Privatize a JavaScript (TypeScript) method at runtime

privatizeMethod.ts

function calledFromInsideClass(className:string):boolean{
    let stackTrace = (new Error()).stack; // Only tested in latest FF and Chrome
    //console.log("stackTrace: " + stackTrace);
    stackTrace = (stackTrace as string).replace(/^Error\s+/, ''); // Sanitize Chrome
    let callerLine = stackTrace.split("\n")[2]; // 1st item is this function, 2nd item is the privatizedMethod, 3rd item is the caller
    return callerLine.includes(className + ".");
}


function privatizeMethod(classFn: any, methodName: string){
    let originalMethod = classFn.prototype[methodName];
    let privatizedMethod = function(){
        if (calledFromInsideClass(classFn.name)){
            console.log("privatized method called from inside");
            return originalMethod.call(this, ...arguments);
        }
        else{
            throw new Error(`privatized method ${originalMethod.name} called`);
        }
    };
    classFn.prototype[methodName] = privatizedMethod;
}


class City{    
    constructor(private name: string,
        private extension: number,
        private population: number){}

    public growCity(percentage: number){
        console.log("growCity invoked");
        this.growExtension(percentage);
        this.growPopulation(percentage);
    }

    public growExtension(percentage: number){
        console.log("growExtension invoked");
        this.extension = this.extension + (this.extension * percentage /100);
    }

    public growPopulation(percentage: number){
        console.log("growPopulation invoked");
        this.population = this.population + (this.population * percentage /100);
    }

    private privateMethod(){
        console.log("I'm a private method");
    }
}

let city = new City("Paris", 105, 2140000);
//city.privateMethod();
//trick to invoke a private method
(city as any).privateMethod();

city.growCity(2);
console.log(JSON.stringify(city, null, "\t"));


//privatize the the City.growPopulation method
privatizeMethod(City, City.prototype.growPopulation.name);


city.growCity(3); //continues to work fine, it invokes internally our new private method
try{
    city.growPopulation(3);
}
catch(ex){
    console.log("EXCEPTION: " + ex.message); ////EXCEPTION: privatized method growPopulation called
}