Reflect.setPrototypeOf() |
If target is not extensible, the
prototype parameter must be the same value as
Object.getPrototypeOf(target) .
|
handler.isExtensible() |
Object.isExtensible() Reflect.isExtensible() |
Object.isExtensible(proxy) must return the same
value as Object.isExtensible(target) .
|
handler.preventExtensions() |
Object.preventExtensions() Reflect.preventExtensions() |
Object.preventExtensions(proxy) only returns
true if
Object.isExtensible(proxy) is
false .
|
handler.getOwnPropertyDescriptor() |
Object.getOwnPropertyDescriptor() Reflect.getOwnPropertyDescriptor() |
|
handler.defineProperty() |
Object.defineProperty() Reflect.defineProperty() |
|
handler.has() |
|
|
handler.get() |
|
|
handler.set() |
|
|
handler.deleteProperty() |
|
A property cannot be deleted if it exists as a non-configurable own
property of target .
|
handler.enumerate() |
|
The enumerate method must return an object. |
handler.ownKeys() |
Object.getOwnPropertyNames() Object.getOwnPropertySymbols() Object.keys() Reflect.ownKeys() |
|
handler.apply() |
proxy(..args) Function.prototype.apply() and
Function.prototype.call() Reflect.apply()
|
There are no invariants for the
handler.apply method.
|
handler.construct() |
new proxy(...args) Reflect.construct() |
The result must be an Object . |
Revocable Proxy
The Proxy.revocable()
method is used to create a revocable Proxy
object. This means that the proxy can be revoked via the function revoke
and switches the proxy off.
Afterwards, any operation on the proxy leads to a TypeError
.
let revocable = Proxy.revocable({}, {
get: function(target, name) {
return '[[' + name + ']]'
}
})
let proxy = revocable.proxy
console.log(proxy.foo) // "[[foo]]"
revocable.revoke()
console.log(proxy.foo) // TypeError is thrown
proxy.foo = 1 // TypeError again
delete proxy.foo // still TypeError
typeof proxy // "object", typeof doesn't trigger any trap
Reflection
Reflect
is a built-in object that provides methods for interceptable JavaScript operations. The methods are the same as those of the proxy handlers.
Reflect
is not a function object.
Reflect
helps with forwarding default operations from the handler to the target
.
With Reflect.has()
for example, you get the in
operator as a function:
Reflect.has(Object, 'assign') // true
A better apply
function
In ES5, you typically use the Function.prototype.apply()
method to call a function with a given this
value and arguments
provided as an array (or an array-like object).
Function.prototype.apply.call(Math.floor, undefined, [1.75])
With Reflect.apply
this becomes less verbose and easier to understand:
Reflect.apply(Math.floor, undefined, [1.75])
// 1
Reflect.apply(String.fromCharCode, undefined, [104, 101, 108, 108, 111])
// "hello"
Reflect.apply(RegExp.prototype.exec, /ab/, ['confabulation']).index
// 4
Reflect.apply(''.charAt, 'ponies', [3])
// "i"
Checking if property definition has been successful
With Object.defineProperty
, which returns an object if successful, or throws a TypeError
otherwise, you would use a try...catch
block to catch any error that occurred while defining a property. Because Reflect.defineProperty
returns a Boolean success status, you can just use an if...else
block here:
if (Reflect.defineProperty(target, property, attributes)) {
// success
} else {
// failure
}