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In our angularjs app we're receiving an array of dates that then need to be filtered out to only return an array full of unique dates (not strings). I tried some of the answers on SO but I think because I'm comparing dates and not strings - I'm yielding different results. Any help on what the best approach is would be very helpful. enter image description here

chris
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2 Answers2

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You are right, date equality is different from string equality. Dates are objects, and the === comparator for objects compares them by reference, not value. Since {} === {} in JS is false, two Dates will never be equal to each other. So, let's just turn the dates into numbers: 

let dates = [new Date(), new Date('1/1/11'), new Date('1/1/11'), new Date('1/4/66')]


let uniqueDates = dates
                   .map(s => s.getTime())
                   .filter((s, i, a) => a.indexOf(s) == i)
                   .map(s => new Date(s));
                   
console.log(uniqueDates);

The logic with indexOf is that every unique value should be present only at its own position; any others are duplicates (s is current item, i is current index, and a is the array itself). Then we map the array back into Dates and return it.

joh04667
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    thank you - the refactor of your code was the cherry on top. I think I was overcomplicating before by dismissing the idea of converting to a string and then back to date – chris May 17 '17 at 23:11
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    No problem! Come to think of it, it's more straightforward to replace `.toString()` with `.getTime()`. That returns the date as a simple number, epoch time. Cleaner and a little more efficient than comparing strings. – joh04667 May 17 '17 at 23:22
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If you're using lodash:

const date1 = new Date();
const date2 = new Date();
const arr = [date1, date1, date2, date2];
const uniqueDates = _.uniqBy(arr, date => date.getTime());

Uses uniqBy to ensure that we compare dates by their value and not just by object identity. See here: Compare two dates with JavaScript

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dbandstra
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