The difference() function in Lodash

The difference() function in Lodash takes two arrays, and returns an array containing all the values in the first array that are not in the second array.

const _ = require('lodash');
const array = [1, 2, 3, 4, 5];
const values = [1, 2, 3];
_.difference(array, values); // returns [4, 5]

The difference() function also supports passing in multiple arrays.
If you pass in 3 or more arrays, Lodash returns an array containing all values that are in the first array, but not in any of the subsequent arrays.

_.difference([1, 2, 3, 4, 5], [1, 3], [2, 4]); // [5]

differenceBy()

The differenceBy() function accepts a third argument, a callback function that transforms each element before calculating the difference.
For example, below is how you can calculate the difference between two sets of numbers by their Math.floor() value.

const _ = require('lodash');
const array = [1.1, 1.2, 2.1, 2.2, 3.3, 4, 5];
const values = [1.4, 2.3, 3.2];
_.differenceBy(array, values, Math.floor); // returns [4, 5]

You can also use this function with arrays of objects.

const _ = require('lodash');
const array = [{property: 1, property: 2}];
const values = [{property: 1}];
_.differenceBy(array, values, 'property'); // returns [{property: 2}]

You can even use it to convert values to strings.
This is convenient for calculating the difference between arrays of Mongoose ObjectIds.

const mongoose = require('mongoose');
const _ = require('lodash');

const oids1 = [
  mongoose.Types.ObjectId('123456789012'),
  mongoose.Types.ObjectId('123456789013'), 
  mongoose.Types.ObjectId('123456789014')
];
const oids2 = [
  mongoose.Types.ObjectId('123456789013'),
  mongoose.Types.ObjectId('123456789014')
];

// Doesn't work! Returns all elements of 'oids1', because ObjectIds
// are objects.
_.difference(oids1, oids2);

// Works! Diffs ObjectIds by their string values.
_.differenceBy(oids1, oids2, id => id.toString()); // [mongoose.Types.ObjectId('123456789012')]

differenceWith()

The differenceWith() function takes a comparator function as a third parameter.
The comparator should accept two values, and return true if the values should be considered equal.
The differenceWith() function is often used to calculate the difference between arrays of objects using Lodash’s deep equality check to compare JavaScript objects.

const _ = require('lodash');
const array = [
  { x: 1, y: 2 },
  { x: 2, y: 1 },
];
const values = [{ x: 2, y: 1 }];
_.differenceWith(array, values, _.isEqual) // returns [{x: 1, y: 2}]