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:

var handler = {
  get: function(target, name) {
    return name in target ? target[name] : 42;
}};
var 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.

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.

var revocable = Proxy.revocable({}, {
  get: function(target, name) {
    return '[[' + name + ']]';
  }
});
var 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
}