JavaScript instanceof and moment.js

Question

I'm attempting to understand types in the JavaScript world. My page is using moment.js. I have a function that sometimes returns a moment() and other times, returns a string (it's legacy code gone wild).

My code kind of looks like this:

var now = getDate();
if (now instanceof moment) {
  console.log('we have a moment.');
} else {
  console.log('we have a string.');
}


function getDate() {
  var result = null;
  // Sometimes result will be a moment(), other times, result will be a string.
  result = moment();
  return result;
}

When I execute the code above, I never get we have a moment.. Even if I manually make set result = moment();. Why is that? Am I misunderstanding instanceof or moment?

Answer 1

First of all, instanceof isn't perfectly reliable.

Second of all, moment() returns instance of Moment class that isn't exposed to user. Following code prove this:

moment().__proto__.constructor // function Moment()
moment().constructor === moment; // false

Third of all, moment provide function moment.isMoment that will solve your problem.

And last, but not least - your code should use consistent return types - always return moment instances or always return strings. It will reduce your pain in future.

You can ensure that you always have moment instance by calling moment function - moment(string) equals in value moment(moment(string)), so you can just always convert your argument to moment instance.