Proxy and Reflect
Miscellaneous: Proxy and Reflect
What is a Proxy in JavaScript?
View Answer:
Here is a simple example of how a Proxy can be used to intercept the get operation on an object...
let target = {
message: 'hello, world'
};
let handler = {
get: function(target, prop, receiver) {
console.log(`GET was called for property ${prop}`);
return Reflect.get(...arguments);
}
};
let proxy = new Proxy(target, handler);
console.log(proxy.message); // logs: 'GET was called for property message' then logs: 'hello, world'
In this example, whenever a property is accessed on the proxy object, it logs a message to the console and then proceeds with the normal operation. The actual get operation is performed using Reflect.get(), which is a built-in function that performs a property access operation.
Proxies are a powerful tool that allow developers to control and redefine fundamental JavaScript operations. However, they should be used carefully, because they can greatly increase the complexity of your code and make debugging more difficult. They are best used for meta-programming tasks, or where you need to handle some complex object-access logic.
What are some common use cases for Proxies?
View Answer:
Data Validation: Proxies can be used to validate incoming data before setting it to an object's properties.
let handler = {
set: function(obj, prop, value) {
if (prop === 'age') {
if (!Number.isInteger(value)) {
throw new TypeError('The age is not an integer');
}
if (value <= 0) {
throw new RangeError('The age must be a positive integer');
}
}
// The default behavior to store the value
obj[prop] = value;
// Indicate success
return true;
}
};
let person = new Proxy({}, handler);
person.age = 100;
console.log(person.age); // 100
How do you create a Proxy in JavaScript?
View Answer:
Here's the basic syntax:
let proxy = new Proxy(target, handler);
Here's a detailed example:
let target = {
name: "John"
};
let handler = {
get: function(target, prop, receiver) {
console.log(`Property "${prop}" has been read.`);
return Reflect.get(...arguments);
},
set: function(target, prop, value, receiver) {
console.log(`Property "${prop}" is being set to "${value}".`);
return Reflect.set(...arguments);
}
};
let proxy = new Proxy(target, handler);
proxy.name; // Outputs: Property "name" has been read.
proxy.age = 25; // Outputs: Property "age" is being set to "25".
In this example, the handler object defines two traps:
- The
gettrap is called when a property on theproxyobject is read. It logs a message to the console and then uses theReflect.get()method to perform the default get operation. - The
settrap is called when a property on theproxyobject is set. It logs a message to the console and then uses theReflect.set()method to perform the default set operation.
Note: Reflect.get() and Reflect.set() are built-in functions that perform the default get and set operations respectively.
Can you explain the function of the proxy object in JavaScript?
View Answer:
Syntax: let proxy = new Proxy(target, handler);
let target = {};
let proxy = new Proxy(target, {}); // empty handler
proxy.test = 5; // writing to proxy (1)
console.log(target.test); // 5, the property appeared in target!
console.log(proxy.test); // 5, we can read it from proxy too (2)
for (let key in proxy) console.log(key); // test, iteration works (3)
What is a trap in the context of a Proxy?
View Answer:
What can we intercept with a JavaScript proxy trap?
View Answer:
let numbers = [0, 1, 2];
numbers = new Proxy(numbers, {
get(target, prop) {
// trapping [[Get]] internal method here
if (prop in target) {
return target[prop];
} else {
return 0; // default value
}
},
});
console.log(numbers[1]); // 1
console.log(numbers[4]); // 0 (no such item)
Does the proxy overwrite the variable when you implement a proxy?
View Answer:
let dictionary = {
Hello: 'Hola',
Bye: 'Adiós',
};
dictionary = new Proxy(dictionary, {
get(target, phrase) {
// intercept reading a property from dictionary
if (phrase in target) {
// if we have it in the dictionary
return target[phrase]; // return the translation
} else {
// otherwise, return the non-translated phrase
return phrase;
}
},
});
// Look up arbitrary phrases in the dictionary!
// At worst, they're not translated.
console.log(dictionary['Hello']); // Hola
console.log(dictionary['Welcome to Proxy']); // Welcome to Proxy (no translation)
Note: You should ever make a reference to the target object after it has been proxied. Otherwise, it can be easy to make mistakes and harder to debug.
Can you explain the function of the proxy set method?
View Answer:
const p = new Proxy(target, {
set: function (target, property, value, receiver) {},
});
let numbers = [];
numbers = new Proxy(numbers, {
// (*)
set(target, prop, val) {
// to intercept property writing
if (typeof val == 'number') {
target[prop] = val;
return true;
} else {
return false;
}
},
});
numbers.push(1); // added successfully
numbers.push(2); // added successfully
console.log('Length is: ' + numbers.length); // 2
numbers.push('test'); // TypeError ('set' on proxy returned false)
console.log('This line is never reached (error in the line above)');
What is meant by invariants in JavaScript proxies?
