ES6 Proxy
Information drawn from
The Proxy object enables you to create a proxy for another object, which can intercept and redefine fundamental operations for that object.
Description
A Proxy is created with two parameters:
- target: the original object which you want to proxy
- handler: an object that defines which operations will be intercepted and how to redefine intercepted operations.
For example, this code defines a simple target with just two properties, and an even simpler handler with no properties:
const target = {
message1: "hello",
message2: "everyone"
};
const handler1 = {};
const proxy1 = new Proxy(target, handler1);
Because the handler is empty, this proxy behaves just like the original target:
console.log(proxy1.message1); // hello
console.log(proxy1.message2); // everyone
To customise the proxy, we define functions on the handler object:
const target = {
message1: "hello",
message2: "everyone"
};
const handler2 = {
get: function(target, prop, receiver) {
return "world";
}
};
const proxy2 = new Proxy(target, handler2);
Here we’ve provided an implementation of the get() handler, which intercepts attempts to access properties in the target.
Handler functions are sometimes called traps, presumably because they trap calls to the target object. The very simple trap in handler2 above redefines all property accessors:
console.log(proxy2.message1); // world
console.log(proxy2.message2); // world
With the help of the Reflect class we can give some accessors the original behavior and redefine others:
const target = {
message1: "hello",
message2: "everyone"
};
const handler3 = {
get: function (target, prop, receiver) {
if (prop === "message2") {
return "world";
}
return Reflect.get(...arguments);
},
};
const proxy3 = new Proxy(target, handler3);
console.log(proxy3.message1); // hello
console.log(proxy3.message2); // world
## Constructor
Proxy() Creates a new Proxy object.
Static methods
Proxy.revocable() Creates a revocable Proxy object.
Examples
//////////
Basic example
In this simple example, the number 37 gets returned as the default value when the property name is not in the object. It is using the get() handler.
const handler = {
get: function(obj, prop) {
return prop in obj ?
obj[prop] :
37;
}
};
const p = new Proxy({}, handler);
p.a = 1;
p.b = undefined;
console.log(p.a, p.b);
// 1, undefined
console.log('c' in p, p.c);
// false, 37
No-op forwarding proxy
In this example, we are using a native JavaScript object to which our proxy will forward all operations that are applied to it.
const target = {};
const p = new Proxy(target, {});
p.a = 37;
// operation forwarded to the target
console.log(target.a);
// 37
// (The operation has been properly forwarded!)
Note that while this “no-op” works for JavaScript objects, it does not work for native browser objects like DOM Elements.
Validation
With a Proxy, you can easily validate the passed value for an object. This example uses the set() handler.
let validator = {
set: function(obj, prop, value) {
if (prop === 'age') {
if (!Number.isInteger(value)) {
throw new TypeError('The age is not an integer');
}
if (value > 200) {
throw new RangeError('The age seems invalid');
}
}
// The default behavior to store the value
obj[prop] = value;
// Indicate success
return true;
}
};
const person = new Proxy({}, validator);
person.age = 100;
console.log(person.age); // 100
person.age = 'young'; // Throws an exception
person.age = 300; // Throws an exception
Extending constructor
A function proxy could easily extend a constructor with a new constructor. This example uses the construct() and apply() handlers.
function extend(sup, base) {
var descriptor = Object.getOwnPropertyDescriptor(
base.prototype, 'constructor'
);
base.prototype = Object.create(sup.prototype);
var handler = {
construct: function(target, args) {
var obj = Object.create(base.prototype);
this.apply(target, obj, args);
return obj;
},
apply: function(target, that, args) {
sup.apply(that, args);
base.apply(that, args);
}
};
var proxy = new Proxy(base, handler);
descriptor.value = proxy;
Object.defineProperty(base.prototype, 'constructor', descriptor);
return proxy;
}
var Person = function(name) {
this.name = name;
};
var Boy = extend(Person, function(name, age) {
this.age = age;
});
Boy.prototype.gender = 'M';
var Peter = new Boy('Peter', 13);
console.log(Peter.gender); // "M"
console.log(Peter.name); // "Peter"
console.log(Peter.age); // 13
### Manipulating DOM nodes
Sometimes you want to toggle the attribute or class name of two different elements. Here’s how using the set() handler.
