How to Find Unique Dates in an Array in JavaScript

1 - Does each Date have a unique "object reference" in memory?.js

const date1 = new Date("3005-01-01 00:00") // Happy New Year! 🎉🎈🎊🥳
console.log(date1)
// Output: Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time)

// The date2 object will have a different object reference.
const date2 = new Date("3005-01-01 00:00") // Happy New Year! 🎉🎈🎊🥳

// The date3 variable will have the same object reference as date1.
const date3 = date1

// It's possible to add duplicate object references to an array:
const dateArray = [date1, date2, date3, date2]
const uniqueDates = Array.from(new Set(dateArray))
// Using Array.from() is the same as [...(new Set(dateArray)]
//
// We're left with the unique object references for the Date objects:
console.log(uniqueDates)
// Output: [Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time), Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time)]
//
// The uniqueDates array contains the date1 and date2 object references:
console.log(`date1: ${uniqueDates[0] === date1}`) // date1: true
console.log(`date2: ${uniqueDates[1] === date2}`) // date2: true

2 - Does each Date have a unique timestamp in milliseconds?.js

const date4 = new Date("3005-01-01 00:00") // Happy New Year! 🎉🎈🎊🥳
console.log(date4)
// Output: Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time)

// The date5 object will have a different object reference
const date5 = new Date("3005-01-01 00:00") // Happy New Year! 🎉🎈🎊🥳

// This array has 2 different object references.
const dateArray2 = [date4, date5]
// Use JSON.stringify() and JSON.parse() in a .map() call with new Date():
const uniqueDates2 = Array.from(
  new Set(dateArray2.map((dateObject) => JSON.stringify(dateObject)))
).map((dateString) => new Date(JSON.parse(dateString)))
// Using Array.from() is the same as [...(new Set(dateArray)]

// Just one Date remains, because they had the same timestamp.
console.log(uniqueDates2)
// Output: [Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time)]

// The uniqueDates array contains a new Date, neither date4 nor date5.
console.log(`date4: ${uniqueDates2[0] === date4}`) // date4: true
console.log(`date5: ${uniqueDates2[0] === date5}`) // date5: false
// The new date has the same timestamp in milliseconds as date4 or date5.
console.log(`${uniqueDates2[0].valueOf() === date4.valueOf()}`) // true

3 - Does each Date produce a unique human-readable string?.js

const date6 = new Date("3005-01-01 00:01") // Happy New Year! 🎉🎈🎊🥳
console.log(date6)
// Output: Date Tue Jan 01 3005 00:01:00 GMT-0500 (Eastern Standard Time)

// Now the date7 object is one minute ahead of the date6 object.
const date7 = new Date("3005-01-01 00:02") // Happy New Year! 🎉🎈🎊🥳
console.log(date7)
// Output: Date Tue Jan 01 3005 00:02:00 GMT-0500 (Eastern Standard Time)

const dateArray3 = [date6, date7]
// Calling .toDateString() on each Date object compares the calendar date.
const uniqueDates3 = Array.from(
  new Set(dateArray3.map((dateObject) => dateObject.toDateString()))
).map((dateString) => new Date(dateString))
// Using Array.from() is the same as [...(new Set(dateArray)]

// Just one Date remains, because they had the same human-readable date.
console.log(uniqueDates3)
// Output: [Date Tue Jan 01 3005 00:00:00 GMT-0500 (Eastern Standard Time)]

// Note that the time was lost for the new Date, so we're back to midnight.
console.log(`${uniqueDates3[0].valueOf() === date6.valueOf()}`) // false