View Answer:
Here are a few examples of such invariants:
1. Non-configurable, non-writable properties: If an object has a property that's non-configurable and non-writable, the set trap for a Proxy must not change the value of that property, or else JavaScript will throw a TypeError.
let target = {};
Object.defineProperty(target, 'prop', {
value: 1,
writable: false,
configurable: false
});
let handler = {
set: function(target, prop, value, receiver) {
target[prop] = value;
return true;
}
};
let proxy = new Proxy(target, handler);
proxy.prop = 2; // TypeError
2. Non-extensibility and defining properties: If an object is non-extensible, you can't add new properties to it. Hence, the defineProperty trap for a Proxy should not successfully add new properties to a non-extensible target object, or else JavaScript will throw a TypeError.
let target = {};
Object.preventExtensions(target);
let handler = {
defineProperty: function(target, prop, descriptor) {
return Reflect.defineProperty(target, prop, descriptor);
}
};
let proxy = new Proxy(target, handler);
Object.defineProperty(proxy, 'prop', { value: 1 }); // TypeError
3. Property descriptors and non-configurability: If a property is non-configurable on a target object, then the getOwnPropertyDescriptor trap must return a descriptor that says the property is non-configurable. If it says the property is configurable, JavaScript will throw a TypeError.
These are just a few examples. The main point is that JavaScript expects certain rules to be respected in the implementation of Proxy handlers, and if these rules are not respected, a TypeError will be thrown.
Is there a way to iterate over an object’s keys using a method that uses the [[OwnPropertyKeys]] internal method?
View Answer:
let user = {
name: 'John',
age: 30,
_password: '***',
};
user = new Proxy(user, {
ownKeys(target) {
return Object.keys(target).filter((key) => !key.startsWith('_'));
},
});
// "ownKeys" filters out _password
for (let key in user) console.log(key); // name, then: age
// same effect on these methods:
console.log(Object.keys(user)); // name,age
console.log(Object.values(user)); // John,30
Do private (#) properties and methods require proxies in a class?
View Answer:
class MyClass {
#privateField = 'Hello, world';
#privateMethod() {
return 'Private method has been called';
}
callPrivateMethod() {
return this.#privateMethod();
}
getPrivateField() {
return this.#privateField;
}
}
let instance = new MyClass();
console.log(instance.callPrivateMethod()); // logs: 'Private method has been called'
console.log(instance.getPrivateField()); // logs: 'Hello, world'
In this example, #privateField and #privateMethod are private to instances of MyClass. They can't be accessed or called from outside the class. The public methods callPrivateMethod and getPrivateField provide a way to interact with the private field and method.
Proxies could be used if you wanted to customize the behavior of property access or method invocation on an object, including private properties or methods. However, please note that as of my knowledge cutoff in September 2021, private fields are not accessible or interceptable by JavaScript proxies, as per the language design. The main use case of a Proxy is to define custom behavior for fundamental operations on objects, such as property lookups or function invocations.
What is the benefit of using a Proxy vs. a wrapper function?
View Answer:
function delay(f, ms) {
return new Proxy(f, {
apply(target, thisArg, args) {
setTimeout(() => target.apply(thisArg, args), ms);
}
});
}
function sayHi(user) {
console.log(`Hello, ${user}!`);
}
sayHi = delay(sayHi, 3000);
console.log(sayHi.length); // this would be 0 if we used a wrapper function
// return 1 (*) proxy forwards "get length" operation to the target
sayHi("John"); // Hello, John! (after 3 seconds)
What is the Reflect API in JavaScript?
View Answer:
Can you explain the function of the Reflect built-in JavaScript object?
View Answer:
// Reflect get() method
const object1 = {
x: 1,
y: 2,
};
console.log(Reflect.get(object1, 'x'));
// expected output: 1
const array1 = ['zero', 'one'];
console.log(Reflect.get(array1, 1));
// expected output: "one"
// Reflect set() method
let user = {};
Reflect.set(user, 'name', 'John');
console.log(user.name); // John
What are some common methods provided by the Reflect API?
View Answer:
How does the Reflect API relate to Proxies in JavaScript?
