Add a duration to a moment (moment.js)

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I think you missed a key point in the documentation for .add()

Mutates the original moment by adding time.

You appear to be treating it as a function that returns the immutable result. Easy mistake to make. 🙂

If you use the return value, it is the same actual object as the one you started with. It’s just returned as a convenience for method chaining.

You can work around this behavior by cloning the moment, as described here.

Also, you cannot just use == to test. You could format each moment to the same output and compare those, or you could just use the .isSame() method.

Your code is now:

var timestring1 = "2013-05-09T00:00:00Z";
var timestring2 = "2013-05-09T02:00:00Z";
var startdate = moment(timestring1);
var expected_enddate = moment(timestring2);
var returned_endate = moment(startdate).add(2, 'hours');  // see the cloning?
returned_endate.isSame(expected_enddate)  // true

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