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May 31
updated on July 3
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Today: yet another ES6 feature that I think most people don’t know about.

“The Proxy object is used to define custom behavior for fundamental operations (e.g. property lookup, assignment, enumeration, function invocation, etc).”

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy
Photo by Alina Grubnyak on Unsplash

Let’s dive right in! 🏄‍♂️

Basic example:

const handler = {
    get: (obj, key) => key in obj ? obj[key] : 42
}

const proxy = new Proxy({a: 1}, handler)

console.log(proxy.a, proxy.b) // 1 42

We start with a plain object {a: 1} and we assign it a handler. Our handler intercepts the get statement of a normal object and we can define our own custom logic for it. In this case we return the value if it exists, otherwise a 42.

We can se that calling proxy.b will output the 42, as it is not set in the root object.

The handler

The second argument of the Proxy function is what is called the handler. This is simply an object with functions that define the logic of the operations, which are called traps (as they get triggered). The available traps are as follows:

  • get
  • set
  • has
  • deleteProperty
  • ownKeys
  • apply
  • construct

There are a few more, but these are the basic ones. Here is the full list.

Some practical examples

var handler = {
	set: (obj, key, value) => {
		if (key === 'wheels')
			if (!Number.isInteger(value) || value % 2 !== 0)
				throw new Error('Wheels need to come in pairs')
		obj[key] = value
	}
}

// 🚗
var car = new Proxy({}, handler)

car.wheels = 4
car.wheels = 3 // Throws an error

Here we can see that we can define constrains for the setter. In this case we say that the key wheels needs to be and integer and a multiple of 2. This is a trivial example, but we can imagine how useful this can be with forms.

const validators = {
	username: /^[A-z]{3,}$/,
	password: /(?=.*[\d]).{8,}/
}

const handler = {
	set: (obj, key, value) => {
		if (key in validators && validators[key].test(value))
			obj[key] = value
		else
			throw new Error('Invalid input')
	}
}

const form = new Proxy({}, handler)

form.username = 'abc'
form.username = 'this contains spaces' // -> Error

form.password = 'atleast1digit'
form.password = 'withoutanydigitthiswillthrowanerror' // -> Error

form.something = true // -> Error, since it does not exist in the validator object

In this example we can validate an object from a form for example. We have a validator object that contains some regex. The Proxy then only allows keys that are inside of the validator and only if they pass the corresponding regex. Vary handy 👌

In real life

This only available in ES6 (ES2015) but ~92% of the browsers support it. Some real life applications that make use of this are MobX (since v5.0).

In MobX this allows the developer to skip the tedious mapStateToProps and mapDispatchToProps and just let MobX handle it. Since with the help of proxies MobX can know what parts of the state are being used by listening on the get trap of the state and can then only render the components when the props they are using in the render() function are being used.

There a small package that does the same for the redux world: https://github.com/dai-shi/reactive-react-redux

I hope this helped and was interesting! 👋

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