Question

196

JavaScript instanceof and moment.js

rated 0 times [ 199] [ 3] / answers: 1 / hits: 21746 / 9 Years ago, mon, august 10, 2015, 12:00:00

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?

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sabrina

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1 Year ago

answered 9 Years ago bruno · Accepted Answer

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.