momentJS var 'not modified' in while loop

[benjaminhoffman]

Noticed that linter yells that my while-loop variable "is not modified in this loop", even though I am (via momentJS). You'll see from the code & screenshot below that I subtract one month with momentJS on each iteration. Here's the relevant part:

    const dateStart = moment('2016-02-01')
    const dateEnd = moment()
    while (dateStart < dateEnd) {
      //
      //subtract 1mnth
      dateEnd.subtract(1, 'month')
    }

Thoughts on this one?

[feross]

This is a limitation of the no-unmodified-loop-condition rule. More info about it here: eslint/eslint#6984

In short, what's happening is that the identity of the dateEnd object isn't actually changing in the loop body, as the subtract method just makes some changes to an internal Date object. But when the condition is executed, it is implicitly being coerced to a Number, which is of course different each time through the loop.

So, you can work around this rule by explicitly calling valueOf():

while (dateStart.valueOf() < dateEnd.valueOf()) {
}

(You could also use d.unix() or d.toDate())

[feross]

There is a proposal to eslint that would let us relax this rule a bit so that any reference to the condition variables in the loop body would be treated as a modification. Then it would only be an error to completely omit a condition variable from the body.

You can follow this here: eslint/eslint#8021

[dcousens]

it is implicitly being coerced to a Number

I always found the nature of Dates doing that really funky and weird