View Answer:
let target = {
message: 'hello, world'
};
let handler = {
get: function(target, prop, receiver) {
console.log(`GET was called for property ${prop}`);
return Reflect.get(...arguments); // Use Reflect API to perform the default operation
}
};
let proxy = new Proxy(target, handler);
console.log(proxy.message); // logs: 'GET was called for property message' then logs: 'hello, world'
In this example, the handler object defines a get method that intercepts all get operations on the proxy object. It logs a message and then uses Reflect.get() to perform the default get operation.
Because every method in the Reflect API corresponds to a Proxy handler method, you can use the Reflect API to easily fallback to the default behavior within a Proxy handler.
Furthermore, using Reflect methods in Proxy handlers helps to maintain the invariants of the JavaScript language, preventing possible TypeErrors that could be thrown if these invariants are violated.
Does the JavaScript proxy built-in object have any limitations?
View Answer:
let map = new Map();
let proxy = new Proxy(map, {});
proxy.set('test', 1); // Error
Built-in Array does not use internal slots. That is for historical reasons, as it appeared so long ago. So, there is no such problem when proxying an array.
Is it possible to proxy an inherited class with private fields in JavaScript?
View Answer:
// Example: Problem
class User {
#name = 'Guest';
getName() {
return this.#name;
}
}
let user = new User();
user = new Proxy(user, {});
console.log(user.getName()); // Error
// Example: Solution
class User {
#name = 'Guest';
getName() {
return this.#name;
}
}
let user = new User();
user = new Proxy(user, {
get(target, prop, receiver) {
let value = Reflect.get(...arguments);
return typeof value == 'function' ? value.bind(target) : value;
},
});
console.log(user.getName()); // Guest
The solution has drawbacks. It exposes the original object to the method, potentially allowing it to be passed further and breaking other proxied functionality.
Can proxies intercept an object used in a strict equality test?
View Answer:
let target = { message: 'hello, world' };
let handler = {
get: function(target, prop, receiver) {
console.log(`GET was called for property ${prop}`);
return Reflect.get(...arguments);
}
};
let proxy = new Proxy(target, handler);
console.log(proxy === target); // logs: false
In this example, the Proxy (proxy) is not strictly equal to the target object (target), and no message is logged to the console because the strict equality operation doesn't trigger the get trap.
Proxies can intercept a range of operations, including property lookup, assignment, function invocation, and more. However, some operations like strict equality check and identity (===) comparison can't be intercepted. It's worth noting that two distinct Proxy instances for the same target are also not strictly equal to each other, because they are different objects.
What is a revocable JavaScript proxy?
View Answer:
Syntax: let {proxy, revoke} = Proxy.revocable(target, handler);
const target = {
name: "John",
age: 30
};
const { proxy, revoke } = Proxy.revocable(target, {});
console.log(proxy.name); // Output: "John"
console.log(proxy.age); // Output: 30
revoke();
console.log(proxy.name); // Output: TypeError: Cannot perform 'get' on a proxy that has been revoked
console.log(proxy.age); // Output: TypeError: Cannot perform 'get' on a proxy that has been revoked
In the above example, we create a revocable proxy using Proxy.revocable(). The proxy variable represents the proxy object, which behaves like the target object. We can access its properties and methods. The revoke() function is used to revoke the proxy, making it inactive. Once revoked, any further access to the proxy will result in a TypeError.
Why would you use a WeakMap when attempting to revoke a proxy?
View Answer:
let revokes = new WeakMap();
let object = {
data: 'Valuable data',
};
let { proxy, revoke } = Proxy.revocable(object, {});
revokes.set(proxy, revoke);
// ..somewhere else in our code..
revoke = revokes.get(proxy);
revoke();
console.log(proxy.data); // Error (revoked)
Can you use a Proxy to create a read-only object?
View Answer:
const target = { message: 'Hello, world' };
const handler = {
set: function(target, prop, value) {
console.log(`Set operation is not allowed on property "${prop}"`);
return true;
}
};
const proxy = new Proxy(target, handler);
console.log(proxy.message); // Outputs: 'Hello, world'
proxy.message = 'Goodbye, world'; // Outputs: 'Set operation is not allowed on property "message"'
console.log(proxy.message); // Still outputs: 'Hello, world'
In this example, the set trap in the handler prevents changes to the target object. When you try to set the message property on the proxy object, it just logs a message to the console and does not change the property.
The set handler returns true to indicate that the assignment was "successful", even though it didn't actually change anything. If it returned false or threw an error, that would indicate that the assignment failed, which could cause issues if the Proxy is used in strict mode.
Remember, this will make all properties of the target object read-only through the proxy, even if they are writable on the target object itself. You can still change the target object directly, because the Proxy doesn't affect the actual target object, it only controls access to it.