Javascript array compare elements

Question

I have an array with dates and I want to take the last value from array for each month. How can I do that? I have this code:

for (var i = 0; i < vm.openDatesToSave.length; i++) {
            var c = vm.openDatesToSave[i].getMonth();
            for (var j = i+1; j < vm.openDatesToSave.length; j++) {
                var d = vm.openDatesToSave[j].getMonth();
                if (d === c) {
                    a.push(vm.openDatesToSave[j]);
                }
            }
        }

For example, I want to take Jan 09, Feb 06 and Mar 14.

is the result of getMonth a string or number or Date object? — uladzimir
– uladzimir, Commented Feb 16, 2018 at 12:44

zabusa · Accepted Answer · 2018-02-16 13:04:17Z

2

var dates = [new Date('2018-05-12'), new Date('2018-04-03'), new Date('2018-05-04')];

var result = dates.reduce((acc, item) => {
   const month = item.getMonth(); 
   acc[month] = item;
   return acc;
},{})

console.log(Object.values(result));

answered Feb 16, 2018 at 13:04

zabusa

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Comments

Shilly · Accepted Answer · 2018-02-16 13:01:20Z

1

var dates = [
	new Date( '2018-02-14' ),
	new Date( '2018-01-17' ),
	new Date( '2018-02-06' ),
	new Date( '2018-01-09' ),
	new Date( '2018-03-14' )
];
// We want to transform an array of dates into a shorter summary. So we use reduce to turn multiple values into less values.
// By using a object as the output, we get an easy summary which can be turned back into an array if needed.
var lastDatesPerMonth = dates.reduce( function( months, date ) {
	// months are zero-based, so + 1
	var month = date.getMonth() + 1;
	// Always overwrite the correct month with the new date.
	months[ month ] = date;
	return months;
}, {} );

console.log( lastDatesPerMonth );

edited Feb 16, 2018 at 13:01

answered Feb 16, 2018 at 12:54

Shilly

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Comments

georg · Accepted Answer · 2018-02-16 13:15:21Z

0

If you're using lodash, then

lastDays = _.map(_.groupBy(dates, d => d.getMonth()), _.last)

otherwise consider using it.

rant: I fail to see why SO keeps preferring convoluted and fragile ad-hoc solutions to an established, efficient and tested library, end rant ;)

edited Feb 16, 2018 at 13:15

answered Feb 16, 2018 at 12:57

georg

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11 Comments

Shilly Over a year ago

Own opinion: Because it teaches people how javascript actually works without abstracting away any of the details. Every time I see a coworker fail to write a simple reduction because his lib of choice doesn't have it builtin, I cry a bit inside. SO opinion: If the original post does not include a library, you are expected to psot a result without a library.

georg Over a year ago

@Shilly: everytime I see a coworker writing a "reduction", although our lib does provide it out of the box, I feel the same ;)

Shilly Over a year ago

That's the advantage of working in a consistent enviroment where everyone uses the same tools. If you have a library, learn to use it completely. If you don't have a library though, you need to know how to solve the issue in raw code. In the end it's the same problem, people not knowing the full power of the tools they use.

georg Over a year ago

@Shilly: that's right, but in day-to-day programming you hardly write "raw" code. You write tools to automate repetitive tasks. You don't write a reducer everytime if you need to group objects, you just create a generic groupBy function and use it henceforth. Still, here on SO, when someone asks about grouping, people are rushing to show off their reduces instead of saying hey, it's a solved problem, here's your generic groupBy.

Shilly Over a year ago

Most of the time these questions get asked by people who have no idea what functional programming is. Compare both our solutions. Yours is obviously way better, but it is also more complex for beginners to even understand how or why it works. It's like those posts suggesting 'use SASS' to people who still use JQuery to change style inline. I learned more on MDN about functional programming from one clear reduce example in JS than from reading the docs of the lib of the moment. Anwyays, you're 100% correct. I just believe that starting programmers should not skip steps while learning.

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