Dynamically return a class constructor in TypeScript

Question

I'm trying to build a mechanism in TypeScript to return a new instance of a class based on a string parameter representing the name of the class. Basically, I'm trying to create a factory, only I can't quite figure out how to do this in TypeScript. In pure JS, I could get away with this by just storing all the class constructors in a key/value object and calling new ClassDictionary[className](); - it's not the most robust solution, but it works in this situation. I can't seem to get the right combination of types and casting to accomplish this in TypeScript. Is there a better way? I haven't been able to find any examples of factory pattern in TypeScript, so right now I've just got a bit switch statement, which is ugly and inflexible.

Answer 1

You can add a function declaration for every class in the dictionary, this can looks like this (Visit: https://www.typescriptlang.org/docs/handbook/declaration-merging.html):

class MyClass { 
    myMethod() { return "my return"; }
}

function factory(className: "Date"): Date;
function factory(className: "Number"): Number;
function factory(className: "MyClass"): MyClass;
function factory(className: string) { 

    var ClassDictionary = {
        "Date": Date,
        "Number": Number,
        "MyClass": MyClass
    };

    return new ClassDictionary[className]();
}

var d = factory("Date");

/* Its a date. */
alert(d.toISOString());

var n = factory("Number");

/* Now a number. */
alert(n.toPrecision());

var c = factory("MyClass");

/* And now MyClass. */
alert(c.myMethod());

Works in the Playground (Sample)

Or in a other way

// ...
var ClassDictionary = { };

ClassDictionary["Date"] = Date;
function factory(className: "Date"): Date;

ClassDictionary["Number"] = Number;
function factory(className: "Number"): Number;

ClassDictionary["MyClass"] = MyClass;
function factory(className: "MyClass"): MyClass;

function factory(className: string) { 
    return new ClassDictionary[className]();
}
// ...

Or with Arguments:

class MyClass {
    constructor(public myReturn: string) { }
    myMethod() { return this.myReturn; }
}

var ClassDictionary = { };

ClassDictionary["Date"] = Date;
function factory(className: "Date"): Date;

ClassDictionary["Number"] = Number;
function factory(className: "Number", value?: number): Number;

ClassDictionary["MyClass"] = MyClass;
function factory(className: "MyClass", myReturn: string): MyClass;

function factory(className: string, valueOrMyReturn?: number | string) { 
   if(valueOrMyReturn) return new ClassDictionary[className](valueOrMyReturn);
    return new ClassDictionary[className]();
}

var d = factory("Date");

/* Its a date. */
alert(d.toISOString());

var n = factory("Number", 42);

/* Now a number. */
alert(n.toPrecision());

var c = factory("MyClass", "This is the answer!");

/* And now MyClass. */
alert(c.myMethod());