let view = new Proxy({
selected: null
},
{
set: function(obj, prop, newval) {
let oldval = obj[prop];
if (prop === 'selected') {
if (oldval) {
oldval.setAttribute('aria-selected', 'false');
}
if (newval) {
newval.setAttribute('aria-selected', 'true');
}
}
// The default behavior to store the value
obj[prop] = newval;
// Indicate success
return true;
}
});
let i1 = view.selected = document.getElementById('item-1'); //giving error here, i1 is null
console.log(i1.getAttribute('aria-selected'));
// 'true'
let i2 = view.selected = document.getElementById('item-2');
console.log(i1.getAttribute('aria-selected'));
// 'false'
console.log(i2.getAttribute('aria-selected'));
// 'true'
Note: even if selected: !null, then giving oldval.setAttribute is not a function
Value correction and an extra property
The products proxy object evaluates the passed value and converts it to an array if needed. The object also supports an extra property called latestBrowser both as a getter and a setter.
let products = new Proxy({
browsers: ['Internet Explorer', 'Netscape']
},
{
get: function(obj, prop) {
// An extra property
if (prop === 'latestBrowser') {
return obj.browsers[obj.browsers.length - 1];
}
// The default behavior to return the value
return obj[prop];
},
set: function(obj, prop, value) {
// An extra property
if (prop === 'latestBrowser') {
obj.browsers.push(value);
return true;
}
// Convert the value if it is not an array
if (typeof value === 'string') {
value = [value];
}
// The default behavior to store the value
obj[prop] = value;
// Indicate success
return true;
}
});
console.log(products.browsers);
// ['Internet Explorer', 'Netscape']
products.browsers = 'Firefox';
// pass a string (by mistake)
console.log(products.browsers);
// ['Firefox'] <- no problem, the value is an array
products.latestBrowser = 'Chrome';
console.log(products.browsers);
// ['Firefox', 'Chrome']
console.log(products.latestBrowser);
// 'Chrome'
Finding an array item object by its property
This proxy extends an array with some utility features. As you see, you can flexibly “define” properties without using Object.defineProperties(). This example can be adapted to find a table row by its cell. In that case, the target will be table.rows.
let products = new Proxy([
{ name: 'Firefox', type: 'browser' },
{ name: 'SeaMonkey', type: 'browser' },
{ name: 'Thunderbird', type: 'mailer' }
],
{
get: function(obj, prop) {
// The default behavior to return the value; prop is usually an integer
if (prop in obj) {
return obj[prop];
}
// Get the number of products; an alias of products.length
if (prop === 'number') {
return obj.length;
}
let result, types = {};
for (let product of obj) {
if (product.name === prop) {
result = product;
}
if (types[product.type]) {
types[product.type].push(product);
} else {
types[product.type] = [product];
}
}
// Get a product by name
if (result) {
return result;
}
// Get products by type
if (prop in types) {
return types[prop];
}
// Get product types
if (prop === 'types') {
return Object.keys(types);
}
return undefined;
}
});
console.log(products[0]); // { name: 'Firefox', type: 'browser' }
console.log(products['Firefox']); // { name: 'Firefox', type: 'browser' }
console.log(products['Chrome']); // undefined
console.log(products.browser); // [{ name: 'Firefox', type: 'browser' }, { name: 'SeaMonkey', type: 'browser' }]
console.log(products.types); // ['browser', 'mailer']
console.log(products.number); // 3
A complete traps list example
Now in order to create a complete sample traps list, for didactic purposes, we will try to proxify a non-native object that is particularly suited to this type of operation: the docCookies global object created by a simple cookie framework.
/*
var docCookies = ... get the "docCookies" object here:
https://reference.codeproject.com/dom/document/cookie/simple_document.cookie_framework
*/
var docCookies = new Proxy(docCookies, {
get: function (oTarget, sKey) {
return oTarget[sKey] || oTarget.getItem(sKey) || undefined;
},
set: function (oTarget, sKey, vValue) {
if (sKey in oTarget) { return false; }
return oTarget.setItem(sKey, vValue);
},
deleteProperty: function (oTarget, sKey) {
if (!sKey in oTarget) { return false; }
return oTarget.removeItem(sKey);
},
enumerate: function (oTarget, sKey) {
return oTarget.keys();
},
ownKeys: function (oTarget, sKey) {
return oTarget.keys();
},
has: function (oTarget, sKey) {
return sKey in oTarget || oTarget.hasItem(sKey);
},
defineProperty: function (oTarget, sKey, oDesc) {
if (oDesc && 'value' in oDesc) { oTarget.setItem(sKey, oDesc.value); }
return oTarget;
},
getOwnPropertyDescriptor: function (oTarget, sKey) {
var vValue = oTarget.getItem(sKey);
return vValue ? {
value: vValue,
writable: true,
enumerable: true,
configurable: false
} : undefined;
},
});
/* Cookies test */
console.log(docCookies.my_cookie1 = 'First value');
console.log(docCookies.getItem('my_cookie1'));
docCookies.setItem('my_cookie1', 'Changed value');
console.log(docCookies.my_cookie1);
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Last update on 01 Dec 2021
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