I searched the web for some way to create instance of "Class" in "reflection" using javaScript but I found nothing.
generally i'm trying to do someting like java code.
Object o = Class.forename("MyClassName");
but in javaScript, in such way that I'll get the instance of class when I have only the name of the class, any idea?
Thankes.
JavaScript doesn't have classes. But if by "class" you mean you have a constructor function:
function MyClassName() {
// do constructor things here
}
But the name of that function is in a variable:
var someclass = "MyClassName";
Then you can instantiate an instance like this:
var obj = new window[someclass]();
The above only works if MyClassName is in the global scope.
Classes in Javascript are nothing but global functions. So you should be able to do this:
var o = new window["MyClassName"];
for example:
var o = new window["Array"];
Keep in mind that this example is just an example. Arrays should be created via [] syntax.
This is working for me:
var obj = eval("new " + className + "()");
Array(); runMaliciousCode();//Now that JavaScript has classes thought I would offer an updated solution.
ES6 introduced an API that makes this extremely trivial but supports all new things JavaScript (modules for example, which weren't addressed in other answers, and doesn't worry about 'window' scope as mentioned in other comments). The Reflect API is well supported across browsers and all the methods are static and aren't limited to just classes, it works with objects in general.
The Reflect API even has convenience methods that are helpful beyond 'reflection-like' implementation. For example, you can quickly determine if an object has a key with:
console.log(Reflect.has(target, <key>))
Which can be considered an alternate of:
console.log(obj[<key>] !== undefined)
To instantiate a new instance of a class via javascript reflection you can call the Reflect.construct(<target>, arugmentsList) method:
animal.js
export default class Animal {
constructor(type) {
this.type = type;
}
get canSwim() {
return ['amphibians', 'fish'].includes(this.type);
}
}
main.js
import Animal from "./animal.js"
const instance = Reflect.construct(Animal, ['bird']);
console.log(Reflect.get(instance, 'canSwim')) //returns false
Just for brevity, you can still use mutators and accessors per normal, I simply used the Reflection.get() method for this example to demonstrate it exists, but the following is synonymous:
const instance = Reflect.construct(Animal, ['bird']);
console.log(instance.canSwim) //returns false
Of course, this works without modules, this example just demonstrates it with modules.
Reference:
Reflect object only for implementing proxy traps. To access properties, use instance.canSwim, to check for properties, use Object.hasOwn(object, 'key') or 'key' in obj, and to construct instances, use new Animal('bird').
new works with arbitrary/dynamic classes just as well: function construct(C) { return new C; }eval and window[name] get at. This is nothing that Reflect helps with anyway ("[The Reflect API] supports all new things JavaScript (modules for example …)" is flat out wrong, Reflect has nothing to do with modules). But once you have a reference to the class, you can replace any Reflect.construct(<target>, arugmentsList) call with new <target>(...arugmentsList) and it works exactly the same. No need for Reflect, you should not use it here - but your answer suggests otherwise.