Reflect in JavaScript Explained!

May 12th, 2021 — 11 min read — Gideon Idoko

Reflect is a built-in global object that was introduced in ES6 and dedicated to allow for effective reflection - the ability of a program to manipulate properties and methods of objects at runtime. The Reflect object provides methods that have the ability to inspect, interact with, and manipulate object properties.

Unlike other global objects, Reflect is not a constructor nor a function, which means, it cannot be used with new and cannot be invoked. All properties and methods of the Reflect object are static like those of Math and JSON objects.

Why is Reflect Important?

Reflect allows you to develop programs that are able to handle dynamic code.
Prior to ES6 (ECMAScript 2015), only a few methods were available to help with reflection, Reflect wraps all the methods used to work with objects into a single object.
The Reflect object makes it easy to interfere with the functionality of an existing object as some of its methods can mutate target objects.
Reflect is a go-to place for various meta-operations on objects like getting and setting the prototype of objects.

Reflect Methods

I'll group the Reflect methods into two (2) categories: Mutating Reflect Methods and Non-Mutating Reflect Methods.

NB💡: The target object (to be manipulated), is always passed to Reflect Methods as the first argument. Arrays and objects can be the target object since they are both typeof object. A TypeError exception is thrown if target object is a non-object except in the case of apply or construct methods where the target is a function. This will be explained later.

For an object, propertyKey is the property of that object while for an array, propertyKey is the index of that array.

Mutating Reflect Methods

These consist of methods that modify the target object, its value, or behavior. They include:

defineProperty()

Reflect.define(target, propertyKey, attribute)

The Reflect.defineProperty() method defines a new property (with propertyKey as key and attribute as its descriptor) on the target object. It returns true if the propertyKey is successfully defined or false if otherwise.

const obj = {
    one: 1,
    two: 2
}; // target object
const arr = ['one', 'two']; // target object

Reflect.defineProperty(obj, 'three', { value: 3 }) ? 
console.log('property defined!') : 
console.log('could not define the propery'); // property defined!
Reflect.defineProperty(arr, 2, { value: 'three' });

console.log(obj.three); // 3
console.log(arr[2]); // three

deleteProperty()

Reflect.deleteProperty(target, propertyKey)

The Reflect.deleteProperty() method deletes propertyKey from the target object. It works like the delete operator, but as a function. It returns true if propertyKey is successfully deleted or false if otherwise.

const obj = {
    one: 1,
    two: 2
}; // target object
const arr = ['one', 'two']; // target object

Reflect.deleteProperty(obj, 'two') ?
console.log("property deleted!") :
console.log("could not delete property"); // property deleted!
Reflect.deleteProperty(arr, 1);

console.log(obj.two); // undefined
console.log(arr[1]); // undefined
console.log(obj); // { one: 1 }
console.log(arr); // [ one, empty ]

The length of an array remains the same after deletion because the Reflect.deleteProperty method, just like the delete operator, removes the value from the index and does not remove the index itself.

apply()

Reflect.apply(target, thisArgument, argumentsList)

The Reflect.apply() method calls the target function with arguments specified by argumentsList, an array-like object.
```
const obj = {
    one: 1,
    two: 2
}; // thisArguments
const arr = ['one', 'two']; // argumentsList

function func(a, b) {
    console.log(this); // { one: 1, two: 2 }
    console.log(a, b); // one two
    return a + ": " + this.one; // `this` references `obj`
}

console.log(Reflect.apply(func, obj, arr)); // one: 1
console.log(Reflect.apply(Math.ceil, undefined, [1.75])); // 2
```
The Reflect.apply() method is not really a mutating method as its target is a function. It works like the Function.prototype.apply().

⚠ If the target is not callable or argumentsList is null or undefined, Reflect.apply() would throw a TypeError although, Function.prototype.apply() would still call the function without any arguments.

construct()

Reflect.construct(target, argumentsList[, newTarget])

The Reflect.construct() method is used to construct an object from the target constructor function. It behaves like the new operator, but as a function. It is the same as called new target(...argumentsList). The argumentsList (array-like object) specifies arguments and the newTarget (optional constructor function) specifies the constructor whose prototype should be used.

function Func(a, b) { this.name = a; } // a = "func"
function JavaScript(a, b) { this.name = b; } // b = "javascript"

const args = ["func", "javascript"];
const obj1 = new Func(...args);
const obj2 = Reflect.construct(Func, args, JavaScript);

console.log(obj1.name); // "func"
console.log(obj2.name); // "func"
console.log(obj1 instanceof Func) // false
console.log(obj2 instanceof Func) // false
console.log(obj1 instanceof JavaScript) // true
console.log(obj2 instanceof JavaScript) // true

⚠ If target or newTarget are not constructors, Reflect.construct() would throw a TypeError.

set()

Reflect.set(target, propertyKey, value[, receiver])

The Reflect.set() method sets the propertyKey of the target object with value. If a property with the name propertyKey already exists, its value will be updated otherwise a new property will be created with the name propertyKey and value value.

