Reflect
The Reflect
namespace object contains static methods for invoking interceptable JavaScript object internal methods. The methods are the same as those of proxy handlers.
Description
Unlike most global objects, Reflect
is not a constructor. You cannot use it with the new
operator or invoke the Reflect
object as a function. All properties and methods of Reflect
are static (just like the Math
object).
The Reflect
object provides a collection of static functions which have the same names as the proxy handler methods.
The major use case of Reflect
is to provide default forwarding behavior in Proxy
handler traps. A trap is used to intercept an operation on an object — it provides a custom implementation for an object internal method. The Reflect
API is used to invoke the corresponding internal method. For example, the code below creates a proxy p
with a deleteProperty
trap that intercepts the [[Delete]]
internal method. Reflect.deleteProperty()
is used to invoke the default [[Delete]]
behavior on targetObject
directly. You can replace it with delete
, but using Reflect
saves you from having to remember the syntax that each internal method corresponds to.
js
const p = new Proxy(
{},
{
deleteProperty(targetObject, property) {
// Custom functionality: log the deletion
console.log("Deleting property:", property);
// Execute the default introspection behavior
return Reflect.deleteProperty(targetObject, property);
},
},
);
The Reflect
methods also allow finer control of how the internal method is invoked. For example, Reflect.construct()
is the only way to construct a target function with a specific new.target
value. If you use the new
operator to invoke a function, the new.target
value is always the function itself. This has important effects with subclassing. For another example, Reflect.get()
allows you to run a getter with a custom this
value, while property accessors always use the current object as the this
value.
Nearly every Reflect
method's behavior can be done with some other syntax or method. Some of these methods have corresponding static methods of the same name on Object
, although they do have some subtle differences. For the exact differences, see the description for each Reflect
method.
Static properties
Reflect[@@toStringTag]
-
The initial value of the
@@toStringTag
property is the string"Reflect"
. This property is used inObject.prototype.toString()
.
Static methods
Reflect.apply()
-
Calls a
target
function with arguments as specified by theargumentsList
parameter. See alsoFunction.prototype.apply()
. Reflect.construct()
-
The
new
operator as a function. Equivalent to callingnew target(...argumentsList)
. Also provides the option to specify a different prototype. Reflect.defineProperty()
-
Similar to
Object.defineProperty()
. Returns a boolean that istrue
if the property was successfully defined. Reflect.deleteProperty()
-
The
delete
operator as a function. Equivalent to callingdelete target[propertyKey]
. Reflect.get()
-
Returns the value of the property. Works like getting a property from an object (
target[propertyKey]
) as a function. Reflect.getOwnPropertyDescriptor()
-
Similar to
Object.getOwnPropertyDescriptor()
. Returns a property descriptor of the given property if it exists on the object,undefined
otherwise. Reflect.getPrototypeOf()
-
Same as
Object.getPrototypeOf()
. Reflect.has()
-
Returns a boolean indicating whether the target has the property. Either as own or inherited. Works like the
in
operator as a function. Reflect.isExtensible()
-
Same as
Object.isExtensible()
. Returns a boolean that istrue
if the target is extensible. Reflect.ownKeys()
-
Returns an array of the target object's own (not inherited) property keys.
Reflect.preventExtensions()
-
Similar to
Object.preventExtensions()
. Returns a boolean that istrue
if the update was successful. Reflect.set()
-
A function that assigns values to properties. Returns a boolean that is
true
if the update was successful. Reflect.setPrototypeOf()
-
A function that sets the prototype of an object. Returns a boolean that is
true
if the update was successful.
Examples
Detecting whether an object contains certain properties
js
const duck = {
name: "Maurice",
color: "white",
greeting() {
console.log(`Quaaaack! My name is ${this.name}`);
},
};
Reflect.has(duck, "color");
// true
Reflect.has(duck, "haircut");
// false
Returning the object's own keys
js
Reflect.ownKeys(duck);
// [ "name", "color", "greeting" ]
Adding a new property to the object
js
Reflect.set(duck, "eyes", "black");
// returns "true" if successful
// "duck" now contains the property "eyes: 'black'"
Specifications
Specification |
---|
ECMAScript Language Specification # sec-reflect-object |
Browser compatibility
BCD tables only load in the browser
See also
- The
Proxy
global object - The
Proxy()
constructor