CODEWITHSUNDEEP
javascriptcoffeescript
Wehrdo
Wehrdo

Reputation: 580

Override Javascript function using Coffeescript

I'm using a library that has a class defined like this:

RoundDown = function () {
    this.prop = 5.5
    this.do_something = function() {
        return "base class: " + this.prop
    };
};

Using CoffeeScript, I want to extend do_something so it will run its original function plus new code. Subclassing it seems like a clean solution, but the base class' function gets called, rather than the overridden function.

# Subclass overriding round function
class RoundUp extends RoundDown
    do_something: () ->
        return super() + " Subclass code"

jsFiddle here.

What is the correct way to do? Thanks!

Upvotes: 2

Views: 442

Answers (2)

George Simms
George Simms

Reputation: 4060

The problem is that RoundDown is setting a field of the instance, not of the prototype. The ideal way to fix this would be

RoundDown = function () {
    this.prop = 5.5
};
RoundDown.prototype.do_something = function() {
    return "base class: " + this.prop
};

But maybe you don't have permission to modify RoundDown, eg it is in a library, so do this

class RoundUp extends RoundDown
  constructor: ->
    super()
    oldDoSomething = @do_something 
    @do_something = ->
      oldDoSomething.call(this) + " Subclass code"

Upvotes: 2

Alex Antonov
Alex Antonov

Reputation: 15156

Using js2coffee converter: js2.coffee:

RoundDown = ->
  @prop = 5.5

  @do_something = ->
    'base class: ' + @prop

Upvotes: 0

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