Adding prototype function to a specific string?

Question

I have a method inside a class that uses lodash's get and looks like this:

get(path, defaultValue = '–') {
  const result = _.get(this, path, defaultValue);
  return result;
}

Assuming that result is a string, is there a chance to add a prototype method isDefault? I've tried implementation below and it does not work:

get(path, defaultValue = '–') {
  const result = _.get(this, path, defaultValue);
  result.isDefault = () => result === defaultValue;
  return result;
}

I know that I could just wrap the result in a class and define the method there, but I also want to just write console.log(result) to get a string printed on my screen.

NB: you're specifically not adding this to the prototype (which affects every instance). You're just trying to add a method as a property of a single instance. — Alnitak
– Alnitak, Commented Oct 11, 2018 at 17:57
maybe this can give u some clues stackoverflow.com/q/6804370/6442877 — Seyed Ali Roshan
– Seyed Ali Roshan, Commented Oct 11, 2018 at 17:58
@Alnitak: Good point, but writing result.prototype.isDefault gives an error — mdmb
– mdmb, Commented Oct 11, 2018 at 18:00
@Ancinek that's because only Function objects (and classes) have a .prototype property. — Alnitak
– Alnitak, Commented Oct 11, 2018 at 19:20

Eugene Tsakh · Accepted Answer · 2018-10-11 18:16:00Z

1

Your string is a primitive string which is not really an instance of string, so you need to wrap your result assignment in String instance:

const result = new String(_.get(this, path, defaultValue));

edited Oct 11, 2018 at 18:16

answered Oct 11, 2018 at 17:55

Eugene Tsakh

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3 Comments

Mark Over a year ago

That works, but it makes result harder to use as if it were a string primitive. For example console.log(result) doesn't log a simple string anymore. You need to explicitly call result.toString().

Eugene Tsakh Over a year ago

@MarkMeyer you also can use __proto__ assignment but I'd not recommend to do this as it is a bad practice: result.__proto__.func = function() { ... };

mdmb Over a year ago

I'm going to accept it as it works the way I expected it to work. Maybe someone can give a deeper insight about this solution - I'm going to have a small research on this tomorrow.

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