“in” operator not working as expected with date strings in IE11

So I came across this odd interaction between Javascript Date objects being converted into Strings and the in operator in IE11. The String that .toLocaleString() returns for a date, doesn't work when checking for keys using the in operator, even through another string with the exact same text works just fine.

in

.toLocaleString()

in

The interaction works correctly in Chrome, but seems to fail in IE11. What am I missing here and is there a way to get around this?

let days =
Monday: ,
Tuesday: ,
Wednesday: ,
Thursday: ,
Friday: ,
Saturday: ,
Sunday:
;

// The example date should be a Monday
let currentDate = new Date('2018/8/20');
let currentDay = currentDate.toLocaleString('en-us',
weekday: 'long'
);

console.log(currentDay);
console.log(typeof currentDay);
console.log(currentDay in days);
console.log('Monday' in days);

Thanks for any input!

console.log(currentDay);

1 Answer
1

This is basically why you should stick to ISO date strings (2018-01-01T00:00Z) and not use arbitrary formats.

2018-01-01T00:00Z

Alternatively, parse the format yourself and use the other overload of the new Date() constructor like so:

new Date()

const currentDate = new Date(2018, 7 /* months start at 0 */, 20 /* days do not */, 0, 0, 0)


Date parsing is bad, and IE is bad, and two bads don't make a good in this case.

It's not the in that's wrong. Your date is invalid in IE.

in

Monday

Monday

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