If the propertyKey already exists, and it has a setter function, receiver (optional object) will be the this value inside the setter function.

A boolean true is returned by the Reflect.set() method if the propertyKey was successfully set otherwise false is returned.

const obj = {
  one: 1,
  two: 2
}; // target object
const arr = ['one', 'two']; // target object

// add a `_one` accessor property to `obj`
Object.defineProperty(obj, '_one', {
  get: function () { return this.one },
  set: function (value) { this.one = ++value }
})

Reflect.set(obj, 'three', 3) ?
console.log("property successfully set") :
console.log("could not set property"); // "property successfully set"
Reflect.set(arr, 1, '2'); // set arr index of 1 to '2'

const receiver = {};
Reflect.set(obj, '_one', 1, receiver);

console.log(obj.three); // 3
console.log(arr[1]); // 2
console.log(obj); // { one: 1, two: 2, three: 3 }
console.log(arr); // [ 'one', '2' ]
console.log(receiver); // { one: 2 }

setPrototypeOf()

Reflect.setPrototypeOf(target, prototype)

The Reflect.setPrototypeOf() method sets the prototype of the target object to prototype (object or null). It returns true if the prototype was successfully set otherwise false is returned.

const obj1 = {}; // target object
const obj2 = { two: 2 }; //target object
const obj3 = { three: 3 } //target object

Object.seal(obj3); //seal `obj3` to make it non-extensible

console.log(Reflect.setPrototypeOf(obj1, { one: 1 })); // true
console.log(Reflect.setPrototypeOf(obj2, null)); // true
console.log(Reflect.setPrototypeOf(obj3, { three: 33})); // false
console.log(obj1.one); // 1

⚠ If the prototype is neither an object nor null, Reflect.setPrototypeOf() would throw a TypeError. If target object is non-extensible a boolean false would be returned.

preventExtensions()

Reflect.preventExtensions(target)

The Reflect.preventExtensions() method is used to prevent new properties from being added to the target object i.e. the target object will be made non-extensible.

const obj = {
  one: 1,
  two: 2
}; // target object
const arr = ['one', 'two']; // target object

Reflect.isExtensible(obj) ?
console.log("successfully made object non-extensible") :
console.log("could not make object non-extensible"); // "successfully made object non-extensible"
Reflect.preventExtensions(arr);

arr[2] = "three"; // this will not happen
console.log(arr); // logs [ 'one', 'two' ] and not [ 'one', 'two', 'three' ]

The Reflect.preventExtensions() method returns true if the target object is successfully made non-extensible otherwise it returns false. Once the target object is made non-extensible, no new properties can be added to it.

Non-Mutating Reflect Methods

These methods do not modify the target object, its value, or behavior. They include:

get()

Reflect.get(target, propertyKey[, receiver])

The Reflect.get() method is used to get a property on an object. It returns the value of the propertyKey property in the target. Normally, this references the target object but the receiver (optional) argument is used as this, if provided and if the property with the same name as propertyKey is a getter function in the target.

const obj = {
    one: 1,
    two: 2
}; // target object

const arr = ['one', 'two']; // target object

//define a getter property
Object.defineProperty(obj, 'three', {
    get() {
        return this.one + this.two;
    }
})

console.log(Reflect.get(obj, 'one')); // 1
console.log(Reflect.get(obj, 'three')); // 3
console.log(Reflect.get(arr, 0)); // one
console.log(Reflect.get(obj, 'three', {
    one: 11,
    two: 22
})); // 33

getOwnPropertyDescriptor()

Reflect.getOwnPropertyDescriptor(target, propertyKey)

The Reflect.getOwnPropertyDescriptor() method returns a property descriptor object that describes the propertyKey of the target object , if found or undefined if not.
```
const obj = {
    one: 1,
    two: 2
}; // target object

const arr = ['one', 'two']; // target object

console.log(Reflect.getOwnPropertyDescriptor(obj, 'one')); 
// yields { value: 1, writable: true, enumerable: true, configurable: true }
console.log(Reflect.getOwnPropertyDescriptor(arr, 1)); 
// yields { value: 'two', writable: true, enumerable: true, configurable: true }
```
The property descriptor object contains:
- value: the value of propertyKey.
- writable: is true if the propertyKey value may be changed otherwise false.
- enumerable: is true if the propertyKey surfaces during enumeration of the properties of the target object otherwise false.
- configurable: is true if the type of propertyKey can be changed or if the propertyKey itself can be deleted from the target object.

getPrototypeOf()

Reflect.getPrototypeOf(target)

The Reflect.getPrototype method returns the prototype of the target object.

class JavaScript {}
const obj = {}; // target object
const arr = ['one', 'two']; // target object
const str = new String(); // target object
const js = new JavaScript();

console.log(Reflect.getPrototypeOf(obj)); // Object { }
console.log(Reflect.getPrototypeOf(arr)); // Array []
console.log(Reflect.getPrototypeOf(str)); // String { "" }
console.log(Reflect.getPrototypeOf(js)); // JavaScript {}

has()

Reflect.has(target, propertyKey)

The Reflect.has() method can be used to check if a propertyKey exists in the target object. It works like the in operator but as a function. It return true if propertyKey exists in the target object, otherwise, false.

const obj = {
    one: 1,
    two: 2
}; // target object
const arr = ['one', 'two']; // target object

console.log(Reflect.has(obj, 'one')); // true
console.log(Reflect.has(obj, 'three')); // false
console.log(Reflect.has(arr, 1)); // true
console.log(Reflect.has(arr, 2)); // false

isExtensible()

Reflect.isExtensible(target)

The Reflect.isExtensible() method is used to check if new properties can be added to the target object.

const obj = {
    one: 1,
    two: 2
}; // target object
const arr = ['one', 'two']; // target object

console.log(Reflect.isExtensible(obj)); // true
console.log(Reflect.isExtensible(arr)); // true

// prevent extensibility
Object.seal(obj);
Object.preventExtensions(arr);

console.log(Reflect.isExtensible(obj)); // false
console.log(Reflect.isExtensible(arr)); // false

ownsKey()

Reflect.ownsKey(target)

The Reflect.ownsKey() method returns an array of the keys of all own properties of the target object. It includes both enumerable and non-enumerable properties.

const obj = {
    one: 1,
    two: 2
}; // target object
const arr = ['one', 'two']; // target object

console.log(Reflect.ownKeys(obj)); // [ 'one', 'two' ]
console.log(Reflect.ownKeys(arr)); // [ '0', '1', 'length' ]

Differences between `Reflect` and `Object` Methods

Some of Reflect object static methods are the same as methods available on Object but there are some differences between these methods. The differences are outline below:

Method Name	Reflect.[Method Name]	Object.[Method Name]
defineProperty()	Returns `true` if property was defined otherwise `false`.	Returns the`target` object if property was defined otherwise `TypeError`.
getOwnPropertyDescriptor()	If `target` is not a primitive object, `TypeError` is returned.	If `target` is not a primitive object, it is coerced to one.
getPrototypeOf()	Throws a `TypeError` for non-objects.	Throws a `TypeError` for non-objects in ES5, but coerces non-objects in ES2015.
setPrototypeOf()	Returns `true` if `prototype` was successfully set, and `false` if it wasn't (including if `prototype` is non-extensible). Throws a `TypeError` if `target` is not an object, or if `prototype` is neither an Object nor null.	Returns the `target` object if `prototype` was set successfully. Throws a `TypeError` if `prototype` is neither Object nor null, or is non-extensible.
preventExtenstions()	Returns `true` if the `target` object has been made non-extensible, and `false` if it has not. Throws a `TypeError` if the `target` object is not an object (a primitive).	Returns the `target` object if it has been made non-extensible. Throws a `TypeError` in ES5 if `target` is not an object (a primitive). In ES2015, treats the `target` as a non-extensible, ordinary object and returns the object itself.
isExtensible()	Throws a `TypeError` if `target` is not an object (a primitive).	Throws a `TypeError` in ES5 if `target` is not an object (a primitive). In ES2015, it will be coerced into a non-extensible, ordinary object and will return `false`.

Conclusion

The need for a single API for reflection gave rise to the creation of the Reflect object. It has been seen that Reflect methods perform the same functionality as some Object static methods, some Function prototype methods, and some operators, however with the Reflect object those functionalities can be performed by just calling its methods. The JavaScript language is still growing so more methods would definitely be added to the Reflect object.

Thanks for reading :)

#javascript#reflect