Create instance from factory in TypeScript

Question

I'm trying to create a factory that creates instance using a string id.
I will let the code explain itself...

class A
{

}

class B
{

}

 class MyFactory{

     private static _instance: MyFactory;

     private _map: { [id: string]: object; } = {};

     private constructor() {
        // Map the id and classes
        this._map["A"] = A;
        this._map["B"] = B;
     }

     public createInstance(id: string) : object {
         let newInstance = null;
         if (this._map[id] != null) {
             newInstance = Object.create(this._map[id]);
             newInstance.constructor.apply(newInstance);
         }

         return newInstance;
     }

     public static get instance() {
         return this._instance || (this._instance = new this());
     }
}

I'm a bit struggling with what should I save in the map and how should I use it to construct a new instance from that class...

score 3 · Accepted Answer · 2017-11-27 11:11:04Z

3

The original example always gives a Function {} object instance because classes were passed as argument for Object.create instead of the prototype property of those classes:

//original
newInstance = Object.create(this._map[id]);
//updated
newInstance = Object.create(this._map[id].prototype);

This updated version would set the prototype to the correct object but the problem is that you are using object as type, the compiler does not allow access to .prototype property. If you change to any, it will compile but you can't use the constructor to initialize the object:

newInstance.constructor.apply(newInstance);
// If in your map you had ordinary functions this would work
// but because they are classes, they result in the run time error ...
// TypeError: Class constructor A cannot be invoked without 'new'

A solution would be changing the annotations from object to any and then use the new operator.

newInstance = new this._map[id]();

If some kind of relation was established between the classes then other kind of casts would be applied for a better type check support. Look for the section about interfaces for constructors in the typescript handbook to see if that would fit the the index declaration for the _map.

edited Nov 27, 2017 at 11:11

answered Nov 26, 2017 at 18:31

user8928802

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2 Comments

Yaron Over a year ago

If I use any for the map value, what do I set there? _map[id] = A; ?

user8928802 Over a year ago

Yes. Classes are Functions and Functions are Objects so they can be used as any other kind of value would - this._map["A"] = A; . Using any as type of a value is not the best solution though because it gives no hint for the compiler to check if an operation on that value is valid.

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