Unable to run object’s function

Question

I am trying to run a function of an object created using a constructor function. However, I am unable to do so as I keep getting and error saying “TypeError: mutant_cat.meow is not a function. (In 'mutant_cat.meow()', 'mutant_cat.meow' is undefined)”. This is my constructor function:

function Cat(legs, sound) {
    this.legs = legs;
    this.sound = sound;
    var meow = function() {
        document.write(sound);
    }
}

And this is where I create the object and attempt to run its function:

var mutant_cat = new Cat(5, "eeeeeee");
mutant_cat.meow();

Any help is greatly appreciated.

You need meow to be on this: this.meow = Better yet, put it on the prototype: Cat.prototype.meow = — Mark
– Mark, Commented Jun 28, 2019 at 17:30

Michael Paccione · Accepted Answer · 2019-06-28 17:32:13Z

2

This should fix it. You need to have the function be a property of the object by using "this".

function Cat(legs, sound) {
    this.legs = legs;
    this.sound = sound;
    this.meow = () => {
        document.write(this.sound);
    }
}

If you expect all your Cats to meow then you are better off using a prototype function as this is memory optimized and a shared function between all Cat instances rather than each Cat having its own duplicate meow function.

You can read more about prototype functions here: https://www.w3schools.com/js/js_object_prototypes.asp

answered Jun 28, 2019 at 17:32

Michael Paccione

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Tom O. · Accepted Answer · 2019-06-28 17:41:08Z

2

It's considered better practice to define your method on the prototype outside the constructor. This way we dont have to define the function for each instance of Cat:

function Cat(legs, sound) {
  this.legs = legs;
  this.sound = sound;
}

//Add the method to the protoype instead of constructor
Cat.prototype.meow = function() {
  console.log(this.sound);
}

var mutant_cat = new Cat(5, "eeeeeee");
mutant_cat.meow();

answered Jun 28, 2019 at 17:41

Tom O.

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