I want user to only set specific properties to an object but as the same time that object should be constructed from custom class.
For example
var row = new Row({ name : 'John Doe', email : '[email protected]' }, Schema);
row
can have methods. But when user is trying to set row.password
, they are not allowed.
One way to do it is using new Proxy
instead of new Row
but then we will loose all cool things we are doing inside Row
class. I want new Row
to return a proxy object with this
reference as a target of proxy.
Anybody have any ideas on this? If you know mongoose
, how mongoose
is doing it?
ES6 implements intercession form of meta programming using Proxies. Similar to ReflectAPI, the Proxy API is another way of implementing meta programming in ES6. The Proxy object is used to define custom behavior for fundamental operations. A proxy object performs some operations on behalf of the real object.
A constructor is a function that is called each time an object is created (also referred to as instantiated). The User constructor creates the properties of the object (this.name, this. age, this. email) and assigns them the value of the parameters passed to it (name, age, email).
A proxy allows you to perform meta-programming operations such as intercepting a call to inspect or change an object's property. The original object the proxy will virtualize.
So Proxy is a wrapper which can be used to intercept fundamental operations like [[Get]] and [[Set]] on an object whereas Reflect provides us minimal wrappers around these fundamental operations like [[Get]] and [[Set]] so that we can call them directly (Usually from inside the trap).
If the proxy is certain to happen for you, one possible solution to limit the set functionality is returning an ES6 Proxy instance.
By default, the constructor in javascript returns this
object automatically but you could define and return a custom behavior by instantiating a proxy on this
as a target. Keep in mind that the set method in proxy should return a boolean value.
MDN: The set method should return a boolean value. Return true to indicate that assignment succeeded. If the set method returns false, and the assignment happened in strict-mode code, a TypeError will be thrown.
class Row { constructor(entry) { // some stuff return new Proxy(this, { set(target, name, value) { let setables = ['name', 'email']; if (!setables.includes(name)) { throw new Error(`Cannot set the ${name} property`); } else { target[name] = value; return true; } } }); } get name() { return this._name; } set name(name) { this._name = name.trim(); } get email() { return this._email; } set email(email) { this._email = email.trim(); } }
So, now you are not allowed to set the non-setable properties according to the proxy.
let row = new Row({ name : 'John Doe', email : '[email protected]' }); row.password = 'blahblahblah'; // Error: Cannot set the password property
It's also possible to have s custom behavior on get method too.
However, beware and take care of overriding the reference that is returned to the calling context.
Note: The sample code has been tested on Node v8.1.3 and modern browsers.
You can do this without using Proxies at all.
In your class constructor you can define the password property like this:
constructor(options, schema) { this.name = options.name; this.email = options.email; Object.defineProperty(this, 'password', { configurable: false, // no re-configuring this.password enumerable: true, // this.password should show up in Object.keys(this) value: options.password, // set the value to options.password writable: false // no changing the value with this.password = ... }); // whatever else you want to do with the Schema }
You can find more information about how to use this on the MDN's Object.defineProperty()
page.
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