How to access prototype property from within javascript class?

Question

I am knew to javascript, please excuse my rather basic question. How can I use a property from within my class, as in the sample code below?

function MyClass() {
  // nothing special
}

MyClass.prototype.myProperty = {
  key1: 'some value 1',
  key2: 'some value 2',
};

My Class.prototype.myFunction = function myFunction() {
  // I need to retrieve the first value of myProperty
  const x = myProperty[Object.keys(myProperty)[0]] + 10; // won't work
  return x;
};

module.exports = MyClass;

Using this.myProperty throws the error Cannot convert undefined or null to object

2 revs · Accepted Answer · 2016-10-18 15:29:05Z

3

Use this.:

MyClass.prototype.myFunction = function myFunction() {
  const x = this.myProperty[Object.keys(this.myProperty)[0]] + 10;
  // -------^^^^^-----------------------^^^^^
  return x;
};

That will get the property from the object. If the object doesn't have its own copy of the property, it will get it from the object's prototype.

Or if you want to get it from the prototype every time, be explicit:

MyClass.prototype.myFunction = function myFunction() {
  const p = MyClass.prototype.myProperty;
  const x = p[Object.keys(p)[0]] + 10;
  return x;
};

Side note: const is a new thing, as of ES2015. If you're using ES2015 (and it looks like you're using NodeJS, which means you can if you're using v6 or later), you can write your class more simply using class notation:

class MyClass {
  constructor() {
    // nothing special
  }
  myFunction() {
    const x = this.myProperty[Object.keys(this.myProperty)[0]] + 10;
    return x;
  }
}
MyClass.prototype.myProperty = {
  key1: 'some value 1',
  key2: 'some value 2',
};

module.exports = MyClass;

Note that if you want myProperty to be on the prototype, you do still need to assign it the "clunky" way. But you may want to create that property in the constructor instead.

Side note 2: Unless you change myProperty later, I strongly recommend using this.myProperty.key1 or this.myProperty.key2 and not using this.myProperty[Object.keys(this.myProperty)[0]], which is hard to read, chaotic (undefined behavior, the order of the keys returned by Object.keys is not specified, not even in ES2015), and extra work.

edited Oct 18, 2016 at 15:29

community wiki

2 revs
T.J. Crowder

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3 Comments

Yalda Over a year ago

Sorry I somehow invalidated the examples in your answer by updating my question! But it seems that this.myProperty is undefined. I guess because I never newed the class???

T.J. Crowder Over a year ago

@Yalda: Show how you're using it.

Yalda Over a year ago

I think I'm closer to find out what the problem is. Actually, I used the same property in myFunction as well as another property. It seems that the error comes from the latter instead. That's a totally different question, so I will accept your answer for the current question :)

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