CODEWITHSUNDEEP
javascriptinheritanceoverridingprototype
Skyler
Skyler

Reputation: 805

How to override a private method in javascript?

I'm trying to use javascript object inheritance where I override a "private" method in the base "class" (in other words, make it a protected method).

Is it possible? Here's my best attempt (which doesn't work)

function Vehicle(color) {
    this.color = color;
}

Vehicle.prototype.drive = function() {
    _getMessage.call(this);
}

function _getMessage() {
    console.log("override this method!")
}


//----------------------------------------------------------------

var Car = (function() {

    Car.prototype = Object.create(Vehicle.prototype);
    Car.prototype.constructor = Car;

    function Car(color) {
        Vehicle.call(this, color)
    }

    function _getMessage() {
        console.log("The " + this.color + " car is moving!")
    }


    return Car;
}());

//----------------------------------------------------------------

$(function() {
    var c = new Car('blue');
    c.drive()

})

https://plnkr.co/edit/ZMB9izK1W9VuFQHPNsvu?p=preview

Upvotes: 2

Views: 8171

Answers (1)

RobG
RobG

Reputation: 147363

You could introduce a privileged method that can change the private one:

// IIFE to create constructor
var Car = (function(){

  // Private method
  function _getMessage(text){
    console.log('original: ' + text);
  }

  // Normal constructor stuff
  function Car(make){
    this.make = make;
  }
  Car.prototype.getMake = function(){
    return this.make;
  }
  Car.prototype.getMessage = function(){
    _getMessage(this.make);
  }

  // Privileged function to access & change private method
  Car.changeGetMessage = function(fn) {
    _getMessage = fn;
  }
  return Car;
}());

// Demonstration
// Create instance
var ford = new Car('Ford');
console.log(ford.getMake());

// Call original method
ford.getMessage();

// Replace original
Car.changeGetMessage(function(text){
  console.log('new message: ' + text);
});

// Existing instances get new method
ford.getMessage();

// Create new instance
var volvo = new Car('Volvo');
console.log(volvo.getMake());

// New instances get method too
volvo.getMessage();

Upvotes: 2

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