Unique Array for dates Javascript

You can do a simple filter with a lookup but you need to convert the dates to something that can be compared, since two objects are never the same in JavaScript, unless it's two references to the exact same object.

const dates = [
  new Date(2016, 09, 30, 10, 35, 40, 0),
  new Date(2016, 09, 30, 10, 35, 40, 0), //same
  new Date(2016, 09, 30, 10, 35, 40, 0), //same
  new Date(1995, 07, 15, 03, 15, 05, 0) //different
];


function filterUniqueDates(data) {
  const lookup = new Set();
  
  return data.filter(date => {
     const serialised = date.getTime();
    if (lookup.has(serialised)) {
      return false;
    } else { 
      lookup.add(serialised);
      return true;
    }
  })
}

console.log(filterUniqueDates(dates));

This can be further generalised, if you want to filter anything by just changing how you determine uniqueness

const dates = [
  new Date(2016, 09, 30, 10, 35, 40, 0),
  new Date(2016, 09, 30, 10, 35, 40, 0), //same
  new Date(2016, 09, 30, 10, 35, 40, 0), //same
  new Date(1995, 07, 15, 03, 15, 05, 0) //different
];

const dateSerialisation = date => date.getTime(); // this is the previous logic for dates, but extracted

//as primitives, these can be compared for uniqueness without anything extra
const numbers = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4];
const strings = ["a", "b", "b", "c", "c", "c"];

const people = [
  {name: "Alice", age: 20},
  {name: "Bob", age: 30},
  {name: "Bob", age: 40}, //technically the same
  {name: "Carol", age: 50},
  {name: "Carol", age: 60}, //technically the same
  {name: "Carol", age: 70} //technically the same
]

//let's assume that a person with the same name is the same person regardless of anything else 
const peopleSerialisation = person => person.name;

/* 
 * this now accepts a transformation function that will be used 
 * to find duplicates. The default is an identity function that simply returns the same item.
 */
function filterUnique(data, canonicalize = x => x) { 
  const lookup = new Set();
  
  return data.filter(item => {
     const serialised = canonicalize(item); //use extract the value by which items are considered unique
    
    if (lookup.has(serialised)) {
      return false;
    } else { 
      lookup.add(serialised);
      return true;
    }
  })
}


console.log("dates", filterUnique(dates, dateSerialisation));
console.log("numbers", filterUnique(numbers));
console.log("strings", filterUnique(strings));
console.log("people", filterUnique(people, peopleSerialisation));

This is using ES6 but it's trivial to convert to ES5 compliant code - removing the fat arrow functions, the default parameter and the new Set() here is what you need:

function filterUnique(data, canonicalize) {
  if (!canonicalize) {
    canonicalize = function(x) { return x; }
  }

  var lookup = {};

  return data.filter(function(item) {
     var serialised = canonicalize(item);

    if (lookup.hasOwnProperty(serialised)) {
      return false;
    } else { 
      lookup[serialised] = true;
      return true;
    }
  })
}

If you compare two dates via ===, you compare the references of the two date objects. Two objects that represent the same date still are different objects.

Instead, compare the timestamps from Date.prototype.getTime():

function isDateInArray(needle, haystack) {
  for (var i = 0; i < haystack.length; i++) {
    if (needle.getTime() === haystack[i].getTime()) {
      return true;
    }
  }
  return false;
}

var dates = [
  new Date('October 1, 2016 12:00:00 GMT+0000'),
  new Date('October 2, 2016 12:00:00 GMT+0000'),
  new Date('October 3, 2016 12:00:00 GMT+0000'),
  new Date('October 2, 2016 12:00:00 GMT+0000')
];

var uniqueDates = [];
for (var i = 0; i < dates.length; i++) {
  if (!isDateInArray(dates[i], uniqueDates)) {
    uniqueDates.push(dates[i]);
  }
}

console.log(uniqueDates);

Optimization and error handling is up to you.


ES6 way:

datesArray.filter((date, i, self) => 
  self.findIndex(d => d.getTime() === date.getTime()) === i
)

Thanks to https://stackoverflow.com/a/36744732/3161291

Tags:

Javascript

Arrays

